man/man9/sleepqueue.9

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.Dd June 19, 2019
.Dt SLEEPQUEUE 9
.Os
.Sh NAME
.Nm init_sleepqueues ,
.Nm sleepq_abort ,
.Nm sleepq_add ,
.Nm sleepq_alloc ,
.Nm sleepq_broadcast ,
.Nm sleepq_free ,
.Nm sleepq_lock ,
.Nm sleepq_lookup ,
.Nm sleepq_release ,
.Nm sleepq_remove ,
.Nm sleepq_signal ,
.Nm sleepq_set_timeout ,
.Nm sleepq_set_timeout_sbt ,
.Nm sleepq_sleepcnt ,
.Nm sleepq_timedwait ,
.Nm sleepq_timedwait_sig ,
.Nm sleepq_type ,
.Nm sleepq_wait ,
.Nm sleepq_wait_sig
.Nd manage the queues of sleeping threads
.Sh SYNOPSIS
.In sys/param.h
.In sys/sleepqueue.h
.Ft void
.Fn init_sleepqueues "void"
.Ft int
.Fn sleepq_abort "struct thread *td"
.Ft void
.Fn sleepq_add "const void *wchan" "struct lock_object *lock" "const char *wmesg" "int flags" "int queue"
.Ft struct sleepqueue *
.Fn sleepq_alloc "void"
.Ft int
.Fn sleepq_broadcast "const void *wchan" "int flags" "int pri" "int queue"
.Ft void
.Fn sleepq_free "struct sleepqueue *sq"
.Ft struct sleepqueue *
.Fn sleepq_lookup "const void *wchan"
.Ft void
.Fn sleepq_lock "const void *wchan"
.Ft void
.Fn sleepq_release "const void *wchan"
.Ft void
.Fn sleepq_remove "struct thread *td" "const void *wchan"
.Ft int
.Fn sleepq_signal "const void *wchan" "int flags" "int pri" "int queue"
.Ft void
.Fn sleepq_set_timeout "const void *wchan" "int timo"
.Ft void
.Fn sleepq_set_timeout_sbt "const void *wchan" "sbintime_t sbt" \
"sbintime_t pr" "int flags"
.Ft u_int
.Fn sleepq_sleepcnt "const void *wchan" "int queue"
.Ft int
.Fn sleepq_timedwait "const void *wchan" "int pri"
.Ft int
.Fn sleepq_timedwait_sig "const void *wchan" "int pri"
.Ft int
.Fn sleepq_type "const void *wchan"
.Ft void
.Fn sleepq_wait "const void *wchan" "int pri"
.Ft int
.Fn sleepq_wait_sig "const void *wchan" "int pri"
.Sh DESCRIPTION
Sleep queues provide a mechanism for suspending execution of a thread until
some condition is met.
Each queue is associated with a specific wait channel when it is active,
and only one queue may be associated with a wait channel at any given point
in time.
The implementation of each wait channel splits its sleepqueue into 2 sub-queues
in order to enable some optimizations on threads' wakeups.
An active queue holds a list of threads that are blocked on the associated
wait channel.
Threads that are not blocked on a wait channel have an associated inactive
sleep queue.
When a thread blocks on a wait channel it donates its inactive sleep queue
to the wait channel.
When a thread is resumed,
the wait channel that it was blocked on gives it an inactive sleep queue for
later use.
.Pp
The
.Fn sleepq_alloc
function allocates an inactive sleep queue and is used to assign a
sleep queue to a thread during thread creation.
The
.Fn sleepq_free
function frees the resources associated with an inactive sleep queue and is
used to free a queue during thread destruction.
.Pp
Active sleep queues are stored in a hash table hashed on the addresses pointed
to by wait channels.
Each bucket in the hash table contains a sleep queue chain.
A sleep queue chain contains a spin mutex and a list of sleep queues that hash
to that specific chain.
Active sleep queues are protected by their chain's spin mutex.
The
.Fn init_sleepqueues
function initializes the hash table of sleep queue chains.
.Pp
The
.Fn sleepq_lock
function locks the sleep queue chain associated with wait channel
.Fa wchan .
.Pp
The
.Fn sleepq_lookup
returns a pointer to the currently active sleep queue for that wait
channel associated with
.Fa wchan
or
.Dv NULL
if there is no active sleep queue associated with
argument
.Fa wchan .
It requires the sleep queue chain associated with
.Fa wchan
to have been locked by a prior call to
.Fn sleepq_lock .
.Pp
The
.Fn sleepq_release
function unlocks the sleep queue chain associated with
.Fn wchan
and is primarily useful when aborting a pending sleep request before one of
the wait functions is called.
.Pp
The
.Fn sleepq_add
function places the current thread on the sleep queue associated with the
wait channel
.Fa wchan .
The sleep queue chain associated with argument
.Fa wchan
must be locked by a prior call to
.Fn sleepq_lock
when this function is called.
If a lock is specified via the
.Fa lock
argument, and if the kernel was compiled with
.Cd "options INVARIANTS" ,
then the sleep queue code will perform extra checks to ensure that
the lock is used by all threads sleeping on
.Fa wchan .
The
.Fa wmesg
parameter should be a short description of
.Fa wchan .
The
.Fa flags
parameter is a bitmask consisting of the type of sleep queue being slept on
and zero or more optional flags.
The
.Fa queue
parameter specifies the sub-queue, in which the contending thread will be
inserted.
.Pp
There are currently three types of sleep queues:
.Pp
.Bl -tag -width ".Dv SLEEPQ_CONDVAR" -compact
.It Dv SLEEPQ_CONDVAR
A sleep queue used to implement condition variables.
.It Dv SLEEPQ_SLEEP
A sleep queue used to implement
.Xr sleep 9 ,
.Xr wakeup 9
and
.Xr wakeup_one 9 .
.It Dv SLEEPQ_PAUSE
A sleep queue used to implement
.Xr pause 9 .
.El
.Pp
There are currently two optional flag:
.Pp
.Bl -tag -width ".Dv SLEEPQ_INTERRUPTIBLE" -compact
.It Dv SLEEPQ_INTERRUPTIBLE
The current thread is entering an interruptible sleep.
.El
.Bl -tag -width ".Dv SLEEPQ_STOP_ON_BDRY" -compact
.It Dv SLEEPQ_STOP_ON_BDRY
When thread is entering an interruptible sleep, do not stop it upon
arrival of stop action, like
.Dv SIGSTOP .
Wake it up instead.
.El
.Pp
A timeout on the sleep may be specified by calling
.Fn sleepq_set_timeout
after
.Fn sleepq_add .
The
.Fa wchan
parameter should be the same value from the preceding call to
.Fn sleepq_add ,
and the sleep queue chain associated with
.Fa wchan
must have been locked by a prior call to
.Fn sleepq_lock .
The
.Fa timo
parameter should specify the timeout value in ticks.
.Pp
.Fn sleepq_set_timeout_sbt
function takes
.Fa sbt
argument instead of
.Fa timo .
It allows to specify relative or absolute wakeup time with higher resolution
in form of
.Vt sbintime_t .
The parameter
.Fa pr
allows to specify wanted absolute event precision.
The parameter
.Fa flags
allows to pass additional
.Fn callout_reset_sbt
flags.
.Pp
Once the thread is ready to suspend,
one of the wait functions is called to put the current thread to sleep
until it is awakened and to context switch to another thread.
The
.Fn sleepq_wait
function is used for non-interruptible sleeps that do not have a timeout.
The
.Fn sleepq_timedwait
function is used for non-interruptible sleeps that have had a timeout set via
.Fn sleepq_set_timeout .
The
.Fn sleepq_wait_sig
function is used for interruptible sleeps that do not have a timeout.
The
.Fn sleepq_timedwait_sig
function is used for interruptible sleeps that do have a timeout set.
The
.Fa wchan
argument to all of the wait functions is the wait channel being slept
on.
The sleep queue chain associated with argument
.Fa wchan
needs to have been locked with a prior call to
.Fn sleepq_lock .
The
.Fa pri
argument is used to set the priority of the thread when it is awakened.
If it is set to zero, the thread's priority is left alone.
.Pp
When the thread is resumed,
the wait functions return a non-zero value if the thread was awakened due to
an interrupt other than a signal or a timeout.
If the sleep timed out, then
.Er EWOULDBLOCK
is returned.
If the sleep was interrupted by something other than a signal,
then some other return value will be returned.
.Pp
A sleeping thread is normally resumed by the
.Fn sleepq_broadcast
and
.Fn sleepq_signal
functions.
The
.Fn sleepq_signal
function awakens the highest priority thread sleeping on a wait channel
(if SLEEPQ_UNFAIR flag is set, thread that went to sleep recently) while
.Fn sleepq_broadcast
awakens all of the threads sleeping on a wait channel.
The
.Fa wchan
argument specifics which wait channel to awaken.
The
.Fa flags
argument must match the sleep queue type contained in the
.Fa flags
argument passed to
.Fn sleepq_add
by the threads sleeping on the wait channel.
If the
.Fa pri
argument does not equal \-1,
then each thread that is awakened will have its priority raised to
.Fa pri
if it has a lower priority.
The sleep queue chain associated with argument
.Fa wchan
must be locked by a prior call to
.Fn sleepq_lock
before calling any of these functions.
The
.Fa queue
argument specifies the sub-queue, from which threads need to be woken up.
.Pp
A thread in an interruptible sleep can be interrupted by another thread via
the
.Fn sleepq_abort
function.
The
.Fa td
argument specifies the thread to interrupt.
An individual thread can also be awakened from sleeping on a specific wait
channel via the
.Fn sleepq_remove
function.
The
.Fa td
argument specifies the thread to awaken and the
.Fa wchan
argument specifies the wait channel to awaken it from.
If the thread
.Fa td
is not blocked on the wait channel
.Fa wchan
then this function will not do anything,
even if the thread is asleep on a different wait channel.
This function should only be used if one of the other functions above is not
sufficient.
One possible use is waking up a specific thread from a widely shared sleep
channel.
.Pp
The
.Fn sleepq_sleepcnt
function offer a simple way to retrieve the number of threads sleeping for
the specified
.Fa queue ,
given a
.Fa wchan .
.Pp
The
.Fn sleepq_type
function returns the type of
.Fa wchan
associated to a sleepqueue.
.Pp
The
.Fn sleepq_abort ,
.Fn sleepq_broadcast ,
and
.Fn sleepq_signal
functions all return a boolean value.
If the return value is true,
then at least one thread was resumed that is currently swapped out.
The caller is responsible for awakening the scheduler process so that the
resumed thread will be swapped back in.
This is done by calling the
.Fn kick_proc0
function after releasing the sleep queue chain lock via a call to
.Fn sleepq_release .
.Pp
The sleep queue interface is currently used to implement the
.Xr sleep 9
and
.Xr condvar 9
interfaces.
Almost all other code in the kernel should use one of those interfaces rather
than manipulating sleep queues directly.
.Sh SEE ALSO
.Xr condvar 9 ,
.Xr runqueue 9 ,
.Xr scheduler 9 ,
.Xr sleep 9 ,
.Xr timeout 9