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    SubjectRe: [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@uged.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@uged.al>
    > > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    > > Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
    > > Link: https://lore.kernel.org/r/20210612112815.61678-1-odin@uged.al
    > > Signed-off-by: Sasha Levin <sashal@kernel.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

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