linux-kernel - Re: [RFC PATCH] sched/fair: Fix impossible migrate

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Message-ID: <20230721105711.nzunqdtdaevmrgyg@airbuntu>
Date:   Fri, 21 Jul 2023 11:57:11 +0100
From:   Qais Yousef <qyousef@...alina.io>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] sched/fair: Fix impossible migrate_util scenario in
 load balance

On 07/20/23 14:31, Vincent Guittot wrote:

> I was trying to reproduce the behavior but I was failing until I
> realized that this code path is used when the 2 groups are not sharing
> their cache. Which topology do you use ? I thought that dynamiQ and
> shares cache between all 8 cpus was the norm for arm64 embedded device
> now

Hmm good question. phantom domains didn't die which I think is what causing
this. I can look if this is for a good reason or just historical artifact.

> 
> Also when you say "the little cluster capacity is very small nowadays
> (around 200 or less)", it is the capacity of 1 core or the cluster ?

I meant one core. So in my case all the littles were busy except for one that
was mostly idle and never pulled a task from mid where two tasks were stuck on
a CPU there. And the logs I have added were showing me that the env->imbalance
was on 150+ range but the task we pull was in the 350+ range.

I should have mentioned that I'm on 5.15 - sorry with Android it's hard to run
mainline on products :( But this code as far as I can tell hasn't changed much.

I can try to find something that runs mainline and reproduce there if you think
my description of the problem is not clear or applicable.

Thanks

--
Qais Yousef