linux-kernel - Re: [PATCH 1/2] sched/fair: Filter out locally-unsolvable misfit imbalances

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Date:   Thu, 22 Apr 2021 20:19:30 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>,
        linux-kernel@...r.kernel.org
Cc:     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>,
        Lingutla Chandrasekhar <clingutla@...eaurora.org>
Subject: Re: [PATCH 1/2] sched/fair: Filter out locally-unsolvable misfit imbalances

On 22/04/21 11:48, Dietmar Eggemann wrote:
> On 15/04/2021 19:58, Valentin Schneider wrote:
>
> [...]
>
>> Aggregate a misfit task's load into sgs->group_misfit_task_load only if
>> env->dst_cpu would grant it a capacity uplift.
>>
>> Note that the aforementioned capacity vs sgc->max_capacity comparison was
>> meant to prevent misfit task downmigration: candidate groups classified as
>> group_misfit_task but with a higher (max) CPU capacity than the destination CPU
>> would be discarded. This change makes it so said group_misfit_task
>> classification can't happen anymore, which may cause some undesired
>> downmigrations.
>>
>> Further tweak find_busiest_queue() to ensure this doesn't happen.
>
> Maybe you can describe shortly here what's the new mechanism is you
> replace the old 'prevent misfit task downmigration' with.
>

Will do.

> Also note
>> find_busiest_queue() can now iterate over CPUs with a higher capacity than
>> the local CPU's, so add a capacity check there.
>
> [...]
>
>> +static inline void update_sg_lb_misfit_stats(struct lb_env *env,
>> +					     struct sched_group *group,
>
> Seems to be not used.
>

Right, that's an update_sg_lb_stats() copy/paste fail :-)

>
>> @@ -9288,6 +9310,8 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>>      if (!sds.busiest)
>>              goto out_balanced;
>>
>> +	env->src_grp_type = busiest->group_type;
>
> Maybe a short comment why you set it here in fbg(). It's only used later
> in fbq() for asym. CPU capacity sd but is set unconditionally.
>

Seeing as almost every other line there has an accompanying comment, I
think I'll do that.

> [...]