man/man9/callout.9

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.Dd September 1, 2021
.Dt CALLOUT 9
.Os
.Sh NAME
.Nm callout_active ,
.Nm callout_deactivate ,
.Nm callout_async_drain ,
.Nm callout_drain ,
.Nm callout_init ,
.Nm callout_init_mtx ,
.Nm callout_init_rm ,
.Nm callout_init_rw ,
.Nm callout_pending ,
.Nm callout_reset ,
.Nm callout_reset_curcpu ,
.Nm callout_reset_on ,
.Nm callout_reset_sbt ,
.Nm callout_reset_sbt_curcpu ,
.Nm callout_reset_sbt_on ,
.Nm callout_schedule ,
.Nm callout_schedule_curcpu ,
.Nm callout_schedule_on ,
.Nm callout_schedule_sbt ,
.Nm callout_schedule_sbt_curcpu ,
.Nm callout_schedule_sbt_on ,
.Nm callout_stop ,
.Nm callout_when
.Nd execute a function after a specified length of time
.Sh SYNOPSIS
.In sys/types.h
.In sys/callout.h
.Bd -literal
typedef void callout_func_t (void *);
.Ed
.Ft int
.Fn callout_active "struct callout *c"
.Ft void
.Fn callout_deactivate "struct callout *c"
.Ft int
.Fn callout_async_drain "struct callout *c" "callout_func_t *drain"
.Ft int
.Fn callout_drain "struct callout *c"
.Ft void
.Fn callout_init "struct callout *c" "int mpsafe"
.Ft void
.Fn callout_init_mtx "struct callout *c" "struct mtx *mtx" "int flags"
.Ft void
.Fn callout_init_rm "struct callout *c" "struct rmlock *rm" "int flags"
.Ft void
.Fn callout_init_rw "struct callout *c" "struct rwlock *rw" "int flags"
.Ft int
.Fn callout_pending "struct callout *c"
.Ft int
.Fo callout_reset
.Fa "struct callout *c"
.Fa "int ticks"
.Fa "callout_func_t *func"
.Fa "void *arg"
.Fc
.Ft int
.Fo callout_reset_curcpu
.Fa "struct callout *c"
.Fa "int ticks"
.Fa "callout_func_t *func"
.Fa "void *arg"
.Fc
.Ft int
.Fo callout_reset_on
.Fa "struct callout *c"
.Fa "int ticks"
.Fa "callout_func_t *func"
.Fa "void *arg"
.Fa "int cpu"
.Fc
.Ft int
.Fo callout_reset_sbt
.Fa "struct callout *c"
.Fa "sbintime_t sbt"
.Fa "sbintime_t pr"
.Fa "callout_func_t *func"
.Fa "void *arg"
.Fa "int flags"
.Fc
.Ft int
.Fo callout_reset_sbt_curcpu
.Fa "struct callout *c"
.Fa "sbintime_t sbt"
.Fa "sbintime_t pr"
.Fa "callout_func_t *func"
.Fa "void *arg"
.Fa "int flags"
.Fc
.Ft int
.Fo callout_reset_sbt_on
.Fa "struct callout *c"
.Fa "sbintime_t sbt"
.Fa "sbintime_t pr"
.Fa "callout_func_t *func"
.Fa "void *arg"
.Fa "int cpu"
.Fa "int flags"
.Fc
.Ft int
.Fn callout_schedule "struct callout *c" "int ticks"
.Ft int
.Fn callout_schedule_curcpu "struct callout *c" "int ticks"
.Ft int
.Fn callout_schedule_on "struct callout *c" "int ticks" "int cpu"
.Ft int
.Fo callout_schedule_sbt
.Fa "struct callout *c"
.Fa "sbintime_t sbt"
.Fa "sbintime_t pr"
.Fa "int flags"
.Fc
.Ft int
.Fo callout_schedule_sbt_curcpu
.Fa "struct callout *c"
.Fa "sbintime_t sbt"
.Fa "sbintime_t pr"
.Fa "int flags"
.Fc
.Ft int
.Fo callout_schedule_sbt_on
.Fa "struct callout *c"
.Fa "sbintime_t sbt"
.Fa "sbintime_t pr"
.Fa "int cpu"
.Fa "int flags"
.Fc
.Ft int
.Fn callout_stop "struct callout *c"
.Ft sbintime_t
.Fo callout_when
.Fa "sbintime_t sbt"
.Fa "sbintime_t precision"
.Fa "int flags"
.Fa "sbintime_t *sbt_res"
.Fa "sbintime_t *precision_res"
.Fc
.Sh DESCRIPTION
The
.Nm callout
API is used to schedule a call to an arbitrary function at a specific
time in the future.
Consumers of this API are required to allocate a callout structure
.Pq struct callout
for each pending function invocation.
This structure stores state about the pending function invocation including
the function to be called and the time at which the function should be invoked.
Pending function calls can be cancelled or rescheduled to a different time.
In addition,
a callout structure may be reused to schedule a new function call after a
scheduled call is completed.
.Pp
Callouts only provide a single-shot mode.
If a consumer requires a periodic timer,
it must explicitly reschedule each function call.
This is normally done by rescheduling the subsequent call within the called
function.
.Pp
Callout functions must not sleep.
They may not acquire sleepable locks,
wait on condition variables,
perform blocking allocation requests,
or invoke any other action that might sleep.
.Pp
Each callout structure must be initialized by
.Fn callout_init ,
.Fn callout_init_mtx ,
.Fn callout_init_rm ,
or
.Fn callout_init_rw
before it is passed to any of the other callout functions.
The
.Fn callout_init
function initializes a callout structure in
.Fa c
that is not associated with a specific lock.
If the
.Fa mpsafe
argument is zero,
the callout structure is not considered to be
.Dq multi-processor safe ;
and the Giant lock will be acquired before calling the callout function
and released when the callout function returns.
.Pp
The
.Fn callout_init_mtx ,
.Fn callout_init_rm ,
and
.Fn callout_init_rw
functions initialize a callout structure in
.Fa c
that is associated with a specific lock.
The lock is specified by the
.Fa mtx ,
.Fa rm ,
or
.Fa rw
parameter.
The associated lock must be held while stopping or rescheduling the
callout.
The callout subsystem acquires the associated lock before calling the
callout function and releases it after the function returns.
If the callout was cancelled while the callout subsystem waited for the
associated lock,
the callout function is not called,
and the associated lock is released.
This ensures that stopping or rescheduling the callout will abort any
previously scheduled invocation.
.Pp
A sleepable read-mostly lock
.Po
one initialized with the
.Dv RM_SLEEPABLE
flag
.Pc
may not be used with
.Fn callout_init_rm .
Similarly, other sleepable lock types such as
.Xr sx 9
and
.Xr lockmgr 9
cannot be used with callouts because sleeping is not permitted in
the callout subsystem.
.Pp
These
.Fa flags
may be specified for
.Fn callout_init_mtx ,
.Fn callout_init_rm ,
or
.Fn callout_init_rw :
.Bl -tag -width ".Dv CALLOUT_RETURNUNLOCKED"
.It Dv CALLOUT_RETURNUNLOCKED
The callout function will release the associated lock itself,
so the callout subsystem should not attempt to unlock it
after the callout function returns.
.It Dv CALLOUT_SHAREDLOCK
The lock is only acquired in read mode when running the callout handler.
This flag is ignored by
.Fn callout_init_mtx .
.El
.Pp
The function
.Fn callout_stop
cancels a callout
.Fa c
if it is currently pending.
If the callout is pending and successfully stopped, then
.Fn callout_stop
returns a value of one.
If the callout is not set, or
has already been serviced, then
negative one is returned.
If the callout is currently being serviced and cannot be stopped,
then zero will be returned.
If the callout is currently being serviced and cannot be stopped, and at the
same time a next invocation of the same callout is also scheduled, then
.Fn callout_stop
unschedules the next run and returns zero.
If the callout has an associated lock,
then that lock must be held when this function is called.
.Pp
The function
.Fn callout_async_drain
is identical to
.Fn callout_stop
with one difference.
When
.Fn callout_async_drain
returns zero it will arrange for the function
.Fa drain
to be called using the same argument given to the
.Fn callout_reset
function.
.Fn callout_async_drain
If the callout has an associated lock,
then that lock must be held when this function is called.
Note that when stopping multiple callouts that use the same lock it is possible
to get multiple return's of zero and multiple calls to the
.Fa drain
function, depending upon which CPU's the callouts are running.
The
.Fa drain
function itself is called from the context of the completing callout
i.e. softclock or hardclock, just like a callout itself.
.Pp
The function
.Fn callout_drain
is identical to
.Fn callout_stop
except that it will wait for the callout
.Fa c
to complete if it is already in progress.
This function MUST NOT be called while holding any
locks on which the callout might block, or deadlock will result.
Note that if the callout subsystem has already begun processing this
callout, then the callout function may be invoked before
.Fn callout_drain
returns.
However, the callout subsystem does guarantee that the callout will be
fully stopped before
.Fn callout_drain
returns.
.Pp
The
.Fn callout_reset
and
.Fn callout_schedule
function families schedule a future function invocation for callout
.Fa c .
If
.Fa c
already has a pending callout,
it is cancelled before the new invocation is scheduled.
These functions return a value of one if a pending callout was cancelled
and zero if there was no pending callout.
If the callout has an associated lock,
then that lock must be held when any of these functions are called.
.Pp
The time at which the callout function will be invoked is determined by
either the
.Fa ticks
argument or the
.Fa sbt ,
.Fa pr ,
and
.Fa flags
arguments.
When
.Fa ticks
is used,
the callout is scheduled to execute after
.Fa ticks Ns No /hz
seconds.
Non-positive values of
.Fa ticks
are silently converted to the value
.Sq 1 .
.Pp
The
.Fa sbt ,
.Fa pr ,
and
.Fa flags
arguments provide more control over the scheduled time including
support for higher resolution times,
specifying the precision of the scheduled time,
and setting an absolute deadline instead of a relative timeout.
The callout is scheduled to execute in a time window which begins at
the time specified in
.Fa sbt
and extends for the amount of time specified in
.Fa pr .
If
.Fa sbt
specifies a time in the past,
the window is adjusted to start at the current time.
A non-zero value for
.Fa pr
allows the callout subsystem to coalesce callouts scheduled close to each
other into fewer timer interrupts,
reducing processing overhead and power consumption.
These
.Fa flags
may be specified to adjust the interpretation of
.Fa sbt
and
.Fa pr :
.Bl -tag -width ".Dv C_DIRECT_EXEC"
.It Dv C_ABSOLUTE
Handle the
.Fa sbt
argument as an absolute time since boot.
By default,
.Fa sbt
is treated as a relative amount of time,
similar to
.Fa ticks .
.It Dv C_DIRECT_EXEC
Run the handler directly from hardware interrupt context instead of from the
softclock thread.
This reduces latency and overhead, but puts more constraints on the callout
function.
Callout functions run in this context may use only spin mutexes for locking
and should be as small as possible because they run with absolute priority.
.It Fn C_PREL
Specifies relative event time precision as binary logarithm of time interval
divided by acceptable time deviation: 1 -- 1/2, 2 -- 1/4, etc.
Note that the larger of
.Fa pr
or this value is used as the length of the time window.
Smaller values
.Pq which result in larger time intervals
allow the callout subsystem to aggregate more events in one timer interrupt.
.It Dv C_PRECALC
The
.Fa sbt
argument specifies the absolute time at which the callout should be run,
and the
.Fa pr
argument specifies the requested precision, which will not be
adjusted during the scheduling process.
The
.Fa sbt
and
.Fa pr
values should be calculated by an earlier call to
.Fn callout_when
which uses the user-supplied
.Fa sbt ,
.Fa pr ,
and
.Fa flags
values.
.It Dv C_HARDCLOCK
Align the timeouts to
.Fn hardclock
calls if possible.
.El
.Pp
The
.Fn callout_reset
functions accept a
.Fa func
argument which identifies the function to be called when the time expires.
It must be a pointer to a function that takes a single
.Fa void *
argument.
Upon invocation,
.Fa func
will receive
.Fa arg
as its only argument.
The
.Fn callout_schedule
functions reuse the
.Fa func
and
.Fa arg
arguments from the previous callout.
Note that one of the
.Fn callout_reset
functions must always be called to initialize
.Fa func
and
.Fa arg
before one of the
.Fn callout_schedule
functions can be used.
.Pp
The callout subsystem provides a softclock thread for each CPU in the system.
Callouts are assigned to a single CPU and are executed by the softclock thread
for that CPU.
Initially,
callouts are assigned to CPU 0.
The
.Fn callout_reset_on ,
.Fn callout_reset_sbt_on ,
.Fn callout_schedule_on
and
.Fn callout_schedule_sbt_on
functions assign the callout to CPU
.Fa cpu .
The
.Fn callout_reset_curcpu ,
.Fn callout_reset_sbt_curpu ,
.Fn callout_schedule_curcpu
and
.Fn callout_schedule_sbt_curcpu
functions assign the callout to the current CPU.
The
.Fn callout_reset ,
.Fn callout_reset_sbt ,
.Fn callout_schedule
and
.Fn callout_schedule_sbt
functions schedule the callout to execute in the softclock thread of the CPU
to which it is currently assigned.
.Pp
Softclock threads are not pinned to their respective CPUs by default.
The softclock thread for CPU 0 can be pinned to CPU 0 by setting the
.Va kern.pin_default_swi
loader tunable to a non-zero value.
Softclock threads for CPUs other than zero can be pinned to their
respective CPUs by setting the
.Va kern.pin_pcpu_swi
loader tunable to a non-zero value.
.Pp
The macros
.Fn callout_pending ,
.Fn callout_active
and
.Fn callout_deactivate
provide access to the current state of the callout.
The
.Fn callout_pending
macro checks whether a callout is
.Em pending ;
a callout is considered
.Em pending
when a timeout has been set but the time has not yet arrived.
Note that once the timeout time arrives and the callout subsystem
starts to process this callout,
.Fn callout_pending
will return
.Dv FALSE
even though the callout function may not have finished
.Pq or even begun
executing.
The
.Fn callout_active
macro checks whether a callout is marked as
.Em active ,
and the
.Fn callout_deactivate
macro clears the callout's
.Em active
flag.
The callout subsystem marks a callout as
.Em active
when a timeout is set and it clears the
.Em active
flag in
.Fn callout_stop
and
.Fn callout_drain ,
but it
.Em does not
clear it when a callout expires normally via the execution of the
callout function.
.Pp
The
.Fn callout_when
function may be used to pre-calculate the absolute time at which the
timeout should be run and the precision of the scheduled run time
according to the required time
.Fa sbt ,
precision
.Fa precision ,
and additional adjustments requested by the
.Fa flags
argument.
Flags accepted by the
.Fn callout_when
function are the same as flags for the
.Fn callout_reset
function.
The resulting time is assigned to the variable pointed to by the
.Fa sbt_res
argument, and the resulting precision is assigned to
.Fa *precision_res .
When passing the results to
.Fa callout_reset ,
add the
.Va C_PRECALC
flag to
.Fa flags ,
to avoid incorrect re-adjustment.
The function is intended for situations where precise time of the callout
run should be known in advance, since
trying to read this time from the callout structure itself after a
.Fn callout_reset
call is racy.
.Ss "Avoiding Race Conditions"
The callout subsystem invokes callout functions from its own thread
context.
Without some kind of synchronization,
it is possible that a callout
function will be invoked concurrently with an attempt to stop or reset
the callout by another thread.
In particular, since callout functions typically acquire a lock as
their first action, the callout function may have already been invoked,
but is blocked waiting for that lock at the time that another thread
tries to reset or stop the callout.
.Pp
There are three main techniques for addressing these
synchronization concerns.
The first approach is preferred as it is the simplest:
.Bl -enum -offset indent
.It
Callouts can be associated with a specific lock when they are initialized
by
.Fn callout_init_mtx ,
.Fn callout_init_rm ,
or
.Fn callout_init_rw .
When a callout is associated with a lock,
the callout subsystem acquires the lock before the callout function is
invoked.
This allows the callout subsystem to transparently handle races between
callout cancellation,
scheduling,
and execution.
Note that the associated lock must be acquired before calling
.Fn callout_stop
or one of the
.Fn callout_reset
or
.Fn callout_schedule
functions to provide this safety.
.Pp
A callout initialized via
.Fn callout_init
with
.Fa mpsafe
set to zero is implicitly associated with the
.Va Giant
mutex.
If
.Va Giant
is held when cancelling or rescheduling the callout,
then its use will prevent races with the callout function.
.It
The return value from
.Fn callout_stop
.Po
or the
.Fn callout_reset
and
.Fn callout_schedule
function families
.Pc
indicates whether or not the callout was removed.
If it is known that the callout was set and the callout function has
not yet executed, then a return value of
.Dv FALSE
indicates that the callout function is about to be called.
For example:
.Bd -literal -offset indent
if (sc->sc_flags & SCFLG_CALLOUT_RUNNING) {
	if (callout_stop(&sc->sc_callout)) {
		sc->sc_flags &= ~SCFLG_CALLOUT_RUNNING;
		/* successfully stopped */
	} else {
		/*
		 * callout has expired and callout
		 * function is about to be executed
		 */
	}
}
.Ed
.It
The
.Fn callout_pending ,
.Fn callout_active
and
.Fn callout_deactivate
macros can be used together to work around the race conditions.
When a callout's timeout is set, the callout subsystem marks the
callout as both
.Em active
and
.Em pending .
When the timeout time arrives, the callout subsystem begins processing
the callout by first clearing the
.Em pending
flag.
It then invokes the callout function without changing the
.Em active
flag, and does not clear the
.Em active
flag even after the callout function returns.
The mechanism described here requires the callout function itself to
clear the
.Em active
flag using the
.Fn callout_deactivate
macro.
The
.Fn callout_stop
and
.Fn callout_drain
functions always clear both the
.Em active
and
.Em pending
flags before returning.
.Pp
The callout function should first check the
.Em pending
flag and return without action if
.Fn callout_pending
returns
.Dv TRUE .
This indicates that the callout was rescheduled using
.Fn callout_reset
just before the callout function was invoked.
If
.Fn callout_active
returns
.Dv FALSE
then the callout function should also return without action.
This indicates that the callout has been stopped.
Finally, the callout function should call
.Fn callout_deactivate
to clear the
.Em active
flag.
For example:
.Bd -literal -offset indent
mtx_lock(&sc->sc_mtx);
if (callout_pending(&sc->sc_callout)) {
	/* callout was reset */
	mtx_unlock(&sc->sc_mtx);
	return;
}
if (!callout_active(&sc->sc_callout)) {
	/* callout was stopped */
	mtx_unlock(&sc->sc_mtx);
	return;
}
callout_deactivate(&sc->sc_callout);
/* rest of callout function */
.Ed
.Pp
Together with appropriate synchronization, such as the mutex used above,
this approach permits the
.Fn callout_stop
and
.Fn callout_reset
functions to be used at any time without races.
For example:
.Bd -literal -offset indent
mtx_lock(&sc->sc_mtx);
callout_stop(&sc->sc_callout);
/* The callout is effectively stopped now. */
.Ed
.Pp
If the callout is still pending then these functions operate normally,
but if processing of the callout has already begun then the tests in
the callout function cause it to return without further action.
Synchronization between the callout function and other code ensures that
stopping or resetting the callout will never be attempted while the
callout function is past the
.Fn callout_deactivate
call.
.Pp
The above technique additionally ensures that the
.Em active
flag always reflects whether the callout is effectively enabled or
disabled.
If
.Fn callout_active
returns false, then the callout is effectively disabled, since even if
the callout subsystem is actually just about to invoke the callout
function, the callout function will return without action.
.El
.Pp
There is one final race condition that must be considered when a
callout is being stopped for the last time.
In this case it may not be safe to let the callout function itself
detect that the callout was stopped, since it may need to access
data objects that have already been destroyed or recycled.
To ensure that the callout is completely finished, a call to
.Fn callout_drain
should be used.
In particular,
a callout should always be drained prior to destroying its associated lock
or releasing the storage for the callout structure.
.Sh RETURN VALUES
The
.Fn callout_active
macro returns the state of a callout's
.Em active
flag.
.Pp
The
.Fn callout_pending
macro returns the state of a callout's
.Em pending
flag.
.Pp
The
.Fn callout_reset
and
.Fn callout_schedule
function families return a value of one if the callout was pending before the new
function invocation was scheduled.
.Pp
The
.Fn callout_stop
and
.Fn callout_drain
functions return a value of one if the callout was still pending when it was
called, a zero if the callout could not be stopped and a negative one is it
was either not running or has already completed.
.Sh HISTORY
.Fx
initially used the long standing
.Bx
linked list
callout mechanism which offered O(n) insertion and removal running time
but did not generate or require handles for untimeout operations.
.Pp
.Fx 3.0
introduced a new set of timeout and untimeout routines from
.Nx
based on the work of
.An Adam M. Costello
and
.An George Varghese ,
published in a technical report entitled
.%T "Redesigning the BSD Callout and Timer Facilities"
and modified for inclusion in
.Fx
by
.An Justin T. Gibbs .
The original work on the data structures used in that implementation
was published by
.An G. Varghese
and
.An A. Lauck
in the paper
.%T "Hashed and Hierarchical Timing Wheels: Data Structures for the Efficient Implementation of a Timer Facility"
in the
.%B "Proceedings of the 11th ACM Annual Symposium on Operating Systems Principles" .
.Pp
.Fx 3.3
introduced the first implementations of
.Fn callout_init ,
.Fn callout_reset ,
and
.Fn callout_stop
which permitted callers to allocate dedicated storage for callouts.
This ensured that a callout would always fire unlike
.Fn timeout
which would silently fail if it was unable to allocate a callout.
.Pp
.Fx 5.0
permitted callout handlers to be tagged as MPSAFE via
.Fn callout_init .
.Pp
.Fx 5.3
introduced
.Fn callout_drain .
.Pp
.Fx 6.0
introduced
.Fn callout_init_mtx .
.Pp
.Fx 8.0
introduced per-CPU callout wheels,
.Fn callout_init_rw ,
and
.Fn callout_schedule .
.Pp
.Fx 9.0
changed the underlying timer interrupts used to drive callouts to prefer
one-shot event timers instead of a periodic timer interrupt.
.Pp
.Fx 10.0
switched the callout wheel to support tickless operation.
These changes introduced
.Vt sbintime_t
and the
.Fn callout_reset_sbt*
family of functions.
.Fx 10.0 also added
.Dv C_DIRECT_EXEC
and
.Fn callout_init_rm .
.Pp
.Fx 10.2
introduced the
.Fn callout_schedule_sbt*
family of functions.
.Pp
.Fx 11.0
introduced
.Fn callout_async_drain .
.Fx 11.1
introduced
.Fn callout_when .
.Fx 13.0
removed
.Vt timeout_t ,
.Fn timeout ,
and
.Fn untimeout .