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* preempt_schedule_irq() loop question
@ 2019-01-16 12:50 Valentin Schneider
  2019-01-16 13:45 ` Peter Zijlstra
  0 siblings, 1 reply; 4+ messages in thread
From: Valentin Schneider @ 2019-01-16 12:50 UTC (permalink / raw)
  To: Ingo Molnar, linux-kernel
  Cc: Peter Zijlstra, Catalin Marinas, Will Deacon, Mark Rutland,
	Julien Thierry

Hi,

I've been wandering around preempt_schedule_irq() in sched/core.c, and
got curious regarding how the arch code calls it.

The main part of preempt_schedule_irq() is:

    do {
	    preempt_disable();
	    local_irq_enable();
	    __schedule(true);
	    local_irq_disable();
	    sched_preempt_enable_no_resched();
    } while (need_resched());

Yet all the arch entry.S I looked at (I stopped after arm64, arm, x86_32,
MIPS, powerpc) wrap the call to preempt_schedule_irq() in another

    do { ... } while (need_resched())

For instance, this is what's done in arm64:

    1:	bl	preempt_schedule_irq		// irq en/disable is done inside
	ldr	x0, [tsk, #TSK_TI_FLAGS]	// get new tasks TI_FLAGS
	tbnz	x0, #TIF_NEED_RESCHED, 1b	// needs rescheduling?


I naively thought this could be attributed to something like
preempt_schedule_irq() historically not having an inner loop, but it seems
to have been there since the beginning of time (or at least up to the point
where the git history stops).

I don't see why we need to have these nested loops - AFAICT the one in
preempt_schedule_irq() would suffice. What am I missing?


^ permalink raw reply	[flat|nested] 4+ messages in thread

end of thread, other threads:[~2019-01-16 15:23 UTC | newest]

Thread overview: 4+ messages (download: mbox.gz / follow: Atom feed)
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2019-01-16 12:50 preempt_schedule_irq() loop question Valentin Schneider
2019-01-16 13:45 ` Peter Zijlstra
2019-01-16 14:57   ` Thomas Gleixner
2019-01-16 15:23     ` Valentin Schneider

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