Commit bfd9b2b5 authored Jan 28, 2015 by Frederic Weisbecker Committed by Ingo Molnar Feb 04, 2015

sched: Pull resched loop to __schedule() callers



__schedule() disables preemption during its job and re-enables it
afterward without doing a preemption check to avoid recursion.

But if an event happens after the context switch which requires
rescheduling, we need to check again if a task of a higher priority
needs the CPU. A preempt irq can raise such a situation. To handle that,
__schedule() loops on need_resched().

But preempt_schedule_*() functions, which call __schedule(), also loop
on need_resched() to handle missed preempt irqs. Hence we end up with
the same loop happening twice.

Lets simplify that by attributing the need_resched() loop responsibility
to all __schedule() callers.

There is a risk that the outer loop now handles reschedules that used
to be handled by the inner loop with the added overhead of caller details
(inc/dec of PREEMPT_ACTIVE, irq save/restore) but assuming those inner
rescheduling loop weren't too frequent, this shouldn't matter. Especially
since the whole preemption path is now losing one loop in any case.

Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/1422404652-29067-2-git-send-email-fweisbec@gmail.com


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 9659e1ee

kernel/sched/core.c

+7 −4

Original line number	Diff line number	Diff line
		@@ -2765,6 +2765,10 @@ again:
		* - explicit schedule() call
		* - return from syscall or exception to user-space
		* - return from interrupt-handler to user-space
		*
		* WARNING: all callers must re-check need_resched() afterward and reschedule
		* accordingly in case an event triggered the need for rescheduling (such as
		* an interrupt waking up a task) while preemption was disabled in __schedule().
		*/
		static void __sched __schedule(void)
		{
		@@ -2773,7 +2777,6 @@ static void __sched __schedule(void)
		struct rq *rq;
		int cpu;

		need_resched:
		preempt_disable();
		cpu = smp_processor_id();
		rq = cpu_rq(cpu);
		@@ -2840,8 +2843,6 @@ need_resched:
		post_schedule(rq);

		sched_preempt_enable_no_resched();
		if (need_resched())
		goto need_resched;
		}

		static inline void sched_submit_work(struct task_struct *tsk)
		@@ -2861,7 +2862,9 @@ asmlinkage __visible void __sched schedule(void)
		struct task_struct *tsk = current;

		sched_submit_work(tsk);
		do {
		__schedule();
		} while (need_resched());
		}
		EXPORT_SYMBOL(schedule);

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