linux-kernel - Re: preempt_schedule

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Message-ID: <20190116134521.GG10803@hirez.programming.kicks-ass.net>
Date:   Wed, 16 Jan 2019 14:45:21 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Julien Thierry <julien.thierry@....com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: preempt_schedule_irq() loop question

On Wed, Jan 16, 2019 at 12:50:42PM +0000, Valentin Schneider wrote:
> 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?

I think you're quite right; but I wasn't doing kernel work back when rml
added the preemptible bits. Ingo, do you have any recollections that far
back?