CALLOUT(9) FreeBSD Kernel Developer's Manual CALLOUT(9)
NAME
callout_active, callout_deactivate, callout_async_drain, callout_drain,
callout_init, callout_init_mtx, callout_init_rm, callout_init_rw,
callout_pending, callout_reset, callout_reset_curcpu, callout_reset_on,
callout_reset_sbt, callout_reset_sbt_curcpu, callout_reset_sbt_on,
callout_schedule, callout_schedule_curcpu, callout_schedule_on,
callout_schedule_sbt, callout_schedule_sbt_curcpu,
callout_schedule_sbt_on, callout_stop, callout_when - execute a function
after a specified length of time
SYNOPSIS
#include <sys/types.h>
#include <sys/callout.h>
typedef void callout_func_t (void *);
int
callout_active(struct callout *c);
void
callout_deactivate(struct callout *c);
int
callout_async_drain(struct callout *c, callout_func_t *drain);
int
callout_drain(struct callout *c);
void
callout_init(struct callout *c, int mpsafe);
void
callout_init_mtx(struct callout *c, struct mtx *mtx, int flags);
void
callout_init_rm(struct callout *c, struct rmlock *rm, int flags);
void
callout_init_rw(struct callout *c, struct rwlock *rw, int flags);
int
callout_pending(struct callout *c);
int
callout_reset(struct callout *c, int ticks, callout_func_t *func,
void *arg);
int
callout_reset_curcpu(struct callout *c, int ticks, callout_func_t *func,
void *arg);
int
callout_reset_on(struct callout *c, int ticks, callout_func_t *func,
void *arg, int cpu);
int
callout_reset_sbt(struct callout *c, sbintime_t sbt, sbintime_t pr,
callout_func_t *func, void *arg, int flags);
int
callout_reset_sbt_curcpu(struct callout *c, sbintime_t sbt,
sbintime_t pr, callout_func_t *func, void *arg, int flags);
int
callout_reset_sbt_on(struct callout *c, sbintime_t sbt, sbintime_t pr,
callout_func_t *func, void *arg, int cpu, int flags);
int
callout_schedule(struct callout *c, int ticks);
int
callout_schedule_curcpu(struct callout *c, int ticks);
int
callout_schedule_on(struct callout *c, int ticks, int cpu);
int
callout_schedule_sbt(struct callout *c, sbintime_t sbt, sbintime_t pr,
int flags);
int
callout_schedule_sbt_curcpu(struct callout *c, sbintime_t sbt,
sbintime_t pr, int flags);
int
callout_schedule_sbt_on(struct callout *c, sbintime_t sbt, sbintime_t pr,
int cpu, int flags);
int
callout_stop(struct callout *c);
sbintime_t
callout_when(sbintime_t sbt, sbintime_t precision, int flags,
sbintime_t *sbt_res, sbintime_t *precision_res);
DESCRIPTION
The 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 (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.
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.
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.
Each callout structure must be initialized by callout_init(),
callout_init_mtx(), callout_init_rm(), or callout_init_rw() before it is
passed to any of the other callout functions. The callout_init()
function initializes a callout structure in c that is not associated with
a specific lock. If the mpsafe argument is zero, the callout structure
is not considered to be "multi-processor safe"; and the Giant lock will
be acquired before calling the callout function and released when the
callout function returns.
The callout_init_mtx(), callout_init_rm(), and callout_init_rw()
functions initialize a callout structure in c that is associated with a
specific lock. The lock is specified by the mtx, rm, or 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.
A sleepable read-mostly lock (one initialized with the RM_SLEEPABLE flag)
may not be used with callout_init_rm(). Similarly, other sleepable lock
types such as sx(9) and lockmgr(9) cannot be used with callouts because
sleeping is not permitted in the callout subsystem.
These flags may be specified for callout_init_mtx(), callout_init_rm(),
or callout_init_rw():
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.
CALLOUT_SHAREDLOCK The lock is only acquired in read mode when
running the callout handler. This flag is
ignored by callout_init_mtx().
The function callout_stop() cancels a callout c if it is currently
pending. If the callout is pending and successfully stopped, then
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 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.
The function callout_async_drain() is identical to callout_stop() with
one difference. When callout_async_drain() returns zero it will arrange
for the function drain to be called using the same argument given to the
callout_reset() function. 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
drain function, depending upon which CPU's the callouts are running. The
drain function itself is called from the context of the completing
callout i.e. softclock or hardclock, just like a callout itself.
The function callout_drain() is identical to callout_stop() except that
it will wait for the callout 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 callout_drain() returns. However, the
callout subsystem does guarantee that the callout will be fully stopped
before callout_drain() returns.
The callout_reset() and callout_schedule() function families schedule a
future function invocation for callout c. If 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.
The time at which the callout function will be invoked is determined by
either the ticks argument or the sbt, pr, and flags arguments. When
ticks is used, the callout is scheduled to execute after ticks/hz
seconds. Non-positive values of ticks are silently converted to the
value `1'.
The sbt, pr, and 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 sbt and extends for the
amount of time specified in pr. If sbt specifies a time in the past, the
window is adjusted to start at the current time. A non-zero value for pr
allows the callout subsystem to coalesce callouts scheduled close to each
other into fewer timer interrupts, reducing processing overhead and power
consumption. These flags may be specified to adjust the interpretation
of sbt and pr:
C_ABSOLUTE Handle the sbt argument as an absolute time since
boot. By default, sbt is treated as a relative amount
of time, similar to ticks.
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.
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 pr or this value is used as the length of
the time window. Smaller values (which result in
larger time intervals) allow the callout subsystem to
aggregate more events in one timer interrupt.
C_PRECALC The sbt argument specifies the absolute time at which
the callout should be run, and the pr argument
specifies the requested precision, which will not be
adjusted during the scheduling process. The sbt and
pr values should be calculated by an earlier call to
callout_when() which uses the user-supplied sbt, pr,
and flags values.
C_HARDCLOCK Align the timeouts to hardclock() calls if possible.
The callout_reset() functions accept a 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 void * argument. Upon invocation, func will
receive arg as its only argument. The callout_schedule() functions reuse
the func and arg arguments from the previous callout. Note that one of
the callout_reset() functions must always be called to initialize func
and arg before one of the callout_schedule() functions can be used.
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 callout_reset_on(), callout_reset_sbt_on(), callout_schedule_on()
and callout_schedule_sbt_on() functions assign the callout to CPU cpu.
The callout_reset_curcpu(), callout_reset_sbt_curpu(),
callout_schedule_curcpu() and callout_schedule_sbt_curcpu() functions
assign the callout to the current CPU. The callout_reset(),
callout_reset_sbt(), callout_schedule() and callout_schedule_sbt()
functions schedule the callout to execute in the softclock thread of the
CPU to which it is currently assigned.
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
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 kern.pin_pcpu_swi loader tunable to a non-zero value.
The macros callout_pending(), callout_active() and callout_deactivate()
provide access to the current state of the callout. The
callout_pending() macro checks whether a callout is pending; a callout is
considered 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, callout_pending() will return
FALSE even though the callout function may not have finished (or even
begun) executing. The callout_active() macro checks whether a callout is
marked as active, and the callout_deactivate() macro clears the callout's
active flag. The callout subsystem marks a callout as active when a
timeout is set and it clears the active flag in callout_stop() and
callout_drain(), but it does not clear it when a callout expires normally
via the execution of the callout function.
The 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 sbt, precision
precision, and additional adjustments requested by the flags argument.
Flags accepted by the callout_when() function are the same as flags for
the callout_reset() function. The resulting time is assigned to the
variable pointed to by the sbt_res argument, and the resulting precision
is assigned to *precision_res. When passing the results to
callout_reset, add the C_PRECALC flag to 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 callout_reset() call is
racy.
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.
There are three main techniques for addressing these synchronization
concerns. The first approach is preferred as it is the simplest:
1. Callouts can be associated with a specific lock when they are
initialized by callout_init_mtx(), callout_init_rm(), or
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 callout_stop() or one of the
callout_reset() or callout_schedule() functions to provide
this safety.
A callout initialized via callout_init() with mpsafe set to
zero is implicitly associated with the Giant mutex. If Giant
is held when cancelling or rescheduling the callout, then its
use will prevent races with the callout function.
2. The return value from callout_stop() (or the callout_reset()
and callout_schedule() function families) 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 FALSE indicates that the callout function is
about to be called. For example:
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
*/
}
}
3. The callout_pending(), callout_active() and
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 active and
pending. When the timeout time arrives, the callout subsystem
begins processing the callout by first clearing the pending
flag. It then invokes the callout function without changing
the active flag, and does not clear the active flag even after
the callout function returns. The mechanism described here
requires the callout function itself to clear the active flag
using the callout_deactivate() macro. The callout_stop() and
callout_drain() functions always clear both the active and
pending flags before returning.
The callout function should first check the pending flag and
return without action if callout_pending() returns TRUE. This
indicates that the callout was rescheduled using
callout_reset() just before the callout function was invoked.
If callout_active() returns FALSE then the callout function
should also return without action. This indicates that the
callout has been stopped. Finally, the callout function
should call callout_deactivate() to clear the active flag.
For example:
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 */
Together with appropriate synchronization, such as the mutex
used above, this approach permits the callout_stop() and
callout_reset() functions to be used at any time without
races. For example:
mtx_lock(&sc->sc_mtx);
callout_stop(&sc->sc_callout);
/* The callout is effectively stopped now. */
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 callout_deactivate() call.
The above technique additionally ensures that the active flag
always reflects whether the callout is effectively enabled or
disabled. If 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.
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 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.
RETURN VALUES
The callout_active() macro returns the state of a callout's active flag.
The callout_pending() macro returns the state of a callout's pending
flag.
The callout_reset() and callout_schedule() function families return a
value of one if the callout was pending before the new function
invocation was scheduled.
The callout_stop() and 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.
HISTORY
FreeBSD initially used the long standing BSD linked list callout
mechanism which offered O(n) insertion and removal running time but did
not generate or require handles for untimeout operations.
FreeBSD 3.0 introduced a new set of timeout and untimeout routines from
NetBSD based on the work of Adam M. Costello and George Varghese,
published in a technical report entitled Redesigning the BSD Callout and
Timer Facilities and modified for inclusion in FreeBSD by Justin T.
Gibbs. The original work on the data structures used in that
implementation was published by G. Varghese and A. Lauck in the paper
Hashed and Hierarchical Timing Wheels: Data Structures for the Efficient
Implementation of a Timer Facility in the Proceedings of the 11th ACM
Annual Symposium on Operating Systems Principles.
FreeBSD 3.3 introduced the first implementations of callout_init(),
callout_reset(), and callout_stop() which permitted callers to allocate
dedicated storage for callouts. This ensured that a callout would always
fire unlike timeout() which would silently fail if it was unable to
allocate a callout.
FreeBSD 5.0 permitted callout handlers to be tagged as MPSAFE via
callout_init().
FreeBSD 5.3 introduced callout_drain().
FreeBSD 6.0 introduced callout_init_mtx().
FreeBSD 8.0 introduced per-CPU callout wheels, callout_init_rw(), and
callout_schedule().
FreeBSD 9.0 changed the underlying timer interrupts used to drive
callouts to prefer one-shot event timers instead of a periodic timer
interrupt.
FreeBSD 10.0 switched the callout wheel to support tickless operation.
These changes introduced sbintime_t and the callout_reset_sbt*() family
of functions. FreeBSD 10.0 also added C_DIRECT_EXEC and
callout_init_rm().
FreeBSD 10.2 introduced the callout_schedule_sbt*() family of functions.
FreeBSD 11.0 introduced callout_async_drain(). FreeBSD 11.1 introduced
callout_when(). FreeBSD 13.0 removed timeout_t, timeout(), and
untimeout().
FreeBSD 13.1-RELEASE-p6 September 1, 2021 FreeBSD 13.1-RELEASE-p6
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