linux-kernel - Re: [PATCH v2 1/1] psi: stop relying on timer_pending for poll

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Message-ID: <YN3pUNgpBjn42f8s@hirez.programming.kicks-ass.net>
Date:   Thu, 1 Jul 2021 18:12:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Suren Baghdasaryan <surenb@...gle.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        matthias.bgg@...il.com, Minchan Kim <minchan@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        YT Chang <yt.chang@...iatek.com>,
        Wenju Xu (许文举) <wenju.xu@...iatek.com>,
        Jonathan JMChen (陳家明) 
        <jonathan.jmchen@...iatek.com>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        kernel-team <kernel-team@...roid.com>,
        SH Chen <show-hong.chen@...iatek.com>
Subject: Re: [PATCH v2 1/1] psi: stop relying on timer_pending for poll_work
 rescheduling

On Thu, Jul 01, 2021 at 09:09:25AM -0700, Suren Baghdasaryan wrote:
> On Thu, Jul 1, 2021 at 1:59 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Wed, Jun 30, 2021 at 01:51:51PM -0700, Suren Baghdasaryan wrote:
> > > +     /* cmpxchg should be called even when !force to set poll_scheduled */
> > > +     if (atomic_cmpxchg(&group->poll_scheduled, 0, 1) && !force)
> > >               return;
> >
> > Why is that a cmpxchg() ?
> 
> We want to set poll_scheduled and proceed with rescheduling the timer
> unless it's already scheduled, so cmpxchg helps us to make that
> decision atomically. Or did I misunderstand your question?

What's wrong with: atomic_xchg(&group->poll_scheduled, 1) ?