.\"	$NetBSD: heartbeat.9,v 1.4 2023/07/07 13:19:30 riastradh Exp $
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.Dd July 6, 2023
.Dt HEARTBEAT 9
.Os
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh NAME
.Nm heartbeat
.Nd periodic checks to ensure CPUs are making progress
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SYNOPSIS
.Fd "options 	HEARTBEAT"
.Fd "options 	HEARTBEAT_MAX_PERIOD_DEFAULT=15"
.\"
.In sys/heartbeat.h
.\"
.Ft void
.Fn heartbeat_start void
.Ft void
.Fn heartbeat void
.Ft void
.Fn heartbeat_suspend void
.Ft void
.Fn heartbeat_resume void
.Fd "#ifdef DDB"
.Ft void
.Fn heartbeat_dump void
.Fd "#endif"
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh DESCRIPTION
The
.Nm
subsystem verifies that soft interrupts
.Pq Xr softint 9
and the system
.Xr timecounter 9
are making progress over time, and panics if they appear stuck.
.Pp
The number of seconds before
.Nm
panics without progress is controlled by the sysctl knob
.Li kern.heartbeat.max_period ,
which defaults to 15.
If set to zero, heartbeat checks are disabled.
.Pp
The periodic hardware timer interrupt handler calls
.Fn heartbeat
every tick on each CPU.
Once per second
.Po
i.e., every
.Xr hz 9
ticks
.Pc ,
.Fn heartbeat
schedules a soft interrupt at priority
.Dv SOFTINT_CLOCK
to advance the current CPU's view of
.Xr time_uptime 9 .
.Pp
.Fn heartbeat
checks whether
.Xr time_uptime 9
has changed, to see if either the
.Xr timecounter 9
or soft intrrupts on the current CPU are stuck.
If it hasn't advanced within
.Li kern.heartbeat.max_period
seconds worth of ticks, or if it has updated and the current CPU's view
of it hasn't been updated by more than
.Li kern.heartbeat.max_period
seconds, then
.Fn heartbeat
panics.
.Pp
.Fn heartbeat
also checks whether the next online CPU has advanced its view of
.Xr time_uptime 9 ,
to see if soft interrupts
.Pq including Xr callout 9
on that CPU are stuck.
If it hasn't updated within
.Li kern.heartbeat.max_period
seconds,
.Fn heartbeat
sends an
.Xr ipi 9
to panic on that CPU.
If that CPU has not acknowledged the
.Xr ipi 9
within one second,
.Fn heartbeat
panics on the current CPU instead.
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh FUNCTIONS
.Bl -tag -width Fn
.It Fn heartbeat
Check for timecounter and soft interrupt progress on this CPU and on
another CPU, and schedule a soft interrupt to advance this CPU's view
of timecounter progress.
.Pp
Called by
.Xr hardclock 9
periodically.
.It Fn heartbeat_dump
Print each CPU's heartbeat counter, uptime cache, and uptime cache
timestamp (in units of heartbeats) to the console.
.Pp
Can be invoked from
.Xr ddb 9
by
.Ql call heartbeat_dump .
.It Fn heartbeat_resume
Resume heartbeat monitoring of the current CPU.
.Pp
Called after a CPU has started running but before it has been
marked online.
.It Fn heartbeat_start
Start monitoring heartbeats systemwide.
.Pp
Called by
.Fn main
in
.Pa sys/kern/init_main.c
as soon as soft interrupts can be established.
.It Fn heartbeat_suspend
Suspend heartbeat monitoring of the current CPU.
.Pp
Called after the current CPU has been marked offline but before it has
stopped running.
.El
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh CODE REFERENCES
The
.Nm
subsystem is implemented in
.Pa sys/kern/kern_heartbeat.c .
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh SEE ALSO
.Xr swwdog 4 ,
.Xr wdogctl 8
.\"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""
.Sh HISTORY
The
.Nm
subsystem first appeared in
.Nx 11.0 .