linux-kernel - Re: [PATCH 5.10 095/146] sched/fair: Correctly insert cfs

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Date:   Tue, 22 Jun 2021 12:14:31 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Odin Ugedal <odin@...d.al>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Sasha Levin <sashal@...nel.org>
Subject: Re: [PATCH 5.10 095/146] sched/fair: Correctly insert cfs_rqs to
 list on unthrottle

On Mon, Jun 21, 2021 at 06:57:58PM +0200, Peter Zijlstra wrote:
> On Mon, Jun 21, 2021 at 06:15:25PM +0200, Greg Kroah-Hartman wrote:
> > From: Odin Ugedal <odin@...d.al>
> > 
> > [ Upstream commit a7b359fc6a37faaf472125867c8dc5a068c90982 ]
> > 
> > Fix 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 happens because when a cfs_rq is throttled, all the descendant
> > cfs_rq's 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()")
> > [vingo: !SMP build fix]
> > Signed-off-by: Odin Ugedal <odin@...d.al>
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
> > Link: https://lore.kernel.org/r/20210612112815.61678-1-odin@uged.al
> > Signed-off-by: Sasha Levin <sashal@...nel.org>
> 
> This one is currently known to cause some LTP fail, fixes are being
> discussed, please hold off for now.

Now dropped from all queues, thanks.

greg k-h