linux-kernel - preempt_schedule

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Date:   Wed, 16 Jan 2019 12:50:42 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Julien Thierry <julien.thierry@....com>
Subject: preempt_schedule_irq() loop question

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?