linux-kernel - [PATCH v2] sched: Document schedule() entry points

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Message-Id: <1344070187-2420-1-git-send-email-penberg@kernel.org>
Date:	Sat,  4 Aug 2012 11:49:47 +0300
From:	Pekka Enberg <penberg@...nel.org>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...nel.org, a.p.zijlstra@...llo.nl,
	Pekka Enberg <penberg@...nel.org>,
	Randy Dunlap <rdunlap@...otime.net>
Subject: [PATCH v2] 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.

Cc: Randy Dunlap <rdunlap@...otime.net>
Explained-by: Ingo Molnar <mingo@...nel.org>
Explained-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
Signed-off-by: Pekka Enberg <penberg@...nel.org>
---
V1 -> V2: Fix funky grammar pointed out by Peter and Randy.

 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..7dc75df 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 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

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