sched: Pull resched loop to __schedule() callers
authorFrederic Weisbecker <fweisbec@gmail.com>
Wed, 28 Jan 2015 00:24:09 +0000 (01:24 +0100)
committerIngo Molnar <mingo@kernel.org>
Wed, 4 Feb 2015 06:52:30 +0000 (07:52 +0100)
commitbfd9b2b5f80e7289fdd50210afe4d9ca5952a865
treedd65ba421f2c4e7081b6797bb6c87c97ad2c6c9c
parent9659e1eeee28f7025b6545934d644d19e9c6e603
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>
kernel/sched/core.c