================
Event Histograms
================
Documentation written by Tom Zanussi
1. Introduction
===============
Histogram triggers are special event triggers that can be used to
aggregate trace event data into histograms. For information on
trace events and event triggers, see Documentation/trace/events.rst.
2. Histogram Trigger Command
============================
A histogram trigger command is an event trigger command that
aggregates event hits into a hash table keyed on one or more trace
event format fields (or stacktrace) and a set of running totals
derived from one or more trace event format fields and/or event
counts (hitcount).
The format of a hist trigger is as follows::
hist:keys=<field1[,field2,...]>[:values=<field1[,field2,...]>]
[:sort=<field1[,field2,...]>][:size=#entries][:pause][:continue]
[:clear][:name=histname1][:<handler>.<action>] [if <filter>]
When a matching event is hit, an entry is added to a hash table
using the key(s) and value(s) named. Keys and values correspond to
fields in the event's format description. Values must correspond to
numeric fields - on an event hit, the value(s) will be added to a
sum kept for that field. The special string 'hitcount' can be used
in place of an explicit value field - this is simply a count of
event hits. If 'values' isn't specified, an implicit 'hitcount'
value will be automatically created and used as the only value.
Keys can be any field, or the special string 'stacktrace', which
will use the event's kernel stacktrace as the key. The keywords
'keys' or 'key' can be used to specify keys, and the keywords
'values', 'vals', or 'val' can be used to specify values. Compound
keys consisting of up to two fields can be specified by the 'keys'
keyword. Hashing a compound key produces a unique entry in the
table for each unique combination of component keys, and can be
useful for providing more fine-grained summaries of event data.
Additionally, sort keys consisting of up to two fields can be
specified by the 'sort' keyword. If more than one field is
specified, the result will be a 'sort within a sort': the first key
is taken to be the primary sort key and the second the secondary
key. If a hist trigger is given a name using the 'name' parameter,
its histogram data will be shared with other triggers of the same
name, and trigger hits will update this common data. Only triggers
with 'compatible' fields can be combined in this way; triggers are
'compatible' if the fields named in the trigger share the same
number and type of fields and those fields also have the same names.
Note that any two events always share the compatible 'hitcount' and
'stacktrace' fields and can therefore be combined using those
fields, however pointless that may be.
'hist' triggers add a 'hist' file to each event's subdirectory.
Reading the 'hist' file for the event will dump the hash table in
its entirety to stdout. If there are multiple hist triggers
attached to an event, there will be a table for each trigger in the
output. The table displayed for a named trigger will be the same as
any other instance having the same name. Each printed hash table
entry is a simple list of the keys and values comprising the entry;
keys are printed first and are delineated by curly braces, and are
followed by the set of value fields for the entry. By default,
numeric fields are displayed as base-10 integers. This can be
modified by appending any of the following modifiers to the field
name:
=========== ==========================================
.hex display a number as a hex value
.sym display an address as a symbol
.sym-offset display an address as a symbol and offset
.syscall display a syscall id as a system call name
.execname display a common_pid as a program name
.log2 display log2 value rather than raw number
.usecs display a common_timestamp in microseconds
=========== ==========================================
Note that in general the semantics of a given field aren't
interpreted when applying a modifier to it, but there are some
restrictions to be aware of in this regard:
- only the 'hex' modifier can be used for values (because values
are essentially sums, and the other modifiers don't make sense
in that context).
- the 'execname' modifier can only be used on a 'common_pid'. The
reason for this is that the execname is simply the 'comm' value
saved for the 'current' process when an event was triggered,
which is the same as the common_pid value saved by the event
tracing code. Trying to apply that comm value to other pid
values wouldn't be correct, and typically events that care save
pid-specific comm fields in the event itself.
A typical usage scenario would be the following to enable a hist
trigger, read its current contents, and then turn it off::
# echo 'hist:keys=skbaddr.hex:vals=len' > \
/sys/kernel/debug/tracing/events/net/netif_rx/trigger
# cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
# echo '!hist:keys=skbaddr.hex:vals=len' > \
/sys/kernel/debug/tracing/events/net/netif_rx/trigger
The trigger file itself can be read to show the details of the
currently attached hist trigger. This information is also displayed
at the top of the 'hist' file when read.
By default, the size of the hash table is 2048 entries. The 'size'
parameter can be used to specify more or fewer than that. The units
are in terms of hashtable entries - if a run uses more entries than
specified, the results will show the number of 'drops', the number
of hits that were ignored. The size should be a power of 2 between
128 and 131072 (any non- power-of-2 number specified will be rounded
up).
The 'sort' parameter can be used to specify a value field to sort
on. The default if unspecified is 'hitcount' and the default sort
order is 'ascending'. To sort in the opposite direction, append
.descending' to the sort key.
The 'pause' parameter can be used to pause an existing hist trigger
or to start a hist trigger but not log any events until told to do
so. 'continue' or 'cont' can be used to start or restart a paused
hist trigger.
The 'clear' parameter will clear the contents of a running hist
trigger and leave its current paused/active state.
Note that the 'pause', 'cont', and 'clear' parameters should be
applied using 'append' shell operator ('>>') if applied to an
existing trigger, rather than via the '>' operator, which will cause
the trigger to be removed through truncation.
- enable_hist/disable_hist
The enable_hist and disable_hist triggers can be used to have one
event conditionally start and stop another event's already-attached
hist trigger. Any number of enable_hist and disable_hist triggers
can be attached to a given event, allowing that event to kick off
and stop aggregations on a host of other events.
The format is very similar to the enable/disable_event triggers::
enable_hist:<system>:<event>[:count]
disable_hist:<system>:<event>[:count]
Instead of enabling or disabling the tracing of the target event
into the trace buffer as the enable/disable_event triggers do, the
enable/disable_hist triggers enable or disable the aggregation of
the target event into a hash table.
A typical usage scenario for the enable_hist/disable_hist triggers
would be to first set up a paused hist trigger on some event,
followed by an enable_hist/disable_hist pair that turns the hist
aggregation on and off when conditions of interest are hit::
# echo 'hist:keys=skbaddr.hex:vals=len:pause' > \
/sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
# echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
/sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
# echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
/sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
The above sets up an initially paused hist trigger which is unpaused
and starts aggregating events when a given program is executed, and
which stops aggregating when the process exits and the hist trigger
is paused again.
The examples below provide a more concrete illustration of the
concepts and typical usage patterns discussed above.
'special' event fields
------------------------
There are a number of 'special event fields' available for use as
keys or values in a hist trigger. These look like and behave as if
they were actual event fields, but aren't really part of the event's
field definition or format file. They are however available for any
event, and can be used anywhere an actual event field could be.
They are:
====================== ==== =======================================
common_timestamp u64 timestamp (from ring buffer) associated
with the event, in nanoseconds. May be
modified by .usecs to have timestamps
interpreted as microseconds.
cpu int the cpu on which the event occurred.
====================== ==== =======================================
Extended error information
--------------------------
For some error conditions encountered when invoking a hist trigger
command, extended error information is available via the
tracing/error_log file. See Error Conditions in
:file:`Documentation/trace/ftrace.rst` for details.
6.2 'hist' trigger examples
---------------------------
The first set of examples creates aggregations using the kmalloc
event. The fields that can be used for the hist trigger are listed
in the kmalloc event's format file::
# cat /sys/kernel/debug/tracing/events/kmem/kmalloc/format
name: kmalloc
ID: 374
format:
field:unsigned short common_type; offset:0; size:2; signed:0;
field:unsigned char common_flags; offset:2; size:1; signed:0;
field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
field:int common_pid; offset:4; size:4; signed:1;
field:unsigned long call_site; offset:8; size:8; signed:0;
field:const void * ptr; offset:16; size:8; signed:0;
field:size_t bytes_req; offset:24; size:8; signed:0;
field:size_t bytes_alloc; offset:32; size:8; signed:0;
field:gfp_t gfp_flags; offset:40; size:4; signed:0;
We'll start by creating a hist trigger that generates a simple table
that lists the total number of bytes requested for each function in
the kernel that made one or more calls to kmalloc::
# echo 'hist:key=call_site:val=bytes_req' > \
/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
This tells the tracing system to create a 'hist' trigger using the
call_site field of the kmalloc event as the key for the table, which
just means that each unique call_site address will have an entry
created for it in the table. The 'val=bytes_req' parameter tells
the hist trigger that for each unique entry (call_site) in the
table, it should keep a running total of the number of bytes
requested by that call_site.
We'll let it run for awhile and then dump the contents of the 'hist'
file in the kmalloc event's subdirectory (for readability, a number
of entries have been omitted)::
# cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
# trigger info: hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]
{ call_site: 18446744072106379007 } hitcount: 1 bytes_req: 176
{ call_site: 18446744071579557049 } hitcount: 1 bytes_req: 1024
{ call_site: 18446744071580608289 } hitcount: 1 bytes_req: 16384
{ call_site: 18446744071581827654 } hitcount: 1 bytes_req: 24
{ call_site: 18446744071580700980 } hitcount: 1 bytes_req: 8
{ call_site: 18446744071579359876 } hitcount: 1 bytes_req: 152
{ call_site: 18446744071580795365 } hitcount: 3 bytes_req: 144
{ call_site: 18446744071581303129 } hitcount: 3 bytes_req: 144
{ call_site: 18446744071580713234 } hitcount: 4 bytes_req: 2560
{ call_site: 18446744071580933750 } hitcount: 4 bytes_req: 736
.
.
.
{ call_site: 18446744072106047046 } hitcount: 69 bytes_req: 5576
{ call_site: 18446744071582116407 } hitcount: 73 bytes_req: 2336
{ call_site: 18446744072106054684 } hitcount: 136 bytes_req: 140504
{ call_site: 18446744072106224230 } hitcount: 136 bytes_req: 19584
{ call_site: 18446744072106078074 } hitcount: 153 bytes_req: 2448
{ call_site: 18446744072106062406 } hitcount: 153 bytes_req: 36720
{ call_site: 18446744071582507929 } hitcount: 153 bytes_req: 37088
{ call_site: 18446744072102520590 } hitcount: 273 bytes_req: 10920
{ call_site: 18446744071582143559 } hitcount: 358 bytes_req: 716
{ call_site: 18446744072106465852 } hitcount: 417 bytes_req: 56712
{ call_site: 18446744072102523378 } hitcount: 485 bytes_req: 27160
{ call_site: 18446744072099568646 } hitcount: 1676 bytes_req: 33520
Totals:
Hits: 4610
Entries: 45
Dropped: 0
The output displays a line for each entry, beginning with the key
specified in the trigger, followed by the value(s) also specified in
the trigger. At the beginning of the output is a line that displays
the trigger info, which can also be displayed by reading the
'trigger' file::
# cat /sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
hist:keys=call_site:vals=bytes_req:sort=hitcount:size=2048 [active]
At the end of the output are a few lines that display the overall
totals for the run. The 'Hits' field shows the total number of
times the event trigger was hit, the 'Entries' field shows the total
number of used entries in the hash table, and the 'Dropped' field
shows the number of hits that were dropped because the number of
used entries for the run exceeded the maximum number of entries
allowed for the table (normally 0, but if not a hint that you may
want to increase the size of the table using the 'size' parameter).
Notice in the above output that there's an extra field, 'hitcount',
which wasn't specified in the trigger. Also notice that in the
trigger info output, there's a parameter, 'sort=hitcount', which
wasn't specified in the trigger either. The reason for that is that
every trigger implicitly keeps a count of the total number of hits
attributed to a given entry, called the 'hitcount'. That hitcount
information is explicitly displayed in the output, and in the
absence of a user-specified sort parameter, is used as the default
sort field.
The value 'hitcount' can be used in place of an explicit value in
the 'values' parameter if you don't really need to have any
particular field summed and are mainly interested in hit
frequencies.
To turn the hist trigger off, simply call up the trigger in the
command history and re-execute it with a '!' prepended::
# echo '!hist:key=call_site:val=bytes_req' > \
/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
Finally, notice that the call_site as displayed in the output above
isn't really very useful. It's an address, but normally addresses
are displayed in hex. To have a numeric field displayed as a hex
value, simply append '.hex' to the field name in the trigger::
# echo 'hist:key=call_site.hex:val=bytes_req' > \
/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
# cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
# trigger info: hist:keys=call_site.hex:vals=bytes_req:sort=hitcount:size=2048 [active]
{ call_site: ffffffffa026b291 } hitcount: 1 bytes_req: 433
{ call_site: ffffffffa07186ff } hitcount: 1 bytes_req: 176
{ call_site: ffffffff811ae721 } hitcount: 1 bytes_req: 16384
{ call_site: ffffffff811c5134 } hitcount: 1 bytes_req: 8
{ call_site: ffffffffa04a9ebb } hitcount: 1 bytes_req: 511
{ call_site: ffffffff8122e0a6 } hitcount: 1 bytes_req: 12
{ call_site: ffffffff8107da84 } hitcount: 1 bytes_req: 152
{ call_site: ffffffff812d8246 } hitcount: 1 bytes_req: 24
{ call_site: ffffffff811dc1e5 } hitcount: 3 bytes_req: 144
{ call_site: ffffffffa02515e8 } hitcount: 3 bytes_req: 648
{ call_site: ffffffff81258159 } hitcount: 3 bytes_req: 144
{ call_site: ffffffff811c80f4 } hitcount: 4 bytes_req: 544
.
.
.
{ call_site: ffffffffa06c7646 } hitcount: 106 bytes_req: 8024
{ call_site: ffffffffa06cb246 } hitcount: 132 bytes_req: 31680
{ call_site: ffffffffa06cef7a } hitcount: 132 bytes_req: 2112
{ call_site: ffffffff8137e399 } hitcount: 132 bytes_req: 23232
{ call_site: ffffffffa06c941c } hitcount: 185 bytes_req: 171360
{ call_site: ffffffffa06f2a66 } hitcount: 185 bytes_req: 26640
{ call_site: ffffffffa036a70e } hitcount: 265 bytes_req: 10600
{ call_site: ffffffff81325447 } hitcount: 292 bytes_req: 584
{ call_site: ffffffffa072da3c } hitcount: 446 bytes_req: 60656
{ call_site: ffffffffa036b1f2 } hitcount: 526 bytes_req: 29456
{ call_site: ffffffffa0099c06 } hitcount: 1780 bytes_req: 35600
Totals:
Hits: 4775
Entries: 46
Dropped: 0
Even that's only marginally more useful - while hex values do look
more like addresses, what users are typically more interested in
when looking at text addresses are the corresponding symbols
instead. To have an address displayed as symbolic value instead,
simply append '.sym' or '.sym-offset' to the field name in the
trigger::
# echo 'hist:key=call_site.sym:val=bytes_req' > \
/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
# cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
# trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=hitcount:size=2048 [active]
{ call_site: [ffffffff810adcb9] syslog_print_all } hitcount: 1 bytes_req: 1024
{ call_site: [ffffffff8154bc62] usb_control_msg } hitcount: 1 bytes_req: 8
{ call_site: [ffffffffa00bf6fe] hidraw_send_report [hid] } hitcount: 1 bytes_req: 7
{ call_site: [ffffffff8154acbe] usb_alloc_urb } hitcount: 1 bytes_req: 192
{ call_site: [ffffffffa00bf1ca] hidraw_report_event [hid] } hitcount: 1 bytes_req: 7
{ call_site: [ffffffff811e3a25] __seq_open_private } hitcount: 1 bytes_req: 40
{ call_site: [ffffffff8109524a] alloc_fair_sched_group } hitcount: 2 bytes_req: 128
{ call_site: [ffffffff811febd5] fsnotify_alloc_group } hitcount: 2 bytes_req: 528
{ call_site: [ffffffff81440f58] __tty_buffer_request_room } hitcount: 2 bytes_req: 2624
{ call_site: [ffffffff81200ba6] inotify_new_group } hitcount: 2 bytes_req: 96
{ call_site: [ffffffffa05e19af] ieee80211_start_tx_ba_session [mac80211] } hitcount: 2 bytes_req: 464
{ call_site: [ffffffff81672406] tcp_get_metrics } hitcount: 2 bytes_req: 304
{ call_site: [ffffffff81097ec2] alloc_rt_sched_group } hitcount: 2 bytes_req: 128
{ call_site: [ffffffff81089b05] sched_create_group } hitcount: 2 bytes_req: 1424
.
.
.
{ call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915] } hitcount: 1185 bytes_req: 123240
{ call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm] } hitcount: 1185 bytes_req: 104280
{ call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915] } hitcount: 1402 bytes_req: 190672
{ call_site: [ffffffff812891ca] ext4_find_extent } hitcount: 1518 bytes_req: 146208
{ call_site: [ffffffffa029070e] drm_vma_node_allow [drm] } hitcount: 1746 bytes_req: 69840
{ call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915] } hitcount: 2021 bytes_req: 792312
{ call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm] } hitcount: 2592 bytes_req: 145152
{ call_site: [ffffffffa0489a66] intel_ring_begin [i915] } hitcount: 2629 bytes_req: 378576
{ call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915] } hitcount: 2629 bytes_req: 3783248
{ call_site: [ffffffff81325607] apparmor_file_alloc_security } hitcount: 5192 bytes_req: 10384
{ call_site: [ffffffffa00b7c06] hid_report_raw_event [hid] } hitcount: 5529 bytes_req: 110584
{ call_site: [ffffffff8131ebf7] aa_alloc_task_context } hitcount: 21943 bytes_req: 702176
{ call_site: [ffffffff8125847d] ext4_htree_store_dirent } hitcount: 55759 bytes_req: 5074265
Totals:
Hits: 109928
Entries: 71
Dropped: 0
Because the default sort key above is 'hitcount', the above shows a
the list of call_sites by increasing hitcount, so that at the bottom
we see the functions that made the most kmalloc calls during the
run. If instead we we wanted to see the top kmalloc callers in
terms of the number of bytes requested rather than the number of
calls, and we wanted the top caller to appear at the top, we can use
the 'sort' parameter, along with the 'descending' modifier::
# echo 'hist:key=call_site.sym:val=bytes_req:sort=bytes_req.descending' > \
/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
# cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
# trigger info: hist:keys=call_site.sym:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]
{ call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915] } hitcount: 2186 bytes_req: 3397464
{ call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915] } hitcount: 1790 bytes_req: 712176
{ call_site: [ffffffff8125847d] ext4_htree_store_dirent } hitcount: 8132 bytes_req: 513135
{ call_site: [ffffffff811e2a1b] seq_buf_alloc } hitcount: 106 bytes_req: 440128
{ call_site: [ffffffffa0489a66] intel_ring_begin [i915] } hitcount: 2186 bytes_req: 314784
{ call_site: [ffffffff812891ca] ext4_find_extent } hitcount: 2174 bytes_req: 208992
{ call_site: [ffffffff811ae8e1] __kmalloc } hitcount: 8 bytes_req: 131072
{ call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915] } hitcount: 859 bytes_req: 116824
{ call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm] } hitcount: 1834 bytes_req: 102704
{ call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915] } hitcount: 972 bytes_req: 101088
{ call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl [drm] } hitcount: 972 bytes_req: 85536
{ call_site: [ffffffffa00b7c06] hid_report_raw_event [hid] } hitcount: 3333 bytes_req: 66664
{ call_site: [ffffffff8137e559] sg_kmalloc } hitcount: 209 bytes_req: 61632
.
.
.
{ call_site: [ffffffff81095225] alloc_fair_sched_group } hitcount: 2 bytes_req: 128
{ call_site: [ffffffff81097ec2] alloc_rt_sched_group } hitcount: 2 bytes_req: 128
{ call_site: [ffffffff812d8406] copy_semundo } hitcount: 2 bytes_req: 48
{ call_site: [ffffffff81200ba6] inotify_new_group } hitcount: 1 bytes_req: 48
{ call_site: [ffffffffa027121a] drm_getmagic [drm] } hitcount: 1 bytes_req: 48
{ call_site: [ffffffff811e3a25] __seq_open_private } hitcount: 1 bytes_req: 40
{ call_site: [ffffffff811c52f4] bprm_change_interp } hitcount: 2 bytes_req: 16
{ call_site: [ffffffff8154bc62] usb_control_msg } hitcount: 1 bytes_req: 8
{ call_site: [ffffffffa00bf1ca] hidraw_report_event [hid] } hitcount: 1 bytes_req: 7
{ call_site: [ffffffffa00bf6fe] hidraw_send_report [hid] } hitcount: 1 bytes_req: 7
Totals:
Hits: 32133
Entries: 81
Dropped: 0
To display the offset and size information in addition to the symbol
name, just use 'sym-offset' instead::
# echo 'hist:key=call_site.sym-offset:val=bytes_req:sort=bytes_req.descending' > \
/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
# cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
# trigger info: hist:keys=call_site.sym-offset:vals=bytes_req:sort=bytes_req.descending:size=2048 [active]
{ call_site: [ffffffffa046041c] i915_gem_execbuffer2+0x6c/0x2c0 [i915] } hitcount: 4569 bytes_req: 3163720
{ call_site: [ffffffffa0489a66] intel_ring_begin+0xc6/0x1f0 [i915] } hitcount: 4569 bytes_req: 657936
{ call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23+0x694/0x1020 [i915] } hitcount: 1519 bytes_req: 472936
{ call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23+0x516/0x1020 [i915] } hitcount: 3050 bytes_req: 211832
{ call_site: [ffffffff811e2a1b] seq_buf_alloc+0x1b/0x50 } hitcount: 34 bytes_req: 148384
{ call_site: [ffffffffa04a580c] intel_crtc_page_flip+0xbc/0x870 [i915] } hitcount: 1385 bytes_req: 144040
{ call_site: [ffffffff811ae8e1] __kmalloc+0x191/0x1b0 } hitcount: 8 bytes_req: 131072
{ call_site: [ffffffffa0287592] drm_mode_page_flip_ioctl+0x282/0x360 [drm] } hitcount: 1385 bytes_req: 121880
{ call_site: [ffffffffa02911f2] drm_modeset_lock_crtc+0x32/0x100 [drm] } hitcount: 1848 bytes_req: 103488
{ call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state+0x2c/0xa0 [i915] } hitcount: 461 bytes_req: 62696
{ call_site: [ffffffffa029070e] drm_vma_node_allow+0x2e/0xd0 [drm] } hitcount: 1541 bytes_req: 61640
{ call_site: [ffffffff815f8d7b] sk_prot_alloc+0xcb/0x1b0 } hitcount: 57 bytes_req: 57456
.
.
.
{ call_site: [ffffffff8109524a] alloc_fair_sched_group+0x5a/0x1a0 } hitcount: 2 bytes_req: 128
{ call_site: [ffffffffa027b921] drm_vm_open_locked+0x31/0xa0 [drm] } hitcount: 3 bytes_req: 96
{ call_site: [ffffffff8122e266] proc_self_follow_link+0x76/0xb0 } hitcount: 8 bytes_req: 96
{ call_site: [ffffffff81213e80] load_elf_binary+0x240/0x1650 } hitcount: 3 bytes_req: 84
{ call_site: [ffffffff8154bc62] usb_control_msg+0x42/0x110 } hitcount: 1 bytes_req: 8
{ call_site: [ffffffffa00bf6fe] hidraw_send_report+0x7e/0x1a0 [hid] } hitcount: 1 bytes_req: 7
{ call_site: [ffffffffa00bf1ca] hidraw_report_event+0x8a/0x120 [hid] } hitcount: 1 bytes_req: 7
Totals:
Hits: 26098
Entries: 64
Dropped: 0
We can also add multiple fields to the 'values' parameter. For
example, we might want to see the total number of bytes allocated
alongside bytes requested, and display the result sorted by bytes
allocated in a descending order::
# echo 'hist:keys=call_site.sym:values=bytes_req,bytes_alloc:sort=bytes_alloc.descending' > \
/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
# cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
# trigger info: hist:keys=call_site.sym:vals=bytes_req,bytes_alloc:sort=bytes_alloc.descending:size=2048 [active]
{ call_site: [ffffffffa046041c] i915_gem_execbuffer2 [i915] } hitcount: 7403 bytes_req: 4084360 bytes_alloc: 5958016
{ call_site: [ffffffff811e2a1b] seq_buf_alloc } hitcount: 541 bytes_req: 2213968 bytes_alloc: 2228224
{ call_site: [ffffffffa0489a66] intel_ring_begin [i915] } hitcount: 7404 bytes_req: 1066176 bytes_alloc: 1421568
{ call_site: [ffffffffa045e7c4] i915_gem_do_execbuffer.isra.23 [i915] } hitcount: 1565 bytes_req: 557368 bytes_alloc: 1037760
{ call_site: [ffffffff8125847d] ext4_htree_store_dirent } hitcount: 9557 bytes_req: 595778 bytes_alloc: 695744
{ call_site: [ffffffffa045e646] i915_gem_do_execbuffer.isra.23 [i915] } hitcount: 5839 bytes_req: 430680 bytes_alloc: 470400
{ call_site: [ffffffffa04c4a3c] intel_plane_duplicate_state [i915] } hitcount: 2388 bytes_req: 324768 bytes_alloc: 458496
{ call_site: [ffffffffa02911f2] drm_modeset_lock_crtc [drm] } hitcount: 3911 bytes_req: 219016 bytes_alloc: 250304
{ call_site: [ffffffff815f8d7b] sk_prot_alloc } hitcount: 235 bytes_req: 236880 bytes_alloc: 240640
{ call_site: [ffffffff8137e559] sg_kmalloc } hitcount: 557 bytes_req: 169024 bytes_alloc: 221760
{ call_site: [ffffffffa00b7c06] hid_report_raw_event [hid] } hitcount: 9378 bytes_req: 187548 bytes_alloc: 206312
{ call_site: [ffffffffa04a580c] intel_crtc_page_flip [i915] } hitcount: 1519 bytes_req: 157976 bytes_alloc: 194432
.
.
.
{ call_site: [ffffffff8109bd3b] sched_autogroup_create_attach } hitcount: 2 bytes_req: 144 bytes_alloc: 192
{ call_site: [ffffffff81097ee8] alloc_rt_sched_group } hitcount: 2 bytes_req: 128 bytes_alloc: 128
{ call_site: [ffffffff8109524a] alloc_fair_sched_group } hitcount: 2 bytes_req: 128 bytes_alloc: 128
{ call_site: [ffffffff81095225] alloc_fair_sched_group } hitcount: 2 bytes_req: 128 bytes_alloc: 128
{ call_site: [ffffffff81097ec2] alloc_rt_sched_group } hitcount: 2 bytes_req: 128 bytes_alloc: 128
{ call_site: [ffffffff81213e80] load_elf_binary } hitcount: 3 bytes_req: 84 bytes_alloc: 96
{ call_site: [ffffffff81079a2e] kthread_create_on_node } hitcount: 1 bytes_req: 56 bytes_alloc: 64
{ call_site: [ffffffffa00bf6fe] hidraw_send_report [hid] } hitcount: 1 bytes_req: 7 bytes_alloc: 8
{ call_site: [ffffffff8154bc62] usb_control_msg } hitcount: 1 bytes_req: 8 bytes_alloc: 8
{ call_site: [ffffffffa00bf1ca] hidraw_report_event [hid] } hitcount: 1 bytes_req: 7 bytes_alloc: 8
Totals:
Hits: 66598
Entries: 65
Dropped: 0
Finally, to finish off our kmalloc example, instead of simply having
the hist trigger display symbolic call_sites, we can have the hist
trigger additionally display the complete set of kernel stack traces
that led to each call_site. To do that, we simply use the special
value 'stacktrace' for the key parameter::
# echo 'hist:keys=stacktrace:values=bytes_req,bytes_alloc:sort=bytes_alloc' > \
/sys/kernel/debug/tracing/events/kmem/kmalloc/trigger
The above trigger will use the kernel stack trace in effect when an
event is triggered as the key for the hash table. This allows the
enumeration of every kernel callpath that led up to a particular
event, along with a running total of any of the event fields for
that event. Here we tally bytes requested and bytes allocated for
every callpath in the system that led up to a kmalloc (in this case
every callpath to a kmalloc for a kernel compile)::
# cat /sys/kernel/debug/tracing/events/kmem/kmalloc/hist
# trigger info: hist:keys=stacktrace:vals=bytes_req,bytes_alloc:sort=bytes_alloc:size=2048 [active]
{ stacktrace:
__kmalloc_track_caller+0x10b/0x1a0
kmemdup+0x20/0x50
hidraw_report_event+0x8a/0x120 [hid]
hid_report_raw_event+0x3ea/0x440 [hid]
hid_input_report+0x112/0x190 [hid]
hid_irq_in+0xc2/0x260 [usbhid]
__usb_hcd_giveback_urb+0x72/0x120
usb_giveback_urb_bh+0x9e/0xe0
tasklet_hi_action+0xf8/0x100
__do_softirq+0x114/0x2c0
irq_exit+0xa5/0xb0
do_IRQ+0x5a/0xf0
ret_from_intr+0x0/0x30
cpuidle_enter+0x17/0x20
cpu_startup_entry+0x315/0x3e0
rest_init+0x7c/0x80
} hitcount: 3 bytes_req: 21 bytes_alloc: 24
{ stacktrace:
__kmalloc_track_caller+0x10b/0x1a0
kmemdup+0x20/0x50
hidraw_report_event+0x8a/0x120 [hid]
hid_report_raw_event+0x3ea/0x440 [hid]
hid_input_report+0x112/0x190 [hid]
hid_irq_in+0xc2/0x260 [usbhid]
__usb_hcd_giveback_urb+0x72/0x120
usb_giveback_urb_bh+0x9e/0xe0
tasklet_hi_action+0xf8/0x100
__do_softirq+0x114/0x2c0
irq_exit+0xa5/0xb0
do_IRQ+0x5a/0xf0
ret_from_intr+0x0/0x30
} hitcount: 3 bytes_req: 21 bytes_alloc: 24
{ stacktrace:
kmem_cache_alloc_trace+0xeb/0x150
aa_alloc_task_context+0x27/0x40
apparmor_cred_prepare+0x1f/0x50
security_prepare_creds+0x16/0x20
prepare_creds+0xdf/0x1a0
SyS_capset+0xb5/0x200
system_call_fastpath+0x12/0x6a
} hitcount: 1 bytes_req: 32 bytes_alloc: 32
.
.
.
{ stacktrace:
__kmalloc+0x11b/0x1b0
i915_gem_execbuffer2+0x6c/0x2c0 [i915]
drm_ioctl+0x349/0x670 [drm]
do_vfs_ioctl+0x2f0/0x4f0
SyS_ioctl+0x81/0xa0
system_call_fastpath+0x12/0x6a
} hitcount: 17726 bytes_req: 13944120 bytes_alloc: 19593808
{ stacktrace:
__kmalloc+0x11b/0x1b0
load_elf_phdrs+0x76/0xa0
load_elf_binary+0x102/0x1650
search_binary_handler+0x97/0x1d0
do_execveat_common.isra.34+0x551/0x6e0
SyS_execve+0x3a/0x50
return_from_execve+0x0/0x23
} hitcount: 33348 bytes_req: 17152128 bytes_alloc: 20226048
{ stacktrace:
kmem_cache_alloc_trace+0xeb/0x150
apparmor_file_alloc_security+0x27/0x40
security_file_alloc+0x16/0x20
get_empty_filp+0x93/0x1c0
path_openat+0x31/0x5f0
do_filp_open+0x3a/0x90
do_sys_open+0x128/0x220
SyS_open+0x1e/0x20
system_call_fastpath+0x12/0x6a
} hitcount: 4766422 bytes_req: 9532844 bytes_alloc: 38131376
{ stacktrace:
__kmalloc+0x11b/0x1b0
seq_buf_alloc+0x1b/0x50
seq_read+0x2cc/0x370
proc_reg_read+0x3d/0x80
__vfs_read+0x28/0xe0
vfs_read+0x86/0x140
SyS_read+0x46/0xb0
system_call_fastpath+0x12/0x6a
} hitcount: 19133 bytes_req: 78368768 bytes_alloc: 78368768
Totals:
Hits: 6085872
Entries: 253
Dropped: 0
If you key a hist trigger on common_pid, in order for example to
gather and display sorted totals for each process, you can use the
special .execname modifier to display the executable names for the
processes in the table rather than raw pids. The example below
keeps a per-process sum of total bytes read::
# echo 'hist:key=common_pid.execname:val=count:sort=count.descending' > \
/sys/kernel/debug/tracing/events/syscalls/sys_enter_read/trigger
# cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_read/hist
# trigger info: hist:keys=common_pid.execname:vals=count:sort=count.descending:size=2048 [active]
{ common_pid: gnome-terminal [ 3196] } hitcount: 280 count: 1093512
{ common_pid: Xorg [ 1309] } hitcount: 525 count: 256640
{ common_pid: compiz [ 2889] } hitcount: 59 count: 254400
{ common_pid: bash [ 8710] } hitcount: 3 count: 66369
{ common_pid: dbus-daemon-lau [ 8703] } hitcount: 49 count: 47739
{ common_pid: irqbalance [ 1252] } hitcount: 27 count: 27648
{ common_pid: 01ifupdown [ 8705] } hitcount: 3 count: 17216
{ common_pid: dbus-daemon [ 772] } hitcount: 10 count: 12396
{ common_pid: Socket Thread [ 8342] } hitcount: 11 count: 11264
{ common_pid: nm-dhcp-client. [ 8701] } hitcount: 6 count: 7424
{ common_pid: gmain [ 1315] } hitcount: 18 count: 6336
.
.
.
{ common_pid: postgres [ 1892] } hitcount: 2 count: 32
{ common_pid: postgres [ 1891] } hitcount: 2 count: 32
{ common_pid: gmain [ 8704] } hitcount: 2 count: 32
{ common_pid: upstart-dbus-br [ 2740] } hitcount: 21 count: 21
{ common_pid: nm-dispatcher.a [ 8696] } hitcount: 1 count: 16
{ common_pid: indicator-datet [ 2904] } hitcount: 1 count: 16
{ common_pid: gdbus [ 2998] } hitcount: 1 count: 16
{ common_pid: rtkit-daemon [ 2052] } hitcount: 1 count: 8
{ common_pid: init [ 1] } hitcount: 2 count: 2
Totals:
Hits: 2116
Entries: 51
Dropped: 0
Similarly, if you key a hist trigger on syscall id, for example to
gather and display a list of systemwide syscall hits, you can use
the special .syscall modifier to display the syscall names rather
than raw ids. The example below keeps a running total of syscall
counts for the system during the run::
# echo 'hist:key=id.syscall:val=hitcount' > \
/sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
# cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
# trigger info: hist:keys=id.syscall:vals=hitcount:sort=hitcount:size=2048 [active]
{ id: sys_fsync [ 74] } hitcount: 1
{ id: sys_newuname [ 63] } hitcount: 1
{ id: sys_prctl [157] } hitcount: 1
{ id: sys_statfs [137] } hitcount: 1
{ id: sys_symlink [ 88] } hitcount: 1
{ id: sys_sendmmsg [307] } hitcount: 1
{ id: sys_semctl [ 66] } hitcount: 1
{ id: sys_readlink [ 89] } hitcount: 3
{ id: sys_bind [ 49] } hitcount: 3
{ id: sys_getsockname [ 51] } hitcount: 3
{ id: sys_unlink [ 87] } hitcount: 3
{ id: sys_rename [ 82] } hitcount: 4
{ id: unknown_syscall [ 58] } hitcount: 4
{ id: sys_connect [ 42] } hitcount: 4
{ id: sys_getpid [ 39] } hitcount: 4
.
.
.
{ id: sys_rt_sigprocmask [ 14] } hitcount: 952
{ id: sys_futex [202] } hitcount: 1534
{ id: sys_write [ 1] } hitcount: 2689
{ id: sys_setitimer [ 38] } hitcount: 2797
{ id: sys_read [ 0] } hitcount: 3202
{ id: sys_select [ 23] } hitcount: 3773
{ id: sys_writev [ 20] } hitcount: 4531
{ id: sys_poll [ 7] } hitcount: 8314
{ id: sys_recvmsg [ 47] } hitcount: 13738
{ id: sys_ioctl [ 16] } hitcount: 21843
Totals:
Hits: 67612
Entries: 72
Dropped: 0
The syscall counts above provide a rough overall picture of system
call activity on the system; we can see for example that the most
popular system call on this system was the 'sys_ioctl' system call.
We can use 'compound' keys to refine that number and provide some
further insight as to which processes exactly contribute to the
overall ioctl count.
The command below keeps a hitcount for every unique combination of
system call id and pid - the end result is essentially a table
that keeps a per-pid sum of system call hits. The results are
sorted using the system call id as the primary key, and the
hitcount sum as the secondary key::
# echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount' > \
/sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
# cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
# trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 [active]
{ id: sys_read [ 0], common_pid: rtkit-daemon [ 1877] } hitcount: 1
{ id: sys_read [ 0], common_pid: gdbus [ 2976] } hitcount: 1
{ id: sys_read [ 0], common_pid: console-kit-dae [ 3400] } hitcount: 1
{ id: sys_read [ 0], common_pid: postgres [ 1865] } hitcount: 1
{ id: sys_read [ 0], common_pid: deja-dup-monito [ 3543] } hitcount: 2
{ id: sys_read [ 0], common_pid: NetworkManager [ 890] } hitcount: 2
{ id: sys_read [ 0], common_pid: evolution-calen [ 3048] } hitcount: 2
{ id: sys_read [ 0], common_pid: postgres [ 1864] } hitcount: 2
{ id: sys_read [ 0], common_pid: nm-applet [ 3022] } hitcount: 2
{ id: sys_read [ 0], common_pid: whoopsie [ 1212] } hitcount: 2
.
.
.
{ id: sys_ioctl [ 16], common_pid: bash [ 8479] } hitcount: 1
{ id: sys_ioctl [ 16], common_pid: bash [ 3472] } hitcount: 12
{ id: sys_ioctl [ 16], common_pid: gnome-terminal [ 3199] } hitcount: 16
{ id: sys_ioctl [ 16], common_pid: Xorg [ 1267] } hitcount: 1808
{ id: sys_ioctl [ 16], common_pid: compiz [ 2994] } hitcount: 5580
.
.
.
{ id: sys_waitid [247], common_pid: upstart-dbus-br [ 2690] } hitcount: 3
{ id: sys_waitid [247], common_pid: upstart-dbus-br [ 2688] } hitcount: 16
{ id: sys_inotify_add_watch [254], common_pid: gmain [ 975] } hitcount: 2
{ id: sys_inotify_add_watch [254], common_pid: gmain [ 3204] } hitcount: 4
{ id: sys_inotify_add_watch [254], common_pid: gmain [ 2888] } hitcount: 4
{ id: sys_inotify_add_watch [254], common_pid: gmain [ 3003] } hitcount: 4
{ id: sys_inotify_add_watch [254], common_pid: gmain [ 2873] } hitcount: 4
{ id: sys_inotify_add_watch [254], common_pid: gmain [ 3196] } hitcount: 6
{ id: sys_openat [257], common_pid: java [ 2623] } hitcount: 2
{ id: sys_eventfd2 [290], common_pid: ibus-ui-gtk3 [ 2760] } hitcount: 4
{ id: sys_eventfd2 [290], common_pid: compiz [ 2994] } hitcount: 6
Totals:
Hits: 31536
Entries: 323
Dropped: 0
The above list does give us a breakdown of the ioctl syscall by
pid, but it also gives us quite a bit more than that, which we
don't really care about at the moment. Since we know the syscall
id for sys_ioctl (16, displayed next to the sys_ioctl name), we
can use that to filter out all the other syscalls::
# echo 'hist:key=id.syscall,common_pid.execname:val=hitcount:sort=id,hitcount if id == 16' > \
/sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/trigger
# cat /sys/kernel/debug/tracing/events/raw_syscalls/sys_enter/hist
# trigger info: hist:keys=id.syscall,common_pid.execname:vals=hitcount:sort=id.syscall,hitcount:size=2048 if id == 16 [active]
{ id: sys_ioctl [ 16], common_pid: gmain [ 2769] } hitcount: 1
{ id: sys_ioctl [ 16], common_pid: evolution-addre [ 8571] } hitcount: 1
{ id: sys_ioctl [ 16], common_pid: gmain [ 3003] } hitcount: 1
{ id: sys_ioctl [ 16], common_pid: gmain [ 2781] } hitcount: 1
{ id: sys_ioctl [ 16], common_pid: gmain [ 2829] } hitcount: 1
{ id: sys_ioctl [ 16], common_pid: bash [ 8726] } hitcount: 1
{ id: sys_ioctl [ 16], common_pid: bash [ 8508] } hitcount: 1
{ id: sys_ioctl [ 16], common_pid: gmain [ 2970] } hitcount: 1
{ id: sys_ioctl [ 16], common_pid: gmain [ 2768] } hitcount: 1
.
.
.
{ id: sys_ioctl [ 16], common_pid: pool [ 8559] } hitcount: 45
{ id: sys_ioctl [ 16], common_pid: pool [ 8555] } hitcount: 48
{ id: sys_ioctl [ 16], common_pid: pool [ 8551] } hitcount: 48
{ id: sys_ioctl [ 16], common_pid: avahi-daemon [ 896] } hitcount: 66
{ id: sys_ioctl [ 16], common_pid: Xorg [ 1267] } hitcount: 26674
{ id: sys_ioctl [ 16], common_pid: compiz [ 2994] } hitcount: 73443
Totals:
Hits: 101162
Entries: 103
Dropped: 0
The above output shows that 'compiz' and 'Xorg' are far and away
the heaviest ioctl callers (which might lead to questions about
whether they really need to be making all those calls and to
possible avenues for further investigation.)
The compound key examples used a key and a sum value (hitcount) to
sort the output, but we can just as easily use two keys instead.
Here's an example where we use a compound key composed of the the
common_pid and size event fields. Sorting with pid as the primary
key and 'size' as the secondary key allows us to display an
ordered summary of the recvfrom sizes, with counts, received by
each process::
# echo 'hist:key=common_pid.execname,size:val=hitcount:sort=common_pid,size' > \
/sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/trigger
# cat /sys/kernel/debug/tracing/events/syscalls/sys_enter_recvfrom/hist
# trigger info: hist:keys=common_pid.execname,size:vals=hitcount:sort=common_pid.execname,size:size=2048 [active]
{ common_pid: smbd [ 784], size: 4 } hitcount: 1
{ common_pid: dnsmasq [ 1412], size: 4096 } hitcount: 672
{ common_pid: postgres [ 1796], size: 1000 } hitcount: 6
{ common_pid: postgres [ 1867], size: 1000 } hitcount: 10
{ common_pid: bamfdaemon [ 2787], size: 28 } hitcount: 2
{ common_pid: bamfdaemon [ 2787], size: 14360 } hitcount: 1
{ common_pid: compiz [ 2994], size: 8 } hitcount: 1
{ common_pid: compiz [ 2994], size: 20 } hitcount: 11
{ common_pid: gnome-terminal [ 3199], size: 4 } hitcount: 2
{ common_pid: firefox [ 8817], size: 4 } hitcount: 1
{ common_pid: firefox [ 8817], size: 8 } hitcount: 5
{ common_pid: firefox [ 8817], size: 588 } hitcount: 2
{ common_pid: firefox [ 8817], size: 628 } hitcount: 1
{ common_pid: firefox [ 8817], size: 6944 } hitcount: 1
{ common_pid: firefox [ 8817], size: 408880 } hitcount: 2
{ common_pid: firefox [ 8822], size: 8 } hitcount: 2
{ common_pid: firefox [ 8822], size: 160 } hitcount: 2
{ common_pid: firefox [ 8822], size: 320 } hitcount: 2
{ common_pid: firefox [ 8822], size: 352 } hitcount: 1
.
.
.
{ common_pid: pool [ 8923], size: 1960 } hitcount: 10
{ common_pid: pool [ 8923], size: 2048 } hitcount: 10
{ common_pid: pool [ 8924], size: 1960 } hitcount: 10
{ common_pid: pool [ 8924], size: 2048 } hitcount: 10
{ common_pid: pool [ 8928], size: 1964 } hitcount: 4
{ common_pid: pool [ 8928], size: 1965 } hitcount: 2
{ common_pid: pool [ 8928], size: 2048 } hitcount: 6
{ common_pid: pool [ 8929], size: 1982 } hitcount: 1
{ common_pid: pool [ 8929], size: 2048 } hitcount: 1
Totals:
Hits: 2016
Entries: 224
Dropped: 0
The above example also illustrates the fact that although a compound
key is treated as a single entity for hashing purposes, the sub-keys
it's composed of can be accessed independently.
The next example uses a string field as the hash key and
demonstrates how you can manually pause and continue a hist trigger.
In this example, we'll aggregate fork counts and don't expect a
large number of entries in the hash table, so we'll drop it to a
much smaller number, say 256::
# echo 'hist:key=child_comm:val=hitcount:size=256' > \
/sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
# cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
# trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]
{ child_comm: dconf worker } hitcount: 1
{ child_comm: ibus-daemon } hitcount: 1
{ child_comm: whoopsie } hitcount: 1
{ child_comm: smbd } hitcount: 1
{ child_comm: gdbus } hitcount: 1
{ child_comm: kthreadd } hitcount: 1
{ child_comm: dconf worker } hitcount: 1
{ child_comm: evolution-alarm } hitcount: 2
{ child_comm: Socket Thread } hitcount: 2
{ child_comm: postgres } hitcount: 2
{ child_comm: bash } hitcount: 3
{ child_comm: compiz } hitcount: 3
{ child_comm: evolution-sourc } hitcount: 4
{ child_comm: dhclient } hitcount: 4
{ child_comm: pool } hitcount: 5
{ child_comm: nm-dispatcher.a } hitcount: 8
{ child_comm: firefox } hitcount: 8
{ child_comm: dbus-daemon } hitcount: 8
{ child_comm: glib-pacrunner } hitcount: 10
{ child_comm: evolution } hitcount: 23
Totals:
Hits: 89
Entries: 20
Dropped: 0
If we want to pause the hist trigger, we can simply append :pause to
the command that started the trigger. Notice that the trigger info
displays as [paused]::
# echo 'hist:key=child_comm:val=hitcount:size=256:pause' >> \
/sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
# cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
# trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [paused]
{ child_comm: dconf worker } hitcount: 1
{ child_comm: kthreadd } hitcount: 1
{ child_comm: dconf worker } hitcount: 1
{ child_comm: gdbus } hitcount: 1
{ child_comm: ibus-daemon } hitcount: 1
{ child_comm: Socket Thread } hitcount: 2
{ child_comm: evolution-alarm } hitcount: 2
{ child_comm: smbd } hitcount: 2
{ child_comm: bash } hitcount: 3
{ child_comm: whoopsie } hitcount: 3
{ child_comm: compiz } hitcount: 3
{ child_comm: evolution-sourc } hitcount: 4
{ child_comm: pool } hitcount: 5
{ child_comm: postgres } hitcount: 6
{ child_comm: firefox } hitcount: 8
{ child_comm: dhclient } hitcount: 10
{ child_comm: emacs } hitcount: 12
{ child_comm: dbus-daemon } hitcount: 20
{ child_comm: nm-dispatcher.a } hitcount: 20
{ child_comm: evolution } hitcount: 35
{ child_comm: glib-pacrunner } hitcount: 59
Totals:
Hits: 199
Entries: 21
Dropped: 0
To manually continue having the trigger aggregate events, append
:cont instead. Notice that the trigger info displays as [active]
again, and the data has changed::
# echo 'hist:key=child_comm:val=hitcount:size=256:cont' >> \
/sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
# cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
# trigger info: hist:keys=child_comm:vals=hitcount:sort=hitcount:size=256 [active]
{ child_comm: dconf worker } hitcount: 1
{ child_comm: dconf worker } hitcount: 1
{ child_comm: kthreadd } hitcount: 1
{ child_comm: gdbus } hitcount: 1
{ child_comm: ibus-daemon } hitcount: 1
{ child_comm: Socket Thread } hitcount: 2
{ child_comm: evolution-alarm } hitcount: 2
{ child_comm: smbd } hitcount: 2
{ child_comm: whoopsie } hitcount: 3
{ child_comm: compiz } hitcount: 3
{ child_comm: evolution-sourc } hitcount: 4
{ child_comm: bash } hitcount: 5
{ child_comm: pool } hitcount: 5
{ child_comm: postgres } hitcount: 6
{ child_comm: firefox } hitcount: 8
{ child_comm: dhclient } hitcount: 11
{ child_comm: emacs } hitcount: 12
{ child_comm: dbus-daemon } hitcount: 22
{ child_comm: nm-dispatcher.a } hitcount: 22
{ child_comm: evolution } hitcount: 35
{ child_comm: glib-pacrunner } hitcount: 59
Totals:
Hits: 206
Entries: 21
Dropped: 0
The previous example showed how to start and stop a hist trigger by
appending 'pause' and 'continue' to the hist trigger command. A
hist trigger can also be started in a paused state by initially
starting the trigger with ':pause' appended. This allows you to
start the trigger only when you're ready to start collecting data
and not before. For example, you could start the trigger in a
paused state, then unpause it and do something you want to measure,
then pause the trigger again when done.
Of course, doing this manually can be difficult and error-prone, but
it is possible to automatically start and stop a hist trigger based
on some condition, via the enable_hist and disable_hist triggers.
For example, suppose we wanted to take a look at the relative
weights in terms of skb length for each callpath that leads to a
netif_receive_skb event when downloading a decent-sized file using
wget.
First we set up an initially paused stacktrace trigger on the
netif_receive_skb event::
# echo 'hist:key=stacktrace:vals=len:pause' > \
/sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
Next, we set up an 'enable_hist' trigger on the sched_process_exec
event, with an 'if filename==/usr/bin/wget' filter. The effect of
this new trigger is that it will 'unpause' the hist trigger we just
set up on netif_receive_skb if and only if it sees a
sched_process_exec event with a filename of '/usr/bin/wget'. When
that happens, all netif_receive_skb events are aggregated into a
hash table keyed on stacktrace::
# echo 'enable_hist:net:netif_receive_skb if filename==/usr/bin/wget' > \
/sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
The aggregation continues until the netif_receive_skb is paused
again, which is what the following disable_hist event does by
creating a similar setup on the sched_process_exit event, using the
filter 'comm==wget'::
# echo 'disable_hist:net:netif_receive_skb if comm==wget' > \
/sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
Whenever a process exits and the comm field of the disable_hist
trigger filter matches 'comm==wget', the netif_receive_skb hist
trigger is disabled.
The overall effect is that netif_receive_skb events are aggregated
into the hash table for only the duration of the wget. Executing a
wget command and then listing the 'hist' file will display the
output generated by the wget command::
$ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz
# cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
# trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]
{ stacktrace:
__netif_receive_skb_core+0x46d/0x990
__netif_receive_skb+0x18/0x60
netif_receive_skb_internal+0x23/0x90
napi_gro_receive+0xc8/0x100
ieee80211_deliver_skb+0xd6/0x270 [mac80211]
ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
ieee80211_rx+0x31d/0x900 [mac80211]
iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
irq_thread_fn+0x20/0x50
irq_thread+0x11f/0x150
kthread+0xd2/0xf0
ret_from_fork+0x42/0x70
} hitcount: 85 len: 28884
{ stacktrace:
__netif_receive_skb_core+0x46d/0x990
__netif_receive_skb+0x18/0x60
netif_receive_skb_internal+0x23/0x90
napi_gro_complete+0xa4/0xe0
dev_gro_receive+0x23a/0x360
napi_gro_receive+0x30/0x100
ieee80211_deliver_skb+0xd6/0x270 [mac80211]
ieee80211_rx_handlers+0xccf/0x22f0 [mac80211]
ieee80211_prepare_and_rx_handle+0x4e7/0xc40 [mac80211]
ieee80211_rx+0x31d/0x900 [mac80211]
iwlagn_rx_reply_rx+0x3db/0x6f0 [iwldvm]
iwl_rx_dispatch+0x8e/0xf0 [iwldvm]
iwl_pcie_irq_handler+0xe3c/0x12f0 [iwlwifi]
irq_thread_fn+0x20/0x50
irq_thread+0x11f/0x150
kthread+0xd2/0xf0
} hitcount: 98 len: 664329
{ stacktrace:
__netif_receive_skb_core+0x46d/0x990
__netif_receive_skb+0x18/0x60
process_backlog+0xa8/0x150
net_rx_action+0x15d/0x340
__do_softirq+0x114/0x2c0
do_softirq_own_stack+0x1c/0x30
do_softirq+0x65/0x70
__local_bh_enable_ip+0xb5/0xc0
ip_finish_output+0x1f4/0x840
ip_output+0x6b/0xc0
ip_local_out_sk+0x31/0x40
ip_send_skb+0x1a/0x50
udp_send_skb+0x173/0x2a0
udp_sendmsg+0x2bf/0x9f0
inet_sendmsg+0x64/0xa0
sock_sendmsg+0x3d/0x50
} hitcount: 115 len: 13030
{ stacktrace:
__netif_receive_skb_core+0x46d/0x990
__netif_receive_skb+0x18/0x60
netif_receive_skb_internal+0x23/0x90
napi_gro_complete+0xa4/0xe0
napi_gro_flush+0x6d/0x90
iwl_pcie_irq_handler+0x92a/0x12f0 [iwlwifi]
irq_thread_fn+0x20/0x50
irq_thread+0x11f/0x150
kthread+0xd2/0xf0
ret_from_fork+0x42/0x70
} hitcount: 934 len: 5512212
Totals:
Hits: 1232
Entries: 4
Dropped: 0
The above shows all the netif_receive_skb callpaths and their total
lengths for the duration of the wget command.
The 'clear' hist trigger param can be used to clear the hash table.
Suppose we wanted to try another run of the previous example but
this time also wanted to see the complete list of events that went
into the histogram. In order to avoid having to set everything up
again, we can just clear the histogram first::
# echo 'hist:key=stacktrace:vals=len:clear' >> \
/sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
Just to verify that it is in fact cleared, here's what we now see in
the hist file::
# cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
# trigger info: hist:keys=stacktrace:vals=len:sort=hitcount:size=2048 [paused]
Totals:
Hits: 0
Entries: 0
Dropped: 0
Since we want to see the detailed list of every netif_receive_skb
event occurring during the new run, which are in fact the same
events being aggregated into the hash table, we add some additional
'enable_event' events to the triggering sched_process_exec and
sched_process_exit events as such::
# echo 'enable_event:net:netif_receive_skb if filename==/usr/bin/wget' > \
/sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
# echo 'disable_event:net:netif_receive_skb if comm==wget' > \
/sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
If you read the trigger files for the sched_process_exec and
sched_process_exit triggers, you should see two triggers for each:
one enabling/disabling the hist aggregation and the other
enabling/disabling the logging of events::
# cat /sys/kernel/debug/tracing/events/sched/sched_process_exec/trigger
enable_event:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
enable_hist:net:netif_receive_skb:unlimited if filename==/usr/bin/wget
# cat /sys/kernel/debug/tracing/events/sched/sched_process_exit/trigger
enable_event:net:netif_receive_skb:unlimited if comm==wget
disable_hist:net:netif_receive_skb:unlimited if comm==wget
In other words, whenever either of the sched_process_exec or
sched_process_exit events is hit and matches 'wget', it enables or
disables both the histogram and the event log, and what you end up
with is a hash table and set of events just covering the specified
duration. Run the wget command again::
$ wget https://www.kernel.org/pub/linux/kernel/v3.x/patch-3.19.xz
Displaying the 'hist' file should show something similar to what you
saw in the last run, but this time you should also see the
individual events in the trace file::
# cat /sys/kernel/debug/tracing/trace
# tracer: nop
#
# entries-in-buffer/entries-written: 183/1426 #P:4
#
# _-----=> irqs-off
# / _----=> need-resched
# | / _---=> hardirq/softirq
# || / _--=> preempt-depth
# ||| / delay
# TASK-PID CPU# |||| TIMESTAMP FUNCTION
# | | | |||| | |
wget-15108 [000] ..s1 31769.606929: netif_receive_skb: dev=lo skbaddr=ffff88009c353100 len=60
wget-15108 [000] ..s1 31769.606999: netif_receive_skb: dev=lo skbaddr=ffff88009c353200 len=60
dnsmasq-1382 [000] ..s1 31769.677652: netif_receive_skb: dev=lo skbaddr=ffff88009c352b00 len=130
dnsmasq-1382 [000] ..s1 31769.685917: netif_receive_skb: dev=lo skbaddr=ffff88009c352200 len=138
##### CPU 2 buffer started ####
irq/29-iwlwifi-559 [002] ..s. 31772.031529: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433d00 len=2948
irq/29-iwlwifi-559 [002] ..s. 31772.031572: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432200 len=1500
irq/29-iwlwifi-559 [002] ..s. 31772.032196: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433100 len=2948
irq/29-iwlwifi-559 [002] ..s. 31772.032761: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d433000 len=2948
irq/29-iwlwifi-559 [002] ..s. 31772.033220: netif_receive_skb: dev=wlan0 skbaddr=ffff88009d432e00 len=1500
.
.
.
The following example demonstrates how multiple hist triggers can be
attached to a given event. This capability can be useful for
creating a set of different summaries derived from the same set of
events, or for comparing the effects of different filters, among
other things::
# echo 'hist:keys=skbaddr.hex:vals=len if len < 0' >> \
/sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
# echo 'hist:keys=skbaddr.hex:vals=len if len > 4096' >> \
/sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
# echo 'hist:keys=skbaddr.hex:vals=len if len == 256' >> \
/sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
# echo 'hist:keys=skbaddr.hex:vals=len' >> \
/sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
# echo 'hist:keys=len:vals=common_preempt_count' >> \
/sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
The above set of commands create four triggers differing only in
their filters, along with a completely different though fairly
nonsensical trigger. Note that in order to append multiple hist
triggers to the same file, you should use the '>>' operator to
append them ('>' will also add the new hist trigger, but will remove
any existing hist triggers beforehand).
Displaying the contents of the 'hist' file for the event shows the
contents of all five histograms::
# cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist
# event histogram
#
# trigger info: hist:keys=len:vals=hitcount,common_preempt_count:sort=hitcount:size=2048 [active]
#
{ len: 176 } hitcount: 1 common_preempt_count: 0
{ len: 223 } hitcount: 1 common_preempt_count: 0
{ len: 4854 } hitcount: 1 common_preempt_count: 0
{ len: 395 } hitcount: 1 common_preempt_count: 0
{ len: 177 } hitcount: 1 common_preempt_count: 0
{ len: 446 } hitcount: 1 common_preempt_count: 0
{ len: 1601 } hitcount: 1 common_preempt_count: 0
.
.
.
{ len: 1280 } hitcount: 66 common_preempt_count: 0
{ len: 116 } hitcount: 81 common_preempt_count: 40
{ len: 708 } hitcount: 112 common_preempt_count: 0
{ len: 46 } hitcount: 221 common_preempt_count: 0
{ len: 1264 } hitcount: 458 common_preempt_count: 0
Totals:
Hits: 1428
Entries: 147
Dropped: 0
# event histogram
#
# trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
#
{ skbaddr: ffff8800baee5e00 } hitcount: 1 len: 130
{ skbaddr: ffff88005f3d5600 } hitcount: 1 len: 1280
{ skbaddr: ffff88005f3d4900 } hitcount: 1 len: 1280
{ skbaddr: ffff88009fed6300 } hitcount: 1 len: 115
{ skbaddr: ffff88009fe0ad00 } hitcount: 1 len: 115
{ skbaddr: ffff88008cdb1900 } hitcount: 1 len: 46
{ skbaddr: ffff880064b5ef00 } hitcount: 1 len: 118
{ skbaddr: ffff880044e3c700 } hitcount: 1 len: 60
{ skbaddr: ffff880100065900 } hitcount: 1 len: 46
{ skbaddr: ffff8800d46bd500 } hitcount: 1 len: 116
{ skbaddr: ffff88005f3d5f00 } hitcount: 1 len: 1280
{ skbaddr: ffff880100064700 } hitcount: 1 len: 365
{ skbaddr: ffff8800badb6f00 } hitcount: 1 len: 60
.
.
.
{ skbaddr: ffff88009fe0be00 } hitcount: 27 len: 24677
{ skbaddr: ffff88009fe0a400 } hitcount: 27 len: 23052
{ skbaddr: ffff88009fe0b700 } hitcount: 31 len: 25589
{ skbaddr: ffff88009fe0b600 } hitcount: 32 len: 27326
{ skbaddr: ffff88006a462800 } hitcount: 68 len: 71678
{ skbaddr: ffff88006a463700 } hitcount: 70 len: 72678
{ skbaddr: ffff88006a462b00 } hitcount: 71 len: 77589
{ skbaddr: ffff88006a463600 } hitcount: 73 len: 71307
{ skbaddr: ffff88006a462200 } hitcount: 81 len: 81032
Totals:
Hits: 1451
Entries: 318
Dropped: 0
# event histogram
#
# trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len == 256 [active]
#
Totals:
Hits: 0
Entries: 0
Dropped: 0
# event histogram
#
# trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len > 4096 [active]
#
{ skbaddr: ffff88009fd2c300 } hitcount: 1 len: 7212
{ skbaddr: ffff8800d2bcce00 } hitcount: 1 len: 7212
{ skbaddr: ffff8800d2bcd700 } hitcount: 1 len: 7212
{ skbaddr: ffff8800d2bcda00 } hitcount: 1 len: 21492
{ skbaddr: ffff8800ae2e2d00 } hitcount: 1 len: 7212
{ skbaddr: ffff8800d2bcdb00 } hitcount: 1 len: 7212
{ skbaddr: ffff88006a4df500 } hitcount: 1 len: 4854
{ skbaddr: ffff88008ce47b00 } hitcount: 1 len: 18636
{ skbaddr: ffff8800ae2e2200 } hitcount: 1 len: 12924
{ skbaddr: ffff88005f3e1000 } hitcount: 1 len: 4356
{ skbaddr: ffff8800d2bcdc00 } hitcount: 2 len: 24420
{ skbaddr: ffff8800d2bcc200 } hitcount: 2 len: 12996
Totals:
Hits: 14
Entries: 12
Dropped: 0
# event histogram
#
# trigger info: hist:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 if len < 0 [active]
#
Totals:
Hits: 0
Entries: 0
Dropped: 0
Named triggers can be used to have triggers share a common set of
histogram data. This capability is mostly useful for combining the
output of events generated by tracepoints contained inside inline
functions, but names can be used in a hist trigger on any event.
For example, these two triggers when hit will update the same 'len'
field in the shared 'foo' histogram data::
# echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
/sys/kernel/debug/tracing/events/net/netif_receive_skb/trigger
# echo 'hist:name=foo:keys=skbaddr.hex:vals=len' > \
/sys/kernel/debug/tracing/events/net/netif_rx/trigger
You can see that they're updating common histogram data by reading
each event's hist files at the same time::
# cat /sys/kernel/debug/tracing/events/net/netif_receive_skb/hist;
cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
# event histogram
#
# trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
#
{ skbaddr: ffff88000ad53500 } hitcount: 1 len: 46
{ skbaddr: ffff8800af5a1500 } hitcount: 1 len: 76
{ skbaddr: ffff8800d62a1900 } hitcount: 1 len: 46
{ skbaddr: ffff8800d2bccb00 } hitcount: 1 len: 468
{ skbaddr: ffff8800d3c69900 } hitcount: 1 len: 46
{ skbaddr: ffff88009ff09100 } hitcount: 1 len: 52
{ skbaddr: ffff88010f13ab00 } hitcount: 1 len: 168
{ skbaddr: ffff88006a54f400 } hitcount: 1 len: 46
{ skbaddr: ffff8800d2bcc500 } hitcount: 1 len: 260
{ skbaddr: ffff880064505000 } hitcount: 1 len: 46
{ skbaddr: ffff8800baf24e00 } hitcount: 1 len: 32
{ skbaddr: ffff88009fe0ad00 } hitcount: 1 len: 46
{ skbaddr: ffff8800d3edff00 } hitcount: 1 len: 44
{ skbaddr: ffff88009fe0b400 } hitcount: 1 len: 168
{ skbaddr: ffff8800a1c55a00 } hitcount: 1 len: 40
{ skbaddr: ffff8800d2bcd100 } hitcount: 1 len: 40
{ skbaddr: ffff880064505f00 } hitcount: 1 len: 174
{ skbaddr: ffff8800a8bff200 } hitcount: 1 len: 160
{ skbaddr: ffff880044e3cc00 } hitcount: 1 len: 76
{ skbaddr: ffff8800a8bfe700 } hitcount: 1 len: 46
{ skbaddr: ffff8800d2bcdc00 } hitcount: 1 len: 32
{ skbaddr: ffff8800a1f64800 } hitcount: 1 len: 46
{ skbaddr: ffff8800d2bcde00 } hitcount: 1 len: 988
{ skbaddr: ffff88006a5dea00 } hitcount: 1 len: 46
{ skbaddr: ffff88002e37a200 } hitcount: 1 len: 44
{ skbaddr: ffff8800a1f32c00 } hitcount: 2 len: 676
{ skbaddr: ffff88000ad52600 } hitcount: 2 len: 107
{ skbaddr: ffff8800a1f91e00 } hitcount: 2 len: 92
{ skbaddr: ffff8800af5a0200 } hitcount: 2 len: 142
{ skbaddr: ffff8800d2bcc600 } hitcount: 2 len: 220
{ skbaddr: ffff8800ba36f500 } hitcount: 2 len: 92
{ skbaddr: ffff8800d021f800 } hitcount: 2 len: 92
{ skbaddr: ffff8800a1f33600 } hitcount: 2 len: 675
{ skbaddr: ffff8800a8bfff00 } hitcount: 3 len: 138
{ skbaddr: ffff8800d62a1300 } hitcount: 3 len: 138
{ skbaddr: ffff88002e37a100 } hitcount: 4 len: 184
{ skbaddr: ffff880064504400 } hitcount: 4 len: 184
{ skbaddr: ffff8800a8bfec00 } hitcount: 4 len: 184
{ skbaddr: ffff88000ad53700 } hitcount: 5 len: 230
{ skbaddr: ffff8800d2bcdb00 } hitcount: 5 len: 196
{ skbaddr: ffff8800a1f90000 } hitcount: 6 len: 276
{ skbaddr: ffff88006a54f900 } hitcount: 6 len: 276
Totals:
Hits: 81
Entries: 42
Dropped: 0
# event histogram
#
# trigger info: hist:name=foo:keys=skbaddr.hex:vals=hitcount,len:sort=hitcount:size=2048 [active]
#
{ skbaddr: ffff88000ad53500 } hitcount: 1 len: 46
{ skbaddr: ffff8800af5a1500 } hitcount: 1 len: 76
{ skbaddr: ffff8800d62a1900 } hitcount: 1 len: 46
{ skbaddr: ffff8800d2bccb00 } hitcount: 1 len: 468
{ skbaddr: ffff8800d3c69900 } hitcount: 1 len: 46
{ skbaddr: ffff88009ff09100 } hitcount: 1 len: 52
{ skbaddr: ffff88010f13ab00 } hitcount: 1 len: 168
{ skbaddr: ffff88006a54f400 } hitcount: 1 len: 46
{ skbaddr: ffff8800d2bcc500 } hitcount: 1 len: 260
{ skbaddr: ffff880064505000 } hitcount: 1 len: 46
{ skbaddr: ffff8800baf24e00 } hitcount: 1 len: 32
{ skbaddr: ffff88009fe0ad00 } hitcount: 1 len: 46
{ skbaddr: ffff8800d3edff00 } hitcount: 1 len: 44
{ skbaddr: ffff88009fe0b400 } hitcount: 1 len: 168
{ skbaddr: ffff8800a1c55a00 } hitcount: 1 len: 40
{ skbaddr: ffff8800d2bcd100 } hitcount: 1 len: 40
{ skbaddr: ffff880064505f00 } hitcount: 1 len: 174
{ skbaddr: ffff8800a8bff200 } hitcount: 1 len: 160
{ skbaddr: ffff880044e3cc00 } hitcount: 1 len: 76
{ skbaddr: ffff8800a8bfe700 } hitcount: 1 len: 46
{ skbaddr: ffff8800d2bcdc00 } hitcount: 1 len: 32
{ skbaddr: ffff8800a1f64800 } hitcount: 1 len: 46
{ skbaddr: ffff8800d2bcde00 } hitcount: 1 len: 988
{ skbaddr: ffff88006a5dea00 } hitcount: 1 len: 46
{ skbaddr: ffff88002e37a200 } hitcount: 1 len: 44
{ skbaddr: ffff8800a1f32c00 } hitcount: 2 len: 676
{ skbaddr: ffff88000ad52600 } hitcount: 2 len: 107
{ skbaddr: ffff8800a1f91e00 } hitcount: 2 len: 92
{ skbaddr: ffff8800af5a0200 } hitcount: 2 len: 142
{ skbaddr: ffff8800d2bcc600 } hitcount: 2 len: 220
{ skbaddr: ffff8800ba36f500 } hitcount: 2 len: 92
{ skbaddr: ffff8800d021f800 } hitcount: 2 len: 92
{ skbaddr: ffff8800a1f33600 } hitcount: 2 len: 675
{ skbaddr: ffff8800a8bfff00 } hitcount: 3 len: 138
{ skbaddr: ffff8800d62a1300 } hitcount: 3 len: 138
{ skbaddr: ffff88002e37a100 } hitcount: 4 len: 184
{ skbaddr: ffff880064504400 } hitcount: 4 len: 184
{ skbaddr: ffff8800a8bfec00 } hitcount: 4 len: 184
{ skbaddr: ffff88000ad53700 } hitcount: 5 len: 230
{ skbaddr: ffff8800d2bcdb00 } hitcount: 5 len: 196
{ skbaddr: ffff8800a1f90000 } hitcount: 6 len: 276
{ skbaddr: ffff88006a54f900 } hitcount: 6 len: 276
Totals:
Hits: 81
Entries: 42
Dropped: 0
And here's an example that shows how to combine histogram data from
any two events even if they don't share any 'compatible' fields
other than 'hitcount' and 'stacktrace'. These commands create a
couple of triggers named 'bar' using those fields::
# echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
/sys/kernel/debug/tracing/events/sched/sched_process_fork/trigger
# echo 'hist:name=bar:key=stacktrace:val=hitcount' > \
/sys/kernel/debug/tracing/events/net/netif_rx/trigger
And displaying the output of either shows some interesting if
somewhat confusing output::
# cat /sys/kernel/debug/tracing/events/sched/sched_process_fork/hist
# cat /sys/kernel/debug/tracing/events/net/netif_rx/hist
# event histogram
#
# trigger info: hist:name=bar:keys=stacktrace:vals=hitcount:sort=hitcount:size=2048 [active]
#
{ stacktrace:
_do_fork+0x18e/0x330
kernel_thread+0x29/0x30
kthreadd+0x154/0x1b0
ret_from_fork+0x3f/0x70
} hitcount: 1
{ stacktrace:
netif_rx_internal+0xb2/0xd0
netif_rx_ni+0x20/0x70
dev_loopback_xmit+0xaa/0xd0
ip_mc_output+0x126/0x240
ip_local_out_sk+0x31/0x40
igmp_send_report+0x1e9/0x230
igmp_timer_expire+0xe9/0x120
call_timer_fn+0x39/0xf0
run_timer_softirq+0x1e1/0x290
__do_softirq+0xfd/0x290
irq_exit+0x98/0xb0
smp_apic_timer_interrupt+0x4a/0x60
apic_timer_interrupt+0x6d/0x80
cpuidle_enter+0x17/0x20
call_cpuidle+0x3b/0x60
cpu_startup_entry+0x22d/0x310
} hitcount: 1
{ stacktrace:
netif_rx_internal+0xb2/0xd0
netif_rx_ni+0x20/0x70
dev_loopback_xmit+0xaa/0xd0
ip_mc_output+0x17f/0x240
ip_local_out_sk+0x31/0x40
ip_send_skb+0x1a/0x50
udp_send_skb+0x13e/0x270
udp_sendmsg+0x2bf/0x980
inet_sendmsg+0x67/0xa0
sock_sendmsg+0x38/0x50
SYSC_sendto+0xef/0x170
SyS_sendto+0xe/0x10
entry_SYSCALL_64_fastpath+0x12/0x6a
} hitcount: 2
{ stacktrace:
netif_rx_internal+0xb2/0xd0
netif_rx+0x1c/0x60
loopback_xmit+0x6c/0xb0
dev_hard_start_xmit+0x219/0x3a0
__dev_queue_xmit+0x415/0x4f0
dev_queue_xmit_sk+0x13/0x20
ip_finish_output2+0x237/0x340
ip_finish_output+0x113/0x1d0
ip_output+0x66/0xc0
ip_local_out_sk+0x31/0x40
ip_send_skb+0x1a/0x50
udp_send_skb+0x16d/0x270
udp_sendmsg+0x2bf/0x980
inet_sendmsg+0x67/0xa0
sock_sendmsg+0x38/0x50
___sys_sendmsg+0x14e/0x270
} hitcount: 76
{ stacktrace:
netif_rx_internal+0xb2/0xd0
netif_rx+0x1c/0x60
loopback_xmit+0x6c/0xb0
dev_hard_start_xmit+0x219/0x3a0
__dev_queue_xmit+0x415/0x4f0
dev_queue_xmit_sk+0x13/0x20
ip_finish_output2+0x237/0x340
ip_finish_output+0x113/0x1d0
ip_output+0x66/0xc0
ip_local_out_sk+0x31/0x40
ip_send_skb+0x1a/0x50
udp_send_skb+0x16d/0x270
udp_sendmsg+0x2bf/0x980
inet_sendmsg+0x67/0xa0
sock_sendmsg+0x38/0x50
___sys_sendmsg+0x269/0x270
} hitcount: 77
{ stacktrace:
netif_rx_internal+0xb2/0xd0
netif_rx+0x1c/0x60
loopback_xmit+0x6c/0xb0
dev_hard_start_xmit+0x219/0x3a0
__dev_queue_xmit+0x415/0x4f0
dev_queue_xmit_sk+0x13/0x20
ip_finish_output2+0x237/0x340
ip_finish_output+0x113/0x1d0
ip_output+0x66/0xc0
ip_local_out_sk+0x31/0x40
ip_send_skb+0x1a/0x50
udp_send_skb+0x16d/0x270
udp_sendmsg+0x2bf/0x980
inet_sendmsg+0x67/0xa0
sock_sendmsg+0x38/0x50
SYSC_sendto+0xef/0x170
} hitcount: 88
{ stacktrace:
_do_fork+0x18e/0x330
SyS_clone+0x19/0x20
entry_SYSCALL_64_fastpath+0x12/0x6a
} hitcount: 244
Totals:
Hits: 489
Entries: 7
Dropped: 0
2.2 Inter-event hist triggers
-----------------------------
Inter-event hist triggers are hist triggers that combine values from
one or more other events and create a histogram using that data. Data
from an inter-event histogram can in turn become the source for
further combined histograms, thus providing a chain of related
histograms, which is important for some applications.
The most important example of an inter-event quantity that can be used
in this manner is latency, which is simply a difference in timestamps
between two events. Although latency is the most important
inter-event quantity, note that because the support is completely
general across the trace event subsystem, any event field can be used
in an inter-event quantity.
An example of a histogram that combines data from other histograms
into a useful chain would be a 'wakeupswitch latency' histogram that
combines a 'wakeup latency' histogram and a 'switch latency'
histogram.
Normally, a hist trigger specification consists of a (possibly
compound) key along with one or more numeric values, which are
continually updated sums associated with that key. A histogram
specification in this case consists of individual key and value
specifications that refer to trace event fields associated with a
single event type.
The inter-event hist trigger extension allows fields from multiple
events to be referenced and combined into a multi-event histogram
specification. In support of this overall goal, a few enabling
features have been added to the hist trigger support:
- In order to compute an inter-event quantity, a value from one
event needs to saved and then referenced from another event. This
requires the introduction of support for histogram 'variables'.
- The computation of inter-event quantities and their combination
require some minimal amount of support for applying simple
expressions to variables (+ and -).
- A histogram consisting of inter-event quantities isn't logically a
histogram on either event (so having the 'hist' file for either
event host the histogram output doesn't really make sense). To
address the idea that the histogram is associated with a
combination of events, support is added allowing the creation of
'synthetic' events that are events derived from other events.
These synthetic events are full-fledged events just like any other
and can be used as such, as for instance to create the
'combination' histograms mentioned previously.
- A set of 'actions' can be associated with histogram entries -
these can be used to generate the previously mentioned synthetic
events, but can also be used for other purposes, such as for
example saving context when a 'max' latency has been hit.
- Trace events don't have a 'timestamp' associated with them, but
there is an implicit timestamp saved along with an event in the
underlying ftrace ring buffer. This timestamp is now exposed as a
a synthetic field named 'common_timestamp' which can be used in
histograms as if it were any other event field; it isn't an actual
field in the trace format but rather is a synthesized value that
nonetheless can be used as if it were an actual field. By default
it is in units of nanoseconds; appending '.usecs' to a
common_timestamp field changes the units to microseconds.
A note on inter-event timestamps: If common_timestamp is used in a
histogram, the trace buffer is automatically switched over to using
absolute timestamps and the "global" trace clock, in order to avoid
bogus timestamp differences with other clocks that aren't coherent
across CPUs. This can be overridden by specifying one of the other
trace clocks instead, using the "clock=XXX" hist trigger attribute,
where XXX is any of the clocks listed in the tracing/trace_clock
pseudo-file.
These features are described in more detail in the following sections.
2.2.1 Histogram Variables
-------------------------
Variables are simply named locations used for saving and retrieving
values between matching events. A 'matching' event is defined as an
event that has a matching key - if a variable is saved for a histogram
entry corresponding to that key, any subsequent event with a matching
key can access that variable.
A variable's value is normally available to any subsequent event until
it is set to something else by a subsequent event. The one exception
to that rule is that any variable used in an expression is essentially
'read-once' - once it's used by an expression in a subsequent event,
it's reset to its 'unset' state, which means it can't be used again
unless it's set again. This ensures not only that an event doesn't
use an uninitialized variable in a calculation, but that that variable
is used only once and not for any unrelated subsequent match.
The basic syntax for saving a variable is to simply prefix a unique
variable name not corresponding to any keyword along with an '=' sign
to any event field.
Either keys or values can be saved and retrieved in this way. This
creates a variable named 'ts0' for a histogram entry with the key
'next_pid'::
# echo 'hist:keys=next_pid:vals=$ts0:ts0=common_timestamp ... >> \
event/trigger
The ts0 variable can be accessed by any subsequent event having the
same pid as 'next_pid'.
Variable references are formed by prepending the variable name with
the '$' sign. Thus for example, the ts0 variable above would be
referenced as '$ts0' in expressions.
Because 'vals=' is used, the common_timestamp variable value above
will also be summed as a normal histogram value would (though for a
timestamp it makes little sense).
The below shows that a key value can also be saved in the same way::
# echo 'hist:timer_pid=common_pid:key=timer_pid ...' >> event/trigger
If a variable isn't a key variable or prefixed with 'vals=', the
associated event field will be saved in a variable but won't be summed
as a value::
# echo 'hist:keys=next_pid:ts1=common_timestamp ...' >> event/trigger
Multiple variables can be assigned at the same time. The below would
result in both ts0 and b being created as variables, with both
common_timestamp and field1 additionally being summed as values::
# echo 'hist:keys=pid:vals=$ts0,$b:ts0=common_timestamp,b=field1 ...' >> \
event/trigger
Note that variable assignments can appear either preceding or
following their use. The command below behaves identically to the
command above::
# echo 'hist:keys=pid:ts0=common_timestamp,b=field1:vals=$ts0,$b ...' >> \
event/trigger
Any number of variables not bound to a 'vals=' prefix can also be
assigned by simply separating them with colons. Below is the same
thing but without the values being summed in the histogram::
# echo 'hist:keys=pid:ts0=common_timestamp:b=field1 ...' >> event/trigger
Variables set as above can be referenced and used in expressions on
another event.
For example, here's how a latency can be calculated::
# echo 'hist:keys=pid,prio:ts0=common_timestamp ...' >> event1/trigger
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp-$ts0 ...' >> event2/trigger
In the first line above, the event's timestamp is saved into the
variable ts0. In the next line, ts0 is subtracted from the second
event's timestamp to produce the latency, which is then assigned into
yet another variable, 'wakeup_lat'. The hist trigger below in turn
makes use of the wakeup_lat variable to compute a combined latency
using the same key and variable from yet another event::
# echo 'hist:key=pid:wakeupswitch_lat=$wakeup_lat+$switchtime_lat ...' >> event3/trigger
2.2.2 Synthetic Events
----------------------
Synthetic events are user-defined events generated from hist trigger
variables or fields associated with one or more other events. Their
purpose is to provide a mechanism for displaying data spanning
multiple events consistent with the existing and already familiar
usage for normal events.
To define a synthetic event, the user writes a simple specification
consisting of the name of the new event along with one or more
variables and their types, which can be any valid field type,
separated by semicolons, to the tracing/synthetic_events file.
For instance, the following creates a new event named 'wakeup_latency'
with 3 fields: lat, pid, and prio. Each of those fields is simply a
variable reference to a variable on another event::
# echo 'wakeup_latency \
u64 lat; \
pid_t pid; \
int prio' >> \
/sys/kernel/debug/tracing/synthetic_events
Reading the tracing/synthetic_events file lists all the currently
defined synthetic events, in this case the event defined above::
# cat /sys/kernel/debug/tracing/synthetic_events
wakeup_latency u64 lat; pid_t pid; int prio
An existing synthetic event definition can be removed by prepending
the command that defined it with a '!'::
# echo '!wakeup_latency u64 lat pid_t pid int prio' >> \
/sys/kernel/debug/tracing/synthetic_events
At this point, there isn't yet an actual 'wakeup_latency' event
instantiated in the event subsystem - for this to happen, a 'hist
trigger action' needs to be instantiated and bound to actual fields
and variables defined on other events (see Section 2.2.3 below on
how that is done using hist trigger 'onmatch' action). Once that is
done, the 'wakeup_latency' synthetic event instance is created.
A histogram can now be defined for the new synthetic event::
# echo 'hist:keys=pid,prio,lat.log2:sort=pid,lat' >> \
/sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
The new event is created under the tracing/events/synthetic/ directory
and looks and behaves just like any other event::
# ls /sys/kernel/debug/tracing/events/synthetic/wakeup_latency
enable filter format hist id trigger
Like any other event, once a histogram is enabled for the event, the
output can be displayed by reading the event's 'hist' file.
2.2.3 Hist trigger 'handlers' and 'actions'
-------------------------------------------
A hist trigger 'action' is a function that's executed (in most cases
conditionally) whenever a histogram entry is added or updated.
When a histogram entry is added or updated, a hist trigger 'handler'
is what decides whether the corresponding action is actually invoked
or not.
Hist trigger handlers and actions are paired together in the general
form:
<handler>.<action>
To specify a handler.action pair for a given event, simply specify
that handler.action pair between colons in the hist trigger
specification.
In theory, any handler can be combined with any action, but in
practice, not every handler.action combination is currently supported;
if a given handler.action combination isn't supported, the hist
trigger will fail with -EINVAL;
The default 'handler.action' if none is explicitly specified is as it
always has been, to simply update the set of values associated with an
entry. Some applications, however, may want to perform additional
actions at that point, such as generate another event, or compare and
save a maximum.
The supported handlers and actions are listed below, and each is
described in more detail in the following paragraphs, in the context
of descriptions of some common and useful handler.action combinations.
The available handlers are:
- onmatch(matching.event) - invoke action on any addition or update
- onmax(var) - invoke action if var exceeds current max
- onchange(var) - invoke action if var changes
The available actions are:
- trace(<synthetic_event_name>,param list) - generate synthetic event
- save(field,...) - save current event fields
- snapshot() - snapshot the trace buffer
The following commonly-used handler.action pairs are available:
- onmatch(matching.event).trace(<synthetic_event_name>,param list)
The 'onmatch(matching.event).trace(<synthetic_event_name>,param
list)' hist trigger action is invoked whenever an event matches
and the histogram entry would be added or updated. It causes the
named synthetic event to be generated with the values given in the
'param list'. The result is the generation of a synthetic event
that consists of the values contained in those variables at the
time the invoking event was hit. For example, if the synthetic
event name is 'wakeup_latency', a wakeup_latency event is
generated using onmatch(event).trace(wakeup_latency,arg1,arg2).
There is also an equivalent alternative form available for
generating synthetic events. In this form, the synthetic event
name is used as if it were a function name. For example, using
the 'wakeup_latency' synthetic event name again, the
wakeup_latency event would be generated by invoking it as if it
were a function call, with the event field values passed in as
arguments: onmatch(event).wakeup_latency(arg1,arg2). The syntax
for this form is:
onmatch(matching.event).<synthetic_event_name>(param list)
In either case, the 'param list' consists of one or more
parameters which may be either variables or fields defined on
either the 'matching.event' or the target event. The variables or
fields specified in the param list may be either fully-qualified
or unqualified. If a variable is specified as unqualified, it
must be unique between the two events. A field name used as a
param can be unqualified if it refers to the target event, but
must be fully qualified if it refers to the matching event. A
fully-qualified name is of the form 'system.event_name.$var_name'
or 'system.event_name.field'.
The 'matching.event' specification is simply the fully qualified
event name of the event that matches the target event for the
onmatch() functionality, in the form 'system.event_name'. Histogram
keys of both events are compared to find if events match. In case
multiple histogram keys are used, they all must match in the specified
order.
Finally, the number and type of variables/fields in the 'param
list' must match the number and types of the fields in the
synthetic event being generated.
As an example the below defines a simple synthetic event and uses
a variable defined on the sched_wakeup_new event as a parameter
when invoking the synthetic event. Here we define the synthetic
event::
# echo 'wakeup_new_test pid_t pid' >> \
/sys/kernel/debug/tracing/synthetic_events
# cat /sys/kernel/debug/tracing/synthetic_events
wakeup_new_test pid_t pid
The following hist trigger both defines the missing testpid
variable and specifies an onmatch() action that generates a
wakeup_new_test synthetic event whenever a sched_wakeup_new event
occurs, which because of the 'if comm == "cyclictest"' filter only
happens when the executable is cyclictest::
# echo 'hist:keys=$testpid:testpid=pid:onmatch(sched.sched_wakeup_new).\
wakeup_new_test($testpid) if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger
Or, equivalently, using the 'trace' keyword syntax:
# echo 'hist:keys=$testpid:testpid=pid:onmatch(sched.sched_wakeup_new).\
trace(wakeup_new_test,$testpid) if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_wakeup_new/trigger
Creating and displaying a histogram based on those events is now
just a matter of using the fields and new synthetic event in the
tracing/events/synthetic directory, as usual::
# echo 'hist:keys=pid:sort=pid' >> \
/sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/trigger
Running 'cyclictest' should cause wakeup_new events to generate
wakeup_new_test synthetic events which should result in histogram
output in the wakeup_new_test event's hist file::
# cat /sys/kernel/debug/tracing/events/synthetic/wakeup_new_test/hist
A more typical usage would be to use two events to calculate a
latency. The following example uses a set of hist triggers to
produce a 'wakeup_latency' histogram.
First, we define a 'wakeup_latency' synthetic event::
# echo 'wakeup_latency u64 lat; pid_t pid; int prio' >> \
/sys/kernel/debug/tracing/synthetic_events
Next, we specify that whenever we see a sched_waking event for a
cyclictest thread, save the timestamp in a 'ts0' variable::
# echo 'hist:keys=$saved_pid:saved_pid=pid:ts0=common_timestamp.usecs \
if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_waking/trigger
Then, when the corresponding thread is actually scheduled onto the
CPU by a sched_switch event (saved_pid matches next_pid), calculate
the latency and use that along with another variable and an event field
to generate a wakeup_latency synthetic event::
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0:\
onmatch(sched.sched_waking).wakeup_latency($wakeup_lat,\
$saved_pid,next_prio) if next_comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger
We also need to create a histogram on the wakeup_latency synthetic
event in order to aggregate the generated synthetic event data::
# echo 'hist:keys=pid,prio,lat:sort=pid,lat' >> \
/sys/kernel/debug/tracing/events/synthetic/wakeup_latency/trigger
Finally, once we've run cyclictest to actually generate some
events, we can see the output by looking at the wakeup_latency
synthetic event's hist file::
# cat /sys/kernel/debug/tracing/events/synthetic/wakeup_latency/hist
- onmax(var).save(field,.. .)
The 'onmax(var).save(field,...)' hist trigger action is invoked
whenever the value of 'var' associated with a histogram entry
exceeds the current maximum contained in that variable.
The end result is that the trace event fields specified as the
onmax.save() params will be saved if 'var' exceeds the current
maximum for that hist trigger entry. This allows context from the
event that exhibited the new maximum to be saved for later
reference. When the histogram is displayed, additional fields
displaying the saved values will be printed.
As an example the below defines a couple of hist triggers, one for
sched_waking and another for sched_switch, keyed on pid. Whenever
a sched_waking occurs, the timestamp is saved in the entry
corresponding to the current pid, and when the scheduler switches
back to that pid, the timestamp difference is calculated. If the
resulting latency, stored in wakeup_lat, exceeds the current
maximum latency, the values specified in the save() fields are
recorded::
# echo 'hist:keys=pid:ts0=common_timestamp.usecs \
if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_waking/trigger
# echo 'hist:keys=next_pid:\
wakeup_lat=common_timestamp.usecs-$ts0:\
onmax($wakeup_lat).save(next_comm,prev_pid,prev_prio,prev_comm) \
if next_comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger
When the histogram is displayed, the max value and the saved
values corresponding to the max are displayed following the rest
of the fields::
# cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
{ next_pid: 2255 } hitcount: 239
common_timestamp-ts0: 0
max: 27
next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/1
{ next_pid: 2256 } hitcount: 2355
common_timestamp-ts0: 0
max: 49 next_comm: cyclictest
prev_pid: 0 prev_prio: 120 prev_comm: swapper/0
Totals:
Hits: 12970
Entries: 2
Dropped: 0
- onmax(var).snapshot()
The 'onmax(var).snapshot()' hist trigger action is invoked
whenever the value of 'var' associated with a histogram entry
exceeds the current maximum contained in that variable.
The end result is that a global snapshot of the trace buffer will
be saved in the tracing/snapshot file if 'var' exceeds the current
maximum for any hist trigger entry.
Note that in this case the maximum is a global maximum for the
current trace instance, which is the maximum across all buckets of
the histogram. The key of the specific trace event that caused
the global maximum and the global maximum itself are displayed,
along with a message stating that a snapshot has been taken and
where to find it. The user can use the key information displayed
to locate the corresponding bucket in the histogram for even more
detail.
As an example the below defines a couple of hist triggers, one for
sched_waking and another for sched_switch, keyed on pid. Whenever
a sched_waking event occurs, the timestamp is saved in the entry
corresponding to the current pid, and when the scheduler switches
back to that pid, the timestamp difference is calculated. If the
resulting latency, stored in wakeup_lat, exceeds the current
maximum latency, a snapshot is taken. As part of the setup, all
the scheduler events are also enabled, which are the events that
will show up in the snapshot when it is taken at some point:
# echo 1 > /sys/kernel/debug/tracing/events/sched/enable
# echo 'hist:keys=pid:ts0=common_timestamp.usecs \
if comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_waking/trigger
# echo 'hist:keys=next_pid:wakeup_lat=common_timestamp.usecs-$ts0: \
onmax($wakeup_lat).save(next_prio,next_comm,prev_pid,prev_prio, \
prev_comm):onmax($wakeup_lat).snapshot() \
if next_comm=="cyclictest"' >> \
/sys/kernel/debug/tracing/events/sched/sched_switch/trigger
When the histogram is displayed, for each bucket the max value
and the saved values corresponding to the max are displayed
following the rest of the fields.
If a snapshot was taken, there is also a message indicating that,
along with the value and event that triggered the global maximum:
# cat /sys/kernel/debug/tracing/events/sched/sched_switch/hist
{ next_pid: 2101 } hitcount: 200
max: 52 next_prio: 120 next_comm: cyclictest \
prev_pid: 0 prev_prio: 120 prev_comm: swapper/6
{ next_pid: 2103 } hitcount: 1326
max: 572 next_prio: 19 next_comm: cyclictest \
prev_pid: 0 prev_prio: 120 prev_comm: swapper/1
{ next_pid: 2102 } hitcount: 1982 \
max: 74 next_prio: 19 next_comm: cyclictest \
prev_pid: 0 prev_prio: 120 prev_comm: swapper/5
Snapshot taken (see tracing/snapshot). Details:
triggering value { onmax($wakeup_lat) }: 572 \
triggered by event with key: { next_pid: 2103 }
Totals:
Hits: 3508
Entries: 3
Dropped: 0
In the above case, the event that triggered the global maximum has
the key with next_pid == 2103. If you look at the bucket that has
2103 as the key, you'll find the additional values save()'d along
with the local maximum for that bucket, which should be the same
as the global maximum (since that was the same value that
triggered the global snapshot).
And finally, looking at the snapshot data should show at or near
the end the event that triggered the snapshot (in this case you
can verify the timestamps between the sched_waking and
sched_switch events, which should match the time displayed in the
global maximum)::
# cat /sys/kernel/debug/tracing/snapshot
<...>-2103 [005] d..3 309.873125: sched_switch: prev_comm=cyclictest prev_pid=2103 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
<idle>-0 [005] d.h3 309.873611: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
<idle>-0 [005] dNh4 309.873613: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
<idle>-0 [005] d..3 309.873616: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
<...>-2102 [005] d..3 309.873625: sched_switch: prev_comm=cyclictest prev_pid=2102 prev_prio=19 prev_state=D ==> next_comm=swapper/5 next_pid=0 next_prio=120
<idle>-0 [005] d.h3 309.874624: sched_waking: comm=cyclictest pid=2102 prio=19 target_cpu=005
<idle>-0 [005] dNh4 309.874626: sched_wakeup: comm=cyclictest pid=2102 prio=19 target_cpu=005
<idle>-0 [005] dNh3 309.874628: sched_waking: comm=cyclictest pid=2103 prio=19 target_cpu=005
<idle>-0 [005] dNh4 309.874630: sched_wakeup: comm=cyclictest pid=2103 prio=19 target_cpu=005
<idle>-0 [005] d..3 309.874633: sched_switch: prev_comm=swapper/5 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2102 next_prio=19
<idle>-0 [004] d.h3 309.874757: sched_waking: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
<idle>-0 [004] dNh4 309.874762: sched_wakeup: comm=gnome-terminal- pid=1699 prio=120 target_cpu=004
<idle>-0 [004] d..3 309.874766: sched_switch: prev_comm=swapper/4 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=gnome-terminal- next_pid=1699 next_prio=120
gnome-terminal--1699 [004] d.h2 309.874941: sched_stat_runtime: comm=gnome-terminal- pid=1699 runtime=180706 [ns] vruntime=1126870572 [ns]
<idle>-0 [003] d.s4 309.874956: sched_waking: comm=rcu_sched pid=9 prio=120 target_cpu=007
<idle>-0 [003] d.s5 309.874960: sched_wake_idle_without_ipi: cpu=7
<idle>-0 [003] d.s5 309.874961: sched_wakeup: comm=rcu_sched pid=9 prio=120 target_cpu=007
<idle>-0 [007] d..3 309.874963: sched_switch: prev_comm=swapper/7 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=rcu_sched next_pid=9 next_prio=120
rcu_sched-9 [007] d..3 309.874973: sched_stat_runtime: comm=rcu_sched pid=9 runtime=13646 [ns] vruntime=22531430286 [ns]
rcu_sched-9 [007] d..3 309.874978: sched_switch: prev_comm=rcu_sched prev_pid=9 prev_prio=120 prev_state=R+ ==> next_comm=swapper/7 next_pid=0 next_prio=120
<...>-2102 [005] d..4 309.874994: sched_migrate_task: comm=cyclictest pid=2103 prio=19 orig_cpu=5 dest_cpu=1
<...>-2102 [005] d..4 309.875185: sched_wake_idle_without_ipi: cpu=1
<idle>-0 [001] d..3 309.875200: sched_switch: prev_comm=swapper/1 prev_pid=0 prev_prio=120 prev_state=S ==> next_comm=cyclictest next_pid=2103 next_prio=19
- onchange(var).save(field,.. .)
The 'onchange(var).save(field,...)' hist trigger action is invoked
whenever the value of 'var' associated with a histogram entry
changes.
The end result is that the trace event fields specified as the
onchange.save() params will be saved if 'var' changes for that
hist trigger entry. This allows context from the event that
changed the value to be saved for later reference. When the
histogram is displayed, additional fields displaying the saved
values will be printed.
- onchange(var).snapshot()
The 'onchange(var).snapshot()' hist trigger action is invoked
whenever the value of 'var' associated with a histogram entry
changes.
The end result is that a global snapshot of the trace buffer will
be saved in the tracing/snapshot file if 'var' changes for any
hist trigger entry.
Note that in this case the changed value is a global variable
associated with current trace instance. The key of the specific
trace event that caused the value to change and the global value
itself are displayed, along with a message stating that a snapshot
has been taken and where to find it. The user can use the key
information displayed to locate the corresponding bucket in the
histogram for even more detail.
As an example the below defines a hist trigger on the tcp_probe
event, keyed on dport. Whenever a tcp_probe event occurs, the
cwnd field is checked against the current value stored in the
$cwnd variable. If the value has changed, a snapshot is taken.
As part of the setup, all the scheduler and tcp events are also
enabled, which are the events that will show up in the snapshot
when it is taken at some point:
# echo 1 > /sys/kernel/debug/tracing/events/sched/enable
# echo 1 > /sys/kernel/debug/tracing/events/tcp/enable
# echo 'hist:keys=dport:cwnd=snd_cwnd: \
onchange($cwnd).save(snd_wnd,srtt,rcv_wnd): \
onchange($cwnd).snapshot()' >> \
/sys/kernel/debug/tracing/events/tcp/tcp_probe/trigger
When the histogram is displayed, for each bucket the tracked value
and the saved values corresponding to that value are displayed
following the rest of the fields.
If a snapshot was taken, there is also a message indicating that,
along with the value and event that triggered the snapshot::
# cat /sys/kernel/debug/tracing/events/tcp/tcp_probe/hist
{ dport: 1521 } hitcount: 8
changed: 10 snd_wnd: 35456 srtt: 154262 rcv_wnd: 42112
{ dport: 80 } hitcount: 23
changed: 10 snd_wnd: 28960 srtt: 19604 rcv_wnd: 29312
{ dport: 9001 } hitcount: 172
changed: 10 snd_wnd: 48384 srtt: 260444 rcv_wnd: 55168
{ dport: 443 } hitcount: 211
changed: 10 snd_wnd: 26960 srtt: 17379 rcv_wnd: 28800
Snapshot taken (see tracing/snapshot). Details::
triggering value { onchange($cwnd) }: 10
triggered by event with key: { dport: 80 }
Totals:
Hits: 414
Entries: 4
Dropped: 0
In the above case, the event that triggered the snapshot has the
key with dport == 80. If you look at the bucket that has 80 as
the key, you'll find the additional values save()'d along with the
changed value for that bucket, which should be the same as the
global changed value (since that was the same value that triggered
the global snapshot).
And finally, looking at the snapshot data should show at or near
the end the event that triggered the snapshot::
# cat /sys/kernel/debug/tracing/snapshot
gnome-shell-1261 [006] dN.3 49.823113: sched_stat_runtime: comm=gnome-shell pid=1261 runtime=49347 [ns] vruntime=1835730389 [ns]
kworker/u16:4-773 [003] d..3 49.823114: sched_switch: prev_comm=kworker/u16:4 prev_pid=773 prev_prio=120 prev_state=R+ ==> next_comm=kworker/3:2 next_pid=135 next_prio=120
gnome-shell-1261 [006] d..3 49.823114: sched_switch: prev_comm=gnome-shell prev_pid=1261 prev_prio=120 prev_state=R+ ==> next_comm=kworker/6:2 next_pid=387 next_prio=120
kworker/3:2-135 [003] d..3 49.823118: sched_stat_runtime: comm=kworker/3:2 pid=135 runtime=5339 [ns] vruntime=17815800388 [ns]
kworker/6:2-387 [006] d..3 49.823120: sched_stat_runtime: comm=kworker/6:2 pid=387 runtime=9594 [ns] vruntime=14589605367 [ns]
kworker/6:2-387 [006] d..3 49.823122: sched_switch: prev_comm=kworker/6:2 prev_pid=387 prev_prio=120 prev_state=R+ ==> next_comm=gnome-shell next_pid=1261 next_prio=120
kworker/3:2-135 [003] d..3 49.823123: sched_switch: prev_comm=kworker/3:2 prev_pid=135 prev_prio=120 prev_state=T ==> next_comm=swapper/3 next_pid=0 next_prio=120
<idle>-0 [004] ..s7 49.823798: tcp_probe: src=10.0.0.10:54326 dest=23.215.104.193:80 mark=0x0 length=32 snd_nxt=0xe3ae2ff5 snd_una=0xe3ae2ecd snd_cwnd=10 ssthresh=2147483647 snd_wnd=28960 srtt=19604 rcv_wnd=29312
3. User space creating a trigger
--------------------------------
Writing into /sys/kernel/tracing/trace_marker writes into the ftrace
ring buffer. This can also act like an event, by writing into the trigger
file located in /sys/kernel/tracing/events/ftrace/print/
Modifying cyclictest to write into the trace_marker file before it sleeps
and after it wakes up, something like this::
static void traceputs(char *str)
{
/* tracemark_fd is the trace_marker file descriptor */
if (tracemark_fd < 0)
return;
/* write the tracemark message */
write(tracemark_fd, str, strlen(str));
}
And later add something like::
traceputs("start");
clock_nanosleep(...);
traceputs("end");
We can make a histogram from this::
# cd /sys/kernel/tracing
# echo 'latency u64 lat' > synthetic_events
# echo 'hist:keys=common_pid:ts0=common_timestamp.usecs if buf == "start"' > events/ftrace/print/trigger
# echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(ftrace.print).latency($lat) if buf == "end"' >> events/ftrace/print/trigger
# echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger
The above created a synthetic event called "latency" and two histograms
against the trace_marker, one gets triggered when "start" is written into the
trace_marker file and the other when "end" is written. If the pids match, then
it will call the "latency" synthetic event with the calculated latency as its
parameter. Finally, a histogram is added to the latency synthetic event to
record the calculated latency along with the pid.
Now running cyclictest with::
# ./cyclictest -p80 -d0 -i250 -n -a -t --tracemark -b 1000
-p80 : run threads at priority 80
-d0 : have all threads run at the same interval
-i250 : start the interval at 250 microseconds (all threads will do this)
-n : sleep with nanosleep
-a : affine all threads to a separate CPU
-t : one thread per available CPU
--tracemark : enable trace mark writing
-b 1000 : stop if any latency is greater than 1000 microseconds
Note, the -b 1000 is used just to make --tracemark available.
Then we can see the histogram created by this with::
# cat events/synthetic/latency/hist
# event histogram
#
# trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
#
{ lat: 107, common_pid: 2039 } hitcount: 1
{ lat: 122, common_pid: 2041 } hitcount: 1
{ lat: 166, common_pid: 2039 } hitcount: 1
{ lat: 174, common_pid: 2039 } hitcount: 1
{ lat: 194, common_pid: 2041 } hitcount: 1
{ lat: 196, common_pid: 2036 } hitcount: 1
{ lat: 197, common_pid: 2038 } hitcount: 1
{ lat: 198, common_pid: 2039 } hitcount: 1
{ lat: 199, common_pid: 2039 } hitcount: 1
{ lat: 200, common_pid: 2041 } hitcount: 1
{ lat: 201, common_pid: 2039 } hitcount: 2
{ lat: 202, common_pid: 2038 } hitcount: 1
{ lat: 202, common_pid: 2043 } hitcount: 1
{ lat: 203, common_pid: 2039 } hitcount: 1
{ lat: 203, common_pid: 2036 } hitcount: 1
{ lat: 203, common_pid: 2041 } hitcount: 1
{ lat: 206, common_pid: 2038 } hitcount: 2
{ lat: 207, common_pid: 2039 } hitcount: 1
{ lat: 207, common_pid: 2036 } hitcount: 1
{ lat: 208, common_pid: 2040 } hitcount: 1
{ lat: 209, common_pid: 2043 } hitcount: 1
{ lat: 210, common_pid: 2039 } hitcount: 1
{ lat: 211, common_pid: 2039 } hitcount: 4
{ lat: 212, common_pid: 2043 } hitcount: 1
{ lat: 212, common_pid: 2039 } hitcount: 2
{ lat: 213, common_pid: 2039 } hitcount: 1
{ lat: 214, common_pid: 2038 } hitcount: 1
{ lat: 214, common_pid: 2039 } hitcount: 2
{ lat: 214, common_pid: 2042 } hitcount: 1
{ lat: 215, common_pid: 2039 } hitcount: 1
{ lat: 217, common_pid: 2036 } hitcount: 1
{ lat: 217, common_pid: 2040 } hitcount: 1
{ lat: 217, common_pid: 2039 } hitcount: 1
{ lat: 218, common_pid: 2039 } hitcount: 6
{ lat: 219, common_pid: 2039 } hitcount: 9
{ lat: 220, common_pid: 2039 } hitcount: 11
{ lat: 221, common_pid: 2039 } hitcount: 5
{ lat: 221, common_pid: 2042 } hitcount: 1
{ lat: 222, common_pid: 2039 } hitcount: 7
{ lat: 223, common_pid: 2036 } hitcount: 1
{ lat: 223, common_pid: 2039 } hitcount: 3
{ lat: 224, common_pid: 2039 } hitcount: 4
{ lat: 224, common_pid: 2037 } hitcount: 1
{ lat: 224, common_pid: 2036 } hitcount: 2
{ lat: 225, common_pid: 2039 } hitcount: 5
{ lat: 225, common_pid: 2042 } hitcount: 1
{ lat: 226, common_pid: 2039 } hitcount: 7
{ lat: 226, common_pid: 2036 } hitcount: 4
{ lat: 227, common_pid: 2039 } hitcount: 6
{ lat: 227, common_pid: 2036 } hitcount: 12
{ lat: 227, common_pid: 2043 } hitcount: 1
{ lat: 228, common_pid: 2039 } hitcount: 7
{ lat: 228, common_pid: 2036 } hitcount: 14
{ lat: 229, common_pid: 2039 } hitcount: 9
{ lat: 229, common_pid: 2036 } hitcount: 8
{ lat: 229, common_pid: 2038 } hitcount: 1
{ lat: 230, common_pid: 2039 } hitcount: 11
{ lat: 230, common_pid: 2036 } hitcount: 6
{ lat: 230, common_pid: 2043 } hitcount: 1
{ lat: 230, common_pid: 2042 } hitcount: 2
{ lat: 231, common_pid: 2041 } hitcount: 1
{ lat: 231, common_pid: 2036 } hitcount: 6
{ lat: 231, common_pid: 2043 } hitcount: 1
{ lat: 231, common_pid: 2039 } hitcount: 8
{ lat: 232, common_pid: 2037 } hitcount: 1
{ lat: 232, common_pid: 2039 } hitcount: 6
{ lat: 232, common_pid: 2040 } hitcount: 2
{ lat: 232, common_pid: 2036 } hitcount: 5
{ lat: 232, common_pid: 2043 } hitcount: 1
{ lat: 233, common_pid: 2036 } hitcount: 5
{ lat: 233, common_pid: 2039 } hitcount: 11
{ lat: 234, common_pid: 2039 } hitcount: 4
{ lat: 234, common_pid: 2038 } hitcount: 2
{ lat: 234, common_pid: 2043 } hitcount: 2
{ lat: 234, common_pid: 2036 } hitcount: 11
{ lat: 234, common_pid: 2040 } hitcount: 1
{ lat: 235, common_pid: 2037 } hitcount: 2
{ lat: 235, common_pid: 2036 } hitcount: 8
{ lat: 235, common_pid: 2043 } hitcount: 2
{ lat: 235, common_pid: 2039 } hitcount: 5
{ lat: 235, common_pid: 2042 } hitcount: 2
{ lat: 235, common_pid: 2040 } hitcount: 4
{ lat: 235, common_pid: 2041 } hitcount: 1
{ lat: 236, common_pid: 2036 } hitcount: 7
{ lat: 236, common_pid: 2037 } hitcount: 1
{ lat: 236, common_pid: 2041 } hitcount: 5
{ lat: 236, common_pid: 2039 } hitcount: 3
{ lat: 236, common_pid: 2043 } hitcount: 9
{ lat: 236, common_pid: 2040 } hitcount: 7
{ lat: 237, common_pid: 2037 } hitcount: 1
{ lat: 237, common_pid: 2040 } hitcount: 1
{ lat: 237, common_pid: 2036 } hitcount: 9
{ lat: 237, common_pid: 2039 } hitcount: 3
{ lat: 237, common_pid: 2043 } hitcount: 8
{ lat: 237, common_pid: 2042 } hitcount: 2
{ lat: 237, common_pid: 2041 } hitcount: 2
{ lat: 238, common_pid: 2043 } hitcount: 10
{ lat: 238, common_pid: 2040 } hitcount: 1
{ lat: 238, common_pid: 2037 } hitcount: 9
{ lat: 238, common_pid: 2038 } hitcount: 1
{ lat: 238, common_pid: 2039 } hitcount: 1
{ lat: 238, common_pid: 2042 } hitcount: 3
{ lat: 238, common_pid: 2036 } hitcount: 7
{ lat: 239, common_pid: 2041 } hitcount: 1
{ lat: 239, common_pid: 2043 } hitcount: 11
{ lat: 239, common_pid: 2037 } hitcount: 11
{ lat: 239, common_pid: 2038 } hitcount: 6
{ lat: 239, common_pid: 2036 } hitcount: 7
{ lat: 239, common_pid: 2040 } hitcount: 1
{ lat: 239, common_pid: 2042 } hitcount: 9
{ lat: 240, common_pid: 2037 } hitcount: 29
{ lat: 240, common_pid: 2043 } hitcount: 15
{ lat: 240, common_pid: 2040 } hitcount: 44
{ lat: 240, common_pid: 2039 } hitcount: 1
{ lat: 240, common_pid: 2041 } hitcount: 2
{ lat: 240, common_pid: 2038 } hitcount: 1
{ lat: 240, common_pid: 2036 } hitcount: 10
{ lat: 240, common_pid: 2042 } hitcount: 13
{ lat: 241, common_pid: 2036 } hitcount: 21
{ lat: 241, common_pid: 2041 } hitcount: 36
{ lat: 241, common_pid: 2037 } hitcount: 34
{ lat: 241, common_pid: 2042 } hitcount: 14
{ lat: 241, common_pid: 2040 } hitcount: 94
{ lat: 241, common_pid: 2039 } hitcount: 12
{ lat: 241, common_pid: 2038 } hitcount: 2
{ lat: 241, common_pid: 2043 } hitcount: 28
{ lat: 242, common_pid: 2040 } hitcount: 109
{ lat: 242, common_pid: 2041 } hitcount: 506
{ lat: 242, common_pid: 2039 } hitcount: 155
{ lat: 242, common_pid: 2042 } hitcount: 21
{ lat: 242, common_pid: 2037 } hitcount: 52
{ lat: 242, common_pid: 2043 } hitcount: 21
{ lat: 242, common_pid: 2036 } hitcount: 16
{ lat: 242, common_pid: 2038 } hitcount: 156
{ lat: 243, common_pid: 2037 } hitcount: 46
{ lat: 243, common_pid: 2039 } hitcount: 40
{ lat: 243, common_pid: 2042 } hitcount: 119
{ lat: 243, common_pid: 2041 } hitcount: 611
{ lat: 243, common_pid: 2036 } hitcount: 69
{ lat: 243, common_pid: 2038 } hitcount: 784
{ lat: 243, common_pid: 2040 } hitcount: 323
{ lat: 243, common_pid: 2043 } hitcount: 14
{ lat: 244, common_pid: 2043 } hitcount: 35
{ lat: 244, common_pid: 2042 } hitcount: 305
{ lat: 244, common_pid: 2039 } hitcount: 8
{ lat: 244, common_pid: 2040 } hitcount: 4515
{ lat: 244, common_pid: 2038 } hitcount: 371
{ lat: 244, common_pid: 2037 } hitcount: 31
{ lat: 244, common_pid: 2036 } hitcount: 114
{ lat: 244, common_pid: 2041 } hitcount: 3396
{ lat: 245, common_pid: 2036 } hitcount: 700
{ lat: 245, common_pid: 2041 } hitcount: 2772
{ lat: 245, common_pid: 2037 } hitcount: 268
{ lat: 245, common_pid: 2039 } hitcount: 472
{ lat: 245, common_pid: 2038 } hitcount: 2758
{ lat: 245, common_pid: 2042 } hitcount: 3833
{ lat: 245, common_pid: 2040 } hitcount: 3105
{ lat: 245, common_pid: 2043 } hitcount: 645
{ lat: 246, common_pid: 2038 } hitcount: 3451
{ lat: 246, common_pid: 2041 } hitcount: 142
{ lat: 246, common_pid: 2037 } hitcount: 5101
{ lat: 246, common_pid: 2040 } hitcount: 68
{ lat: 246, common_pid: 2043 } hitcount: 5099
{ lat: 246, common_pid: 2039 } hitcount: 5608
{ lat: 246, common_pid: 2042 } hitcount: 3723
{ lat: 246, common_pid: 2036 } hitcount: 4738
{ lat: 247, common_pid: 2042 } hitcount: 312
{ lat: 247, common_pid: 2043 } hitcount: 2385
{ lat: 247, common_pid: 2041 } hitcount: 452
{ lat: 247, common_pid: 2038 } hitcount: 792
{ lat: 247, common_pid: 2040 } hitcount: 78
{ lat: 247, common_pid: 2036 } hitcount: 2375
{ lat: 247, common_pid: 2039 } hitcount: 1834
{ lat: 247, common_pid: 2037 } hitcount: 2655
{ lat: 248, common_pid: 2037 } hitcount: 36
{ lat: 248, common_pid: 2042 } hitcount: 11
{ lat: 248, common_pid: 2038 } hitcount: 122
{ lat: 248, common_pid: 2036 } hitcount: 135
{ lat: 248, common_pid: 2039 } hitcount: 26
{ lat: 248, common_pid: 2041 } hitcount: 503
{ lat: 248, common_pid: 2043 } hitcount: 66
{ lat: 248, common_pid: 2040 } hitcount: 46
{ lat: 249, common_pid: 2037 } hitcount: 29
{ lat: 249, common_pid: 2038 } hitcount: 1
{ lat: 249, common_pid: 2043 } hitcount: 29
{ lat: 249, common_pid: 2039 } hitcount: 8
{ lat: 249, common_pid: 2042 } hitcount: 56
{ lat: 249, common_pid: 2040 } hitcount: 27
{ lat: 249, common_pid: 2041 } hitcount: 11
{ lat: 249, common_pid: 2036 } hitcount: 27
{ lat: 250, common_pid: 2038 } hitcount: 1
{ lat: 250, common_pid: 2036 } hitcount: 30
{ lat: 250, common_pid: 2040 } hitcount: 19
{ lat: 250, common_pid: 2043 } hitcount: 22
{ lat: 250, common_pid: 2042 } hitcount: 20
{ lat: 250, common_pid: 2041 } hitcount: 1
{ lat: 250, common_pid: 2039 } hitcount: 6
{ lat: 250, common_pid: 2037 } hitcount: 48
{ lat: 251, common_pid: 2037 } hitcount: 43
{ lat: 251, common_pid: 2039 } hitcount: 1
{ lat: 251, common_pid: 2036 } hitcount: 12
{ lat: 251, common_pid: 2042 } hitcount: 2
{ lat: 251, common_pid: 2041 } hitcount: 1
{ lat: 251, common_pid: 2043 } hitcount: 15
{ lat: 251, common_pid: 2040 } hitcount: 3
{ lat: 252, common_pid: 2040 } hitcount: 1
{ lat: 252, common_pid: 2036 } hitcount: 12
{ lat: 252, common_pid: 2037 } hitcount: 21
{ lat: 252, common_pid: 2043 } hitcount: 14
{ lat: 253, common_pid: 2037 } hitcount: 21
{ lat: 253, common_pid: 2039 } hitcount: 2
{ lat: 253, common_pid: 2036 } hitcount: 9
{ lat: 253, common_pid: 2043 } hitcount: 6
{ lat: 253, common_pid: 2040 } hitcount: 1
{ lat: 254, common_pid: 2036 } hitcount: 8
{ lat: 254, common_pid: 2043 } hitcount: 3
{ lat: 254, common_pid: 2041 } hitcount: 1
{ lat: 254, common_pid: 2042 } hitcount: 1
{ lat: 254, common_pid: 2039 } hitcount: 1
{ lat: 254, common_pid: 2037 } hitcount: 12
{ lat: 255, common_pid: 2043 } hitcount: 1
{ lat: 255, common_pid: 2037 } hitcount: 2
{ lat: 255, common_pid: 2036 } hitcount: 2
{ lat: 255, common_pid: 2039 } hitcount: 8
{ lat: 256, common_pid: 2043 } hitcount: 1
{ lat: 256, common_pid: 2036 } hitcount: 4
{ lat: 256, common_pid: 2039 } hitcount: 6
{ lat: 257, common_pid: 2039 } hitcount: 5
{ lat: 257, common_pid: 2036 } hitcount: 4
{ lat: 258, common_pid: 2039 } hitcount: 5
{ lat: 258, common_pid: 2036 } hitcount: 2
{ lat: 259, common_pid: 2036 } hitcount: 7
{ lat: 259, common_pid: 2039 } hitcount: 7
{ lat: 260, common_pid: 2036 } hitcount: 8
{ lat: 260, common_pid: 2039 } hitcount: 6
{ lat: 261, common_pid: 2036 } hitcount: 5
{ lat: 261, common_pid: 2039 } hitcount: 7
{ lat: 262, common_pid: 2039 } hitcount: 5
{ lat: 262, common_pid: 2036 } hitcount: 5
{ lat: 263, common_pid: 2039 } hitcount: 7
{ lat: 263, common_pid: 2036 } hitcount: 7
{ lat: 264, common_pid: 2039 } hitcount: 9
{ lat: 264, common_pid: 2036 } hitcount: 9
{ lat: 265, common_pid: 2036 } hitcount: 5
{ lat: 265, common_pid: 2039 } hitcount: 1
{ lat: 266, common_pid: 2036 } hitcount: 1
{ lat: 266, common_pid: 2039 } hitcount: 3
{ lat: 267, common_pid: 2036 } hitcount: 1
{ lat: 267, common_pid: 2039 } hitcount: 3
{ lat: 268, common_pid: 2036 } hitcount: 1
{ lat: 268, common_pid: 2039 } hitcount: 6
{ lat: 269, common_pid: 2036 } hitcount: 1
{ lat: 269, common_pid: 2043 } hitcount: 1
{ lat: 269, common_pid: 2039 } hitcount: 2
{ lat: 270, common_pid: 2040 } hitcount: 1
{ lat: 270, common_pid: 2039 } hitcount: 6
{ lat: 271, common_pid: 2041 } hitcount: 1
{ lat: 271, common_pid: 2039 } hitcount: 5
{ lat: 272, common_pid: 2039 } hitcount: 10
{ lat: 273, common_pid: 2039 } hitcount: 8
{ lat: 274, common_pid: 2039 } hitcount: 2
{ lat: 275, common_pid: 2039 } hitcount: 1
{ lat: 276, common_pid: 2039 } hitcount: 2
{ lat: 276, common_pid: 2037 } hitcount: 1
{ lat: 276, common_pid: 2038 } hitcount: 1
{ lat: 277, common_pid: 2039 } hitcount: 1
{ lat: 277, common_pid: 2042 } hitcount: 1
{ lat: 278, common_pid: 2039 } hitcount: 1
{ lat: 279, common_pid: 2039 } hitcount: 4
{ lat: 279, common_pid: 2043 } hitcount: 1
{ lat: 280, common_pid: 2039 } hitcount: 3
{ lat: 283, common_pid: 2036 } hitcount: 2
{ lat: 284, common_pid: 2039 } hitcount: 1
{ lat: 284, common_pid: 2043 } hitcount: 1
{ lat: 288, common_pid: 2039 } hitcount: 1
{ lat: 289, common_pid: 2039 } hitcount: 1
{ lat: 300, common_pid: 2039 } hitcount: 1
{ lat: 384, common_pid: 2039 } hitcount: 1
Totals:
Hits: 67625
Entries: 278
Dropped: 0
Note, the writes are around the sleep, so ideally they will all be of 250
microseconds. If you are wondering how there are several that are under
250 microseconds, that is because the way cyclictest works, is if one
iteration comes in late, the next one will set the timer to wake up less that
250. That is, if an iteration came in 50 microseconds late, the next wake up
will be at 200 microseconds.
But this could easily be done in userspace. To make this even more
interesting, we can mix the histogram between events that happened in the
kernel with trace_marker::
# cd /sys/kernel/tracing
# echo 'latency u64 lat' > synthetic_events
# echo 'hist:keys=pid:ts0=common_timestamp.usecs' > events/sched/sched_waking/trigger
# echo 'hist:keys=common_pid:lat=common_timestamp.usecs-$ts0:onmatch(sched.sched_waking).latency($lat) if buf == "end"' > events/ftrace/print/trigger
# echo 'hist:keys=lat,common_pid:sort=lat' > events/synthetic/latency/trigger
The difference this time is that instead of using the trace_marker to start
the latency, the sched_waking event is used, matching the common_pid for the
trace_marker write with the pid that is being woken by sched_waking.
After running cyclictest again with the same parameters, we now have::
# cat events/synthetic/latency/hist
# event histogram
#
# trigger info: hist:keys=lat,common_pid:vals=hitcount:sort=lat:size=2048 [active]
#
{ lat: 7, common_pid: 2302 } hitcount: 640
{ lat: 7, common_pid: 2299 } hitcount: 42
{ lat: 7, common_pid: 2303 } hitcount: 18
{ lat: 7, common_pid: 2305 } hitcount: 166
{ lat: 7, common_pid: 2306 } hitcount: 1
{ lat: 7, common_pid: 2301 } hitcount: 91
{ lat: 7, common_pid: 2300 } hitcount: 17
{ lat: 8, common_pid: 2303 } hitcount: 8296
{ lat: 8, common_pid: 2304 } hitcount: 6864
{ lat: 8, common_pid: 2305 } hitcount: 9464
{ lat: 8, common_pid: 2301 } hitcount: 9213
{ lat: 8, common_pid: 2306 } hitcount: 6246
{ lat: 8, common_pid: 2302 } hitcount: 8797
{ lat: 8, common_pid: 2299 } hitcount: 8771
{ lat: 8, common_pid: 2300 } hitcount: 8119
{ lat: 9, common_pid: 2305 } hitcount: 1519
{ lat: 9, common_pid: 2299 } hitcount: 2346
{ lat: 9, common_pid: 2303 } hitcount: 2841
{ lat: 9, common_pid: 2301 } hitcount: 1846
{ lat: 9, common_pid: 2304 } hitcount: 3861
{ lat: 9, common_pid: 2302 } hitcount: 1210
{ lat: 9, common_pid: 2300 } hitcount: 2762
{ lat: 9, common_pid: 2306 } hitcount: 4247
{ lat: 10, common_pid: 2299 } hitcount: 16
{ lat: 10, common_pid: 2306 } hitcount: 333
{ lat: 10, common_pid: 2303 } hitcount: 16
{ lat: 10, common_pid: 2304 } hitcount: 168
{ lat: 10, common_pid: 2302 } hitcount: 240
{ lat: 10, common_pid: 2301 } hitcount: 28
{ lat: 10, common_pid: 2300 } hitcount: 95
{ lat: 10, common_pid: 2305 } hitcount: 18
{ lat: 11, common_pid: 2303 } hitcount: 5
{ lat: 11, common_pid: 2305 } hitcount: 8
{ lat: 11, common_pid: 2306 } hitcount: 221
{ lat: 11, common_pid: 2302 } hitcount: 76
{ lat: 11, common_pid: 2304 } hitcount: 26
{ lat: 11, common_pid: 2300 } hitcount: 125
{ lat: 11, common_pid: 2299 } hitcount: 2
{ lat: 12, common_pid: 2305 } hitcount: 3
{ lat: 12, common_pid: 2300 } hitcount: 6
{ lat: 12, common_pid: 2306 } hitcount: 90
{ lat: 12, common_pid: 2302 } hitcount: 4
{ lat: 12, common_pid: 2303 } hitcount: 1
{ lat: 12, common_pid: 2304 } hitcount: 122
{ lat: 13, common_pid: 2300 } hitcount: 12
{ lat: 13, common_pid: 2301 } hitcount: 1
{ lat: 13, common_pid: 2306 } hitcount: 32
{ lat: 13, common_pid: 2302 } hitcount: 5
{ lat: 13, common_pid: 2305 } hitcount: 1
{ lat: 13, common_pid: 2303 } hitcount: 1
{ lat: 13, common_pid: 2304 } hitcount: 61
{ lat: 14, common_pid: 2303 } hitcount: 4
{ lat: 14, common_pid: 2306 } hitcount: 5
{ lat: 14, common_pid: 2305 } hitcount: 4
{ lat: 14, common_pid: 2304 } hitcount: 62
{ lat: 14, common_pid: 2302 } hitcount: 19
{ lat: 14, common_pid: 2300 } hitcount: 33
{ lat: 14, common_pid: 2299 } hitcount: 1
{ lat: 14, common_pid: 2301 } hitcount: 4
{ lat: 15, common_pid: 2305 } hitcount: 1
{ lat: 15, common_pid: 2302 } hitcount: 25
{ lat: 15, common_pid: 2300 } hitcount: 11
{ lat: 15, common_pid: 2299 } hitcount: 5
{ lat: 15, common_pid: 2301 } hitcount: 1
{ lat: 15, common_pid: 2304 } hitcount: 8
{ lat: 15, common_pid: 2303 } hitcount: 1
{ lat: 15, common_pid: 2306 } hitcount: 6
{ lat: 16, common_pid: 2302 } hitcount: 31
{ lat: 16, common_pid: 2306 } hitcount: 3
{ lat: 16, common_pid: 2300 } hitcount: 5
{ lat: 17, common_pid: 2302 } hitcount: 6
{ lat: 17, common_pid: 2303 } hitcount: 1
{ lat: 18, common_pid: 2304 } hitcount: 1
{ lat: 18, common_pid: 2302 } hitcount: 8
{ lat: 18, common_pid: 2299 } hitcount: 1
{ lat: 18, common_pid: 2301 } hitcount: 1
{ lat: 19, common_pid: 2303 } hitcount: 4
{ lat: 19, common_pid: 2304 } hitcount: 5
{ lat: 19, common_pid: 2302 } hitcount: 4
{ lat: 19, common_pid: 2299 } hitcount: 3
{ lat: 19, common_pid: 2306 } hitcount: 1
{ lat: 19, common_pid: 2300 } hitcount: 4
{ lat: 19, common_pid: 2305 } hitcount: 5
{ lat: 20, common_pid: 2299 } hitcount: 2
{ lat: 20, common_pid: 2302 } hitcount: 3
{ lat: 20, common_pid: 2305 } hitcount: 1
{ lat: 20, common_pid: 2300 } hitcount: 2
{ lat: 20, common_pid: 2301 } hitcount: 2
{ lat: 20, common_pid: 2303 } hitcount: 3
{ lat: 21, common_pid: 2305 } hitcount: 1
{ lat: 21, common_pid: 2299 } hitcount: 5
{ lat: 21, common_pid: 2303 } hitcount: 4
{ lat: 21, common_pid: 2302 } hitcount: 7
{ lat: 21, common_pid: 2300 } hitcount: 1
{ lat: 21, common_pid: 2301 } hitcount: 5
{ lat: 21, common_pid: 2304 } hitcount: 2
{ lat: 22, common_pid: 2302 } hitcount: 5
{ lat: 22, common_pid: 2303 } hitcount: 1
{ lat: 22, common_pid: 2306 } hitcount: 3
{ lat: 22, common_pid: 2301 } hitcount: 2
{ lat: 22, common_pid: 2300 } hitcount: 1
{ lat: 22, common_pid: 2299 } hitcount: 1
{ lat: 22, common_pid: 2305 } hitcount: 1
{ lat: 22, common_pid: 2304 } hitcount: 1
{ lat: 23, common_pid: 2299 } hitcount: 1
{ lat: 23, common_pid: 2306 } hitcount: 2
{ lat: 23, common_pid: 2302 } hitcount: 6
{ lat: 24, common_pid: 2302 } hitcount: 3
{ lat: 24, common_pid: 2300 } hitcount: 1
{ lat: 24, common_pid: 2306 } hitcount: 2
{ lat: 24, common_pid: 2305 } hitcount: 1
{ lat: 24, common_pid: 2299 } hitcount: 1
{ lat: 25, common_pid: 2300 } hitcount: 1
{ lat: 25, common_pid: 2302 } hitcount: 4
{ lat: 26, common_pid: 2302 } hitcount: 2
{ lat: 27, common_pid: 2305 } hitcount: 1
{ lat: 27, common_pid: 2300 } hitcount: 1
{ lat: 27, common_pid: 2302 } hitcount: 3
{ lat: 28, common_pid: 2306 } hitcount: 1
{ lat: 28, common_pid: 2302 } hitcount: 4
{ lat: 29, common_pid: 2302 } hitcount: 1
{ lat: 29, common_pid: 2300 } hitcount: 2
{ lat: 29, common_pid: 2306 } hitcount: 1
{ lat: 29, common_pid: 2304 } hitcount: 1
{ lat: 30, common_pid: 2302 } hitcount: 4
{ lat: 31, common_pid: 2302 } hitcount: 6
{ lat: 32, common_pid: 2302 } hitcount: 1
{ lat: 33, common_pid: 2299 } hitcount: 1
{ lat: 33, common_pid: 2302 } hitcount: 3
{ lat: 34, common_pid: 2302 } hitcount: 2
{ lat: 35, common_pid: 2302 } hitcount: 1
{ lat: 35, common_pid: 2304 } hitcount: 1
{ lat: 36, common_pid: 2302 } hitcount: 4
{ lat: 37, common_pid: 2302 } hitcount: 6
{ lat: 38, common_pid: 2302 } hitcount: 2
{ lat: 39, common_pid: 2302 } hitcount: 2
{ lat: 39, common_pid: 2304 } hitcount: 1
{ lat: 40, common_pid: 2304 } hitcount: 2
{ lat: 40, common_pid: 2302 } hitcount: 5
{ lat: 41, common_pid: 2304 } hitcount: 1
{ lat: 41, common_pid: 2302 } hitcount: 8
{ lat: 42, common_pid: 2302 } hitcount: 6
{ lat: 42, common_pid: 2304 } hitcount: 1
{ lat: 43, common_pid: 2302 } hitcount: 3
{ lat: 43, common_pid: 2304 } hitcount: 4
{ lat: 44, common_pid: 2302 } hitcount: 6
{ lat: 45, common_pid: 2302 } hitcount: 5
{ lat: 46, common_pid: 2302 } hitcount: 5
{ lat: 47, common_pid: 2302 } hitcount: 7
{ lat: 48, common_pid: 2301 } hitcount: 1
{ lat: 48, common_pid: 2302 } hitcount: 9
{ lat: 49, common_pid: 2302 } hitcount: 3
{ lat: 50, common_pid: 2302 } hitcount: 1
{ lat: 50, common_pid: 2301 } hitcount: 1
{ lat: 51, common_pid: 2302 } hitcount: 2
{ lat: 51, common_pid: 2301 } hitcount: 1
{ lat: 61, common_pid: 2302 } hitcount: 1
{ lat: 110, common_pid: 2302 } hitcount: 1
Totals:
Hits: 89565
Entries: 158
Dropped: 0
This doesn't tell us any information about how late cyclictest may have
woken up, but it does show us a nice histogram of how long it took from
the time that cyclictest was woken to the time it made it into user space.