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.Dd April 21, 2015
.Dt KTHREAD 9
.Os
.Sh NAME
.Nm kthread_create ,
.Nm kthread_exit ,
.Nm kthread_join ,
.Nm kthread_fpu_enter ,
.Nm kthread_fpu_exit
.Nd kernel threads
.Sh SYNOPSIS
.In sys/kthread.h
.Ft int
.Fn kthread_create "pri_t pri" "int flags" "struct cpu_info *ci" \
"void (*func)(void *)" "void *arg" "lwp_t **newlp" "const char *fmt" "..."
.Ft void
.Fn kthread_exit "int ecode"
.Ft int
.Fn kthread_join "lwp_t *l"
.Ft int
.Fn kthread_fpu_enter
.Ft void
.Fn kthread_fpu_exit "int s"
.Sh DESCRIPTION
Kernel threads are light-weight processes which execute entirely
within the kernel.
.Pp
Any process can request the creation of a new kernel thread.
Kernel threads are not swapped out during memory congestion.
The VM space and limits are shared with proc0 (usually swapper).
.Pp
If the machine has any per-CPU floating-point units or SIMD vector
units that are normally available to user threads, they can be used by
kthreads between
.Fn kthread_fpu_enter
and
.Fn kthread_fpu_exit .
.Sh FUNCTIONS
.Bl -tag -width compact
.It Fn kthread_create "pri" "flags" "ci" "func" "arg" "newlp" "fmt" "..."
Create a kernel thread.
The arguments are as follows.
.Bl -tag -width compact
.It Fa pri
Priority level for the thread.
If no priority level is desired specify
.Dv PRI_NONE ,
causing
.Fn kthread_create
to select the default priority level.
.It Fa flags
Flags that can be logically ORed together to alter the thread's behaviour.
.It Fa ci
If
.No non- Ns Dv NULL ,
the thread will be created bound to the CPU specified by
.Fa ci ,
meaning that it will only ever execute on that CPU.
By default, the threads are free to execute on any CPU in the system.
.It Fa func
A function to be called when the thread begins executing.
This function must not return.
If the thread runs to completion, it must call
.Fn kthread_exit
to properly terminate itself.
.It Fa arg
An argument to be passed to
.Fn func .
May be
.Dv NULL
if not required.
.It Fa newlp
A pointer to receive the new LWP structure for the kernel thread.
May be
.Dv NULL ,
unless
.Dv KTHREAD_MUSTJOIN
is specified in
.Fa flags .
.It Fa fmt
A string containing format information used to display the kernel
thread name.
Must not be
.Dv NULL .
.El
.Pp
The following
.Va flags
are defined.
.Bl -tag -width KTHREAD_MUSTJOIN
.It Dv KTHREAD_IDLE
Causes the thread to be created in the
.Dv LSIDL
(idle) state.
By default, the threads are created in the
.Dv LSRUN
(runnable) state, meaning they will begin execution shortly after creation.
.It Dv KTHREAD_MPSAFE
Specifies that the thread does its own locking and so is multiprocessor safe.
If not specified, the global kernel lock will be held whenever the thread is
running (unless explicitly dropped by the thread).
.It Dv KTHREAD_INTR
Specifies that the thread services device interrupts.
This flag is intended for kernel internal use and should not normally be
specified.
.It Dv KTHREAD_TS
Causes the kthread to be created in the
.Dv SCHED_OTHER
class (timeshared).
The thread's priority will be dynamically adjusted by the scheduler.
Increased activity by the kthread will cause its priority to fall;
decreased activity will cause its priority to rise.
By default, kthreads are created in the
.Dv SCHED_RR
class, with a fixed
priority specified by
.Ar pri .
Threads in the
.Dv SCHED_RR
class do not have their priority dynamically
adjusted by the scheduler.
.It Dv KTHREAD_MUSTJOIN
Indicates that created kthread must be joined.
In such case
.Fn kthread_exit
will wait until
.Fn kthread_join
will be called.
.El
.It Fn kthread_exit "ecode"
Exit from a kernel thread.
Must only be called by a kernel thread.
.It Fn kthread_join "l"
Suspend execution of calling thread until the target kthread terminates.
Conceptually the function can be compared to the user space
.Xr pthread_join 3 ,
however it must be called only once for kernel thread which was
created using the
.Dv KTHREAD_MUSTJOIN
flag and would wait on
.Fa kthread_exit .
.It Fn kthread_fpu_enter
Allow the current kthread to use any machine-dependent per-CPU
floating-point units or SIMD vector units normally available to user
threads.
Returns a cookie that must be passed to
.Fn kthread_fpu_exit
when done.
.Pp
Matching pairs of
.Fn kthread_fpu_enter
and
.Fn kthread_fpu_exit
may be nested.
.It Fn kthread_fpu_exit "s"
Restore the current kthread's access to machine-dependent per-CPU
floating-point units or SIMD vector units to what it was before the
call to
.Fn kthread_fpu_enter
that returned
.Fa s .
.Pp
On the last
.Fn kthread_fpu_exit ,
zero all the units' registers to avoid leaking secrets \(em such units
are often used for cryptography.
.El
.Sh RETURN VALUES
Upon successful completion,
.Fn kthread_create
returns 0.
Otherwise, the following error values are returned:
.Bl -tag -width [EAGAIN]
.It Bq Er EAGAIN
The limit on the total number of system processes would be exceeded;
or the limit
.Dv RLIMIT_NPROC
on the total number of processes under execution by this
user id would be exceeded.
.El
.Sh CODE REFERENCES
The kthread framework itself is implemented within the file
.Pa sys/kern/kern_kthread.c .
Data structures and function prototypes for the framework are located in
.Pa sys/sys/kthread.h .
.Sh SEE ALSO
.Xr condvar 9 ,
.Xr driver 9 ,
.Xr softint 9 ,
.Xr workqueue 9
.Sh HISTORY
The kthread framework appeared in
.Nx 1.4 .