linux-kernel - Re: [PATCH v2] sched/fair: reduce long-tail newly idle balance cost

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Message-ID: <YFoACa0+cGBDxWSF@hirez.programming.kicks-ass.net>
Date:   Tue, 23 Mar 2021 15:49:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Aubrey Li <aubrey.li@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>
Subject: Re: [PATCH v2] sched/fair: reduce long-tail newly idle balance cost

On Tue, Mar 23, 2021 at 02:44:57PM +0100, Vincent Guittot wrote:
> Hi Aurey,
> 
> On Tue, 16 Mar 2021 at 05:27, Li, Aubrey <aubrey.li@...ux.intel.com> wrote:
> >
> > On 2021/2/24 16:15, Aubrey Li wrote:
> > > A long-tail load balance cost is observed on the newly idle path,
> > > this is caused by a race window between the first nr_running check
> > > of the busiest runqueue and its nr_running recheck in detach_tasks.
> > >
> > > Before the busiest runqueue is locked, the tasks on the busiest
> > > runqueue could be pulled by other CPUs and nr_running of the busiest
> > > runqueu becomes 1 or even 0 if the running task becomes idle, this
> > > causes detach_tasks breaks with LBF_ALL_PINNED flag set, and triggers
> > > load_balance redo at the same sched_domain level.
> > >
> > > In order to find the new busiest sched_group and CPU, load balance will
> > > recompute and update the various load statistics, which eventually leads
> > > to the long-tail load balance cost.
> > >
> > > This patch clears LBF_ALL_PINNED flag for this race condition, and hence
> > > reduces the long-tail cost of newly idle balance.
> >
> > Ping...
> 
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

Thanks!