linux-kernel - Re: [PATCH v5] sched/fair: Correctly insert cfs

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Date:   Mon, 14 Jun 2021 15:10:21 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Odin Ugedal <odin@...d.al>, 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@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5] sched/fair: Correctly insert cfs_rq's to list on
 unthrottle

On Mon, Jun 14, 2021 at 02:01:55PM +0200, Vincent Guittot wrote:
> On Sat, 12 Jun 2021 at 13:31, Odin Ugedal <odin@...d.al> wrote:
> >
> > This fixes an issue where fairness is decreased since cfs_rq's can
> > end up not being decayed properly. For two sibling control groups with
> > the same priority, this can often lead to a load ratio of 99/1 (!!).
> >
> > This happen because when a cfs_rq is throttled, all the descendant cfs_rq's
> 
> s/happen/happens/
> 
> > will be removed from the leaf list. When they initial cfs_rq is
> > unthrottled, it will currently only re add descendant cfs_rq's if they
> > have one or more entities enqueued. This is not a perfect heuristic.
> >
> > Instead, we insert all cfs_rq's that contain one or more enqueued
> > entities, or it its load is not completely decayed.
> >
> > Can often lead to situations like this for equally weighted control
> > groups:
> >
> > $ ps u -C stress
> > USER         PID %CPU %MEM    VSZ   RSS TTY      STAT START   TIME COMMAND
> > root       10009 88.8  0.0   3676   100 pts/1    R+   11:04   0:13 stress --cpu 1
> > root       10023  3.0  0.0   3676   104 pts/1    R+   11:04   0:00 stress --cpu 1
> >
> > Fixes: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
> > Signed-off-by: Odin Ugedal <odin@...d.al>
> 
> minor typo in the commit message otherwise
> 
> Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>

Thanks!