linux-kernel - Re: [PATCH v3 1/7] sched/fair: Ignore percpu threads for imbalance pulls

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Date:   Tue, 16 Mar 2021 20:06:09 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org
Cc:     Lingutla Chandrasekhar <clingutla@...eaurora.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Quentin Perret <qperret@...gle.com>,
        Pavan Kondeti <pkondeti@...eaurora.org>,
        Rik van Riel <riel@...riel.com>
Subject: Re: [PATCH v3 1/7] sched/fair: Ignore percpu threads for imbalance
 pulls

On 16/03/2021 18:31, Valentin Schneider wrote:
> On 16/03/21 16:49, Dietmar Eggemann wrote:
>> On 11/03/2021 13:05, Valentin Schneider wrote:
>>> From: Lingutla Chandrasekhar <clingutla@...eaurora.org>
>>>
>>> In load balancing, when balancing group is unable to pull task
>>> due to ->cpus_ptr constraints from busy group, then it sets
>>> LBF_SOME_PINNED to lb env flags, as a consequence, sgc->imbalance
>>> is set for its parent domain level. which makes the group
>>> classified as imbalance to get help from another balancing cpu.
>>>
>>> Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and
>>
>> Does it have to be a big.LITTLE system? I assume this issue also happens
>> on an SMP system.
>>
> 
> Aye, though the consequences are "worse" on asym CPU capacity systems.

I can only think of higher group_type 'group_imbalanced' eclipses
'group_misfit_task' here?

> 
>>> CPUs 2-3 as Bigs with below scenario:
>>> - CPU0 doing newly_idle balancing
>>> - CPU1 running percpu kworker and RT task (small tasks)
>>
>> What's the role of the small RT task here in the story?
>>
> 
> I don't think it matters much here.

Chandra already mentioned that this is part of the story, namely to
start trying to move task on lb MC CPU1->CPU0 (if (busiest->nr_running >
1)).

[...]

>> This sentence mentioned per-cpu threads (and so does the patch name) but
>> the implementation (only) deals with per-cpu kernel threads. IMHO, it
>> would be good to align this.
>>
> 
> Tell you what, I'll go for:
> 1) how can pcpu kthreads cause LBF_SOME_PINNED
> 2) why we may not want this, but still ignore !kthread pcpu tasks
> 3) why this is even more important for big.LITTLE

LGTM.