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From: Valentin Schneider <valentin.schneider@arm.com>
To: Ingo Molnar <mingo@kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Julien Thierry <julien.thierry@arm.com>
Subject: preempt_schedule_irq() loop question
Date: Wed, 16 Jan 2019 12:50:42 +0000
Message-ID: <cc989920-a13b-d53b-db83-1584a7f53edc@arm.com> (raw)

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?


             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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