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From: Valentin Schneider <>
To: Ingo Molnar <>,
	linux-kernel <>
Cc: Peter Zijlstra <>,
	Catalin Marinas <>,
	Will Deacon <>,
	Mark Rutland <>,
	Julien Thierry <>
Subject: preempt_schedule_irq() loop question
Date: Wed, 16 Jan 2019 12:50:42 +0000
Message-ID: <> (raw)


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 {
    } 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?

             reply index

Thread overview: 4+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-16 12:50 Valentin Schneider [this message]
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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