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

@@ -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);
-	__schedule();
+	do {
+		__schedule();
+	} while (need_resched());
 }
 EXPORT_SYMBOL(schedule);