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

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Message-ID: <ZLqNmpQdiTC2fio5@vingu-book>
Date:   Fri, 21 Jul 2023 15:52:26 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Qais Yousef <qyousef@...alina.io>
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

Le vendredi 21 juil. 2023 à 11:57:11 (+0100), Qais Yousef a écrit :
> 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'm not able to reproduce your problem with v6.5-rc2 and without phantom domain,
which is expected because we share cache and weight is 1 so we use the path

		if (busiest->group_weight == 1 || sds->prefer_sibling) {
			/*
			 * When prefer sibling, evenly spread running tasks on
			 * groups.
			 */
			env->migration_type = migrate_task;
			env->imbalance = sibling_imbalance(env, sds, busiest, local);
		} else {

> 
> 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