linux-kernel - Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock

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Date:   Mon, 3 May 2021 21:17:17 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Don <joshdon@...gle.com>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        "Hyser,Chris" <chris.hyser@...cle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...e.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Don Hiatt <dhiatt@...italocean.com>
Subject: Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock

On Thu, Apr 29, 2021 at 01:11:54PM -0700, Josh Don wrote:
> On Wed, Apr 28, 2021 at 2:13 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Tue, Apr 27, 2021 at 04:30:02PM -0700, Josh Don wrote:
> >
> > > Also, did you mean to have a preempt_enable_no_resched() rather than
> > > prempt_enable() in raw_spin_rq_trylock?
> >
> > No, trylock really needs to be preempt_enable(), because it can have
> > failed, at which point it will not have incremented the preemption count
> > and our decrement can hit 0, at which point we really should reschedule.
> 
> Ah yes, makes sense. Did you want to use preempt_enable_no_resched()
> at all then? No chance of preempt_count() being 1 at the point of
> enable, as you comment below.

preempt_enable_no_resched() avoids the branch which we know we'll never
take. It generates slightly saner code. It's not much, but every little
bit helps, right? ;-)

> > ( I'm suffering a cold and am really quite slow atm )
> 
> No worries, hope it's a mild one.

It's not the super popular one from '19, and the family seems to be
mostly recovered again, so all's well.

> > How's this then?
> 
> Looks good to me (other than the synchronize_sched()->synchronize_rcu()).
> 
> For these locking patches,
> Reviewed-by: Josh Don <joshdon@...gle.com>

Thanks!, I'll go fold them into the proper place and update the repo.