linux-kernel - Re: preempt_schedule

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Date:   Wed, 16 Jan 2019 15:57:28 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
cc:     Valentin Schneider <valentin.schneider@....com>,
        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>
Subject: Re: preempt_schedule_irq() loop question

On Wed, 16 Jan 2019, Peter Zijlstra wrote:
> 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?

I just went back in the history tree and had to figure out that it's my
fault :)

preempt_schedule_irq() was introduced to plug a stupid race, but I did not
notice (and obviously nobody else) that this made the extra loop in the
entry code pointless. So it's just histerical raisins without any value.

Thanks,

	tglx