On 07/30/2012 11:15 PM, Pekka Enberg wrote:
> This patch adds a comment on top of the schedule() function to explain
> to scheduler newbies how the main scheduler function is entered.
>
> Explained-by: Ingo Molnar <[email protected]>
> Explained-by: Peter Zijlstra <[email protected]>
> Signed-off-by: Pekka Enberg <[email protected]>
> ---
> kernel/sched/core.c | 34 ++++++++++++++++++++++++++++++++++
> 1 files changed, 34 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 468bdd4..9f31bbd 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3361,6 +3361,40 @@ pick_next_task(struct rq *rq)
>
> /*
> * __schedule() is the main scheduler function.
> + *
> + * The main means of driving the scheduler and thus entering this function
> are:
> + *
> + * 1. Explicit blocking: mutex, semaphore, waitqueue, etc.
> + *
> + * 2. TIF_NEED_RESCHED flag is checked on interrupt and userspace return
> + * paths. For example, see arch/x86/entry_64.S.
> + *
> + * To drive preemption between tasks, the scheduler sets the flag is set
eh?
> + * in timer interrupt handler scheduler_tick().
> + *
> + * 3. Wakeups don't really cause entry into schedule(). They add a
> + * task to the run-queue and that's it.
> + *
> + * Now, if the new task added to the run-queue preempts the current
> + * task, then the wakeup sets TIF_NEED_RESCHED and schedule() gets
> + * called on the nearest possible occasion:
> + *
> + * - If the kernel is preemptible (CONFIG_PREEMPT=y):
> + *
> + * - in syscall or exception context, at the next outmost
> + * preempt_enable(). (this might be as soon as the wake_up()'s
> + * spin_unlock()!)
> + *
> + * - in IRQ context, return from interrupt-handler to
> + * preemptible context
> + *
> + * - If the kernel is not preemptible (CONFIG_PREEMPT is not set)
> + * then at the next:
> + *
> + * - cond_resched() call
> + * - explicit schedule() call
> + * - return from syscall or exception to user-space
> + * - return from interrupt-handler to user-space
> */
> static void __sched __schedule(void)
> {
--
~Randy
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