SLEEPQUEUE(9)

💤 SLEEPQUEUE(9) 💤

ok so this is the Kernel Developer's Manual page for SLEEPQUEUE(9) — yes bestie, we're in kernel land now, put your hair up. It's all about managing the queues of sleeping threads 😴 and honestly the way this whole thing works is lowkey iconic once you get it.

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✨ NAME ✨

init_sleepqueues, sleepq_abort, sleepq_add, sleepq_alloc, sleepq_broadcast, sleepq_free, sleepq_lock, sleepq_lookup, sleepq_release, sleepq_remove, sleepq_signal, sleepq_set_timeout, sleepq_set_timeout_sbt, sleepq_sleepcnt, sleepq_timedwait, sleepq_timedwait_sig, sleepq_type, sleepq_wait, sleepq_wait_sig — the whole squad, and their entire job is: manage the queues of sleeping threads 🫶

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🌸 SYNOPSIS 🌸

#include <sys/param.h>
#include <sys/sleepqueue.h>

void
init_sleepqueues(void);

int
sleepq_abort(struct thread *td);

void
sleepq_add(const void *wchan, struct lock_object *lock,
    const char *wmesg, int flags, int queue);

struct sleepqueue *
sleepq_alloc(void);

int
sleepq_broadcast(const void *wchan, int flags, int pri, int queue);

void
sleepq_free(struct sleepqueue *sq);

struct sleepqueue *
sleepq_lookup(const void *wchan);

void
sleepq_lock(const void *wchan);

void
sleepq_release(const void *wchan);

void
sleepq_remove(struct thread *td, const void *wchan);

int
sleepq_signal(const void *wchan, int flags, int pri, int queue);

void
sleepq_set_timeout(const void *wchan, int timo);

void
sleepq_set_timeout_sbt(const void *wchan, sbintime_t sbt, sbintime_t pr,
    int flags);

u_int
sleepq_sleepcnt(const void *wchan, int queue);

int
sleepq_timedwait(const void *wchan, int pri);

int
sleepq_timedwait_sig(const void *wchan, int pri);

int
sleepq_type(const void *wchan);

void
sleepq_wait(const void *wchan, int pri);

int
sleepq_wait_sig(const void *wchan, int pri);

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💅 DESCRIPTION 💅

the whole concept, explained 🧚

Sleep queues are the mechanism that lets the kernel suspend execution of a thread until some condition is met. That's the entire point of them, no cap.

Each queue is associated with a specific wait channel whenever it's active — and here's the rule: only one queue may be associated with a wait channel at any given point in time. periodt.

Here's the cute part: the implementation of each wait channel splits its sleepqueue into 2 sub-queues so it can do 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.

And the swap is honestly so generous: when a thread blocks on a wait channel, it donates its inactive sleep queue to the wait channel 🎁. When the thread is resumed, the wait channel it was blocked on gives it an inactive sleep queue for later use. mutual aid fr fr.

The sleepq_alloc() function allocates an inactive sleep queue and is used to assign a sleep queue to a thread during thread creation. The sleepq_free() function frees the resources associated with an inactive sleep queue and is used to free a queue during thread destruction. birth and death of a queue, it's giving circle of life 🦋

the hash table situation 🗂️

Active sleep queues live 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 init_sleepqueues() function initializes the hash table of sleep queue chains. that's her job.

the locking girlies 🔐

The sleepq_lock() function locks the sleep queue chain associated with wait channel wchan.

The sleepq_lookup() returns a pointer to the currently active sleep queue for that wait channel associated with wchan, or NULL if there is no active sleep queue associated with argument wchan. ⚠️ It requires the sleep queue chain associated with wchan to have been locked by a prior call to sleepq_lock().

The sleepq_release() function unlocks the sleep queue chain associated with wchan and is primarily useful when aborting a pending sleep request before one of the wait functions is called.

sleepq_add() — getting on the list 📝

The sleepq_add() function places the current thread on the sleep queue associated with the wait channel wchan. The sleep queue chain associated with argument wchan must be locked by a prior call to sleepq_lock() when this function is called.

If a lock is specified via the lock argument, and if the kernel was compiled with options INVARIANTS, then the sleep queue code will perform extra checks to make sure that the lock is used by all threads sleeping on wchan. (she's checking receipts 🔍)

• The wmesg parameter should be a short description of wchan.
• The flags parameter is a bitmask consisting of the type of sleep queue being slept on and zero or more optional flags.
• The queue parameter specifies the sub-queue, in which the contending thread will be inserted.

the three types of sleep queues 🌈

• SLEEPQ_CONDVAR — a sleep queue used to implement condition variables.
• SLEEPQ_SLEEP — a sleep queue used to implement sleep(9), wakeup(9) and wakeup_one(9).
• SLEEPQ_PAUSE — a sleep queue used to implement pause(9).

the two optional flags 🎀

• SLEEPQ_INTERRUPTIBLE — the current thread is entering an interruptible sleep.
• SLEEPQ_STOP_ON_BDRY — when thread is entering an interruptible sleep, do not stop it upon arrival of stop action, like SIGSTOP. Wake it up instead. (she said no thanks to being stopped 💀)

timeouts ⏰

A timeout on the sleep may be specified by calling sleepq_set_timeout() after sleepq_add().

• The wchan parameter should be the same value from the preceding call to sleepq_add().
• The sleep queue chain associated with wchan must have been locked by a prior call to sleepq_lock().
• The timo parameter should specify the timeout value in ticks.

sleepq_set_timeout_sbt() function takes sbt argument instead of timo. It allows to specify relative or absolute wakeup time with higher resolution in form of sbintime_t. The parameter pr allows to specify wanted absolute event precision. The parameter flags allows to pass additional callout_reset_sbt() flags. fancy precision, we love her 💅

the wait functions 😴

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. Pick your fighter:

• sleepq_wait() — for non-interruptible sleeps that do not have a timeout.
• sleepq_timedwait() — for non-interruptible sleeps that have had a timeout set via sleepq_set_timeout().
• sleepq_wait_sig() — for interruptible sleeps that do not have a timeout.
• sleepq_timedwait_sig() — for interruptible sleeps that do have a timeout set.

For all of them:

• The wchan argument is the wait channel being slept on.
• The sleep queue chain associated with argument wchan needs to have been locked with a prior call to sleepq_lock().
• The 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.

what they return 🎯

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 EWOULDBLOCK is returned.
• If the sleep was interrupted by something other than a signal, then some other return value will be returned.

waking threads up 📣

A sleeping thread is normally resumed by the sleepq_broadcast() and sleepq_signal() functions.

• sleepq_signal() awakens the highest priority thread sleeping on a wait channel — (if the SLEEPQ_UNFAIR flag is set, it wakes the thread that went to sleep recently instead, oop 🙈).
• sleepq_broadcast() awakens all of the threads sleeping on a wait channel.

Details, because details matter:

• The wchan argument specifies which wait channel to awaken.
• The flags argument must match the sleep queue type contained in the flags argument passed to sleepq_add() by the threads sleeping on the wait channel.
• If the pri argument does not equal -1, then each thread that is awakened will have its priority raised to pri if it has a lower priority.
• The sleep queue chain associated with argument wchan must be locked by a prior call to sleepq_lock() before calling any of these functions.
• The queue argument specifies the sub-queue, from which threads need to be woken up.

interrupting & removing individual threads ✂️

A thread in an interruptible sleep can be interrupted by another thread via the sleepq_abort() function. The td argument specifies the thread to interrupt.

An individual thread can also be awakened from sleeping on a specific wait channel via the sleepq_remove() function. The td argument specifies the thread to awaken and the wchan argument specifies the wait channel to awaken it from.

⚠️ If the thread td is not blocked on the wait channel wchan, then this function will not do anything, even if the thread is asleep on a different wait channel. read that twice bestie.

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.

little helpers 🧸

The sleepq_sleepcnt() function offers a simple way to retrieve the number of threads sleeping for the specified queue, given a wchan.

The sleepq_type() function returns the type of wchan associated to a sleepqueue.

return values & the kick_proc0() thing 🔥

The sleepq_abort(), sleepq_broadcast(), and 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 kick_proc0() function after releasing the sleep queue chain lock via a call to sleepq_release().

do not forget the kick_proc0(), she will simply not come back otherwise 😭

one last warning 🫶

The sleep queue interface is currently used to implement the sleep(9) and condvar(9) interfaces. Almost all other code in the kernel should use one of those interfaces rather than manipulating sleep queues directly. yeah. just use the wrapper, bestie.

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👑 SEE ALSO 👑

callout(9), condvar(9), runqueue(9), scheduler(9), sleep(9)

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and that's the page!! ✨ this one's from Debian, dated June 19, 2019, filed under SLEEPQUEUE(9) — go take a nap, you've earned it 💤🎀💖