linux-kernel - Re: [PATCH v3] sched: async unthrottling for cfs bandwidth

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Message-ID: <Y3tn5wz6TjsqfGTA@hirez.programming.kicks-ass.net>
Date:   Mon, 21 Nov 2022 12:58:31 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     Josh Don <joshdon@...gle.com>, 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>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Michal Koutný <mkoutny@...e.com>,
        Christian Brauner <brauner@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>
Subject: Re: [PATCH v3] sched: async unthrottling for cfs bandwidth

On Sun, Nov 20, 2022 at 10:22:40AM +0800, Chengming Zhou wrote:
> > +               if (cfs_rq->runtime_remaining > 0) {
> > +                       if (cpu_of(rq) != this_cpu ||
> > +                           SCHED_WARN_ON(local_unthrottle)) {
> > +                               unthrottle_cfs_rq_async(cfs_rq);
> > +                       } else {
> > +                               local_unthrottle = cfs_rq;
> > +                       }
> > +               } else {
> > +                       throttled = true;
> > +               }
> 
> Hello,
> 
> I don't get the point why local unthrottle is put after all the remote cpus,
> since this list is FIFO? (earliest throttled cfs_rq is at the head)

Let the local completion time for a CPU be W. Then if we queue a remote
work after the local synchronous work, the lower bound for total
completion is at least 2W.

OTOH, if we first queue all remote work and then process the local
synchronous work, the lower bound for total completion is W.

The practical difference is that all relevant CPUs get unthrottled
rougly at the same point in time, unlike with the original case, where
some CPUs have the opportunity to consume W runtime while another is
still throttled.