sched: Document schedule() entry points
This patch adds a comment on top of the schedule() function to explain to scheduler newbies how the main scheduler function is entered. Acked-by: Randy Dunlap <rdunlap@xenotime.net> Explained-by: Ingo Molnar <mingo@kernel.org> Explained-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Pekka Enberg <penberg@kernel.org> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> Link: http://lkml.kernel.org/r/1344070187-2420-1-git-send-email-penberg@kernel.org Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
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@ -3367,6 +3367,40 @@ pick_next_task(struct rq *rq)
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/*
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* __schedule() is the main scheduler function.
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*
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* The main means of driving the scheduler and thus entering this function are:
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*
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* 1. Explicit blocking: mutex, semaphore, waitqueue, etc.
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*
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* 2. TIF_NEED_RESCHED flag is checked on interrupt and userspace return
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* paths. For example, see arch/x86/entry_64.S.
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*
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* To drive preemption between tasks, the scheduler sets the flag in timer
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* interrupt handler scheduler_tick().
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*
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* 3. Wakeups don't really cause entry into schedule(). They add a
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* task to the run-queue and that's it.
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*
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* Now, if the new task added to the run-queue preempts the current
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* task, then the wakeup sets TIF_NEED_RESCHED and schedule() gets
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* called on the nearest possible occasion:
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*
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* - If the kernel is preemptible (CONFIG_PREEMPT=y):
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*
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* - in syscall or exception context, at the next outmost
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* preempt_enable(). (this might be as soon as the wake_up()'s
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* spin_unlock()!)
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*
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* - in IRQ context, return from interrupt-handler to
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* preemptible context
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*
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* - If the kernel is not preemptible (CONFIG_PREEMPT is not set)
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* then at the next:
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*
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* - cond_resched() call
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* - explicit schedule() call
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* - return from syscall or exception to user-space
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* - return from interrupt-handler to user-space
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*/
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static void __sched __schedule(void)
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{
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