linux-kernel - Re: [PATCH v3 0/4] sched/pelt: Relax the sync of *_sum with *

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Message-ID: <61e9f5b0-39cf-c584-73ec-6a5ac8581bb7@arm.com>
Date:   Wed, 12 Jan 2022 16:26:17 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
        bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
        linux-kernel@...r.kernel.org, rickyiu@...gle.com, odin@...d.al
Cc:     sachinp@...ux.vnet.ibm.com, naresh.kamboju@...aro.org
Subject: Re: [PATCH v3 0/4] sched/pelt: Relax the sync of *_sum with *_avg

On 11/01/2022 14:46, Vincent Guittot wrote:
> Rick reported performance regressions in bugzilla because of cpu
> frequency being lower than before:
>     https://bugzilla.kernel.org/show_bug.cgi?id=215045
> 
> He bisected the problem to:
> commit 1c35b07e6d39 ("sched/fair: Ensure _sum and _avg values stay consistent")
> 
> More details are available in commit message of patch 1.
> 
> This patchset reverts the commit above and adds several checks when
> propagating the changes in the hierarchy to make sure that we still have
> coherent util_avg and util_sum.
> 
> Dietmar found a simple way to reproduce the WARN fixed by 
> commit 1c35b07e6d39 ("sched/fair: Ensure _sum and _avg values stay consistent")
> by looping on hackbench in several different sched group levels.
> 
> This patchset as run on the reproducer with success but it probably needs
> more tests by people who faced the WARN before.
> 
> The changes done on util_sum have been also applied to runnable_sum and
> load_sum which faces the same rounding problem although this has not been
> reflected in measurable performance impact.
> 
> Changes for v3:
> - split patch 1 in 2 patches
>   - One to fix rick's regression
>   - One to apply same changes in other places
> - some typos
> - move main comment so it appears in the 1st patch 
> 
> Changes for v2:
> - fix wrong update of load_sum
> - move a change from patch 3 to patch 2
> - update patch 3 commit message
> 
> Vincent Guittot (4):
>   sched/pelt: Relax the sync of util_sum with util_avg
>   sched/pelt: Continue to relax the sync of util_sum with util_avg
>   sched/pelt: Relax the sync of runnable_sum with runnable_avg
>   sched/pelt: Relax the sync of load_sum with load_avg
> 
>  kernel/sched/fair.c | 113 +++++++++++++++++++++++++++++---------------
>  1 file changed, 75 insertions(+), 38 deletions(-)

LGTM. Just a couple of questions on the patch header of 1/4.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>