linux-kernel - Re: sched: Reenable interrupts in do sched

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Date:   Tue, 20 Oct 2020 16:07:32 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Subject: Re: sched: Reenable interrupts in do sched_yield()

On Tue, 20 Oct 2020 20:02:55 +0200
Thomas Gleixner <tglx@...utronix.de> wrote:

> On Tue, Oct 20 2020 at 11:38, Steven Rostedt wrote:
> > On Tue, 20 Oct 2020 16:46:55 +0200
> > Thomas Gleixner <tglx@...utronix.de> wrote:
> >  
> >> -	/*
> >> -	 * Since we are going to call schedule() anyway, there's
> >> -	 * no need to preempt or enable interrupts:  
> >
> > I think the above comment still makes sense, just needs to be tweeked:
> >
> > 	/*
> > 	 * Since we are going to call schedule() anyway, there's
> > 	 * no need to allow preemption after releasing the rq lock.  
> >> -	 */  
> >
> > Especially, since we are now enabling interrupts, which is likely to
> > trigger a preemption.  
> 
> sched_preempt_enable_no_resched() still enables preemption. It just
> avoids the check. And it still allows preemption when an interrupt
> triggering preemption happens between sched_preempt_enable_no_resched()
> and __schedule() disabling preemption/interrupts.
> 
> So no, your new variant is just differently bogus and misleading.

What I wrote wasn't exactly what I meant. What I meant to have:

	/*
	 * Since we are going to call schedule() anyways, there's
	 * no need to do the preemption check when the rq_lock is released.
	 */

That is, to document why we have the preempt_disable() before the unlock:

	preempt_disable();
	rq_unlock_irq(rq, &rf);
	sched_preempt_enable_no_resched();


-- Steve