[PATCH] sched: Document schedule() entry points

[PATCH] sched: Document schedule() entry points

[Pekka Enberg]

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 <mingo@kernel.org>
Explained-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Pekka Enberg <penberg@kernel.org>
---
 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
+ *      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)
 {
-- 
1.7.7.6

Re: [PATCH] sched: Document schedule() entry points

[Randy Dunlap]

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 <mingo@kernel.org>
> Explained-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> Signed-off-by: Pekka Enberg <penberg@kernel.org>
> ---
>  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