linux-kernel - Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload

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Date:   Wed, 29 Nov 2023 13:16:30 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Pierre Gondois <pierre.gondois@....com>,
        linux-kernel@...r.kernel.org
Cc:     Qais Yousef <qyousef@...alina.io>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>
Subject: Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload

On 29/11/2023 11:48, Pierre Gondois wrote:
> 
> 
> On 11/29/23 09:55, Dietmar Eggemann wrote:
>> On 24/11/2023 16:33, Pierre Gondois wrote:

[...]

>>> Testing workload ([1]):
>>> Spawn 6 CPU-bound tasks. During the first 100ms (step 1), each tasks
>>> is affine to a CPU, except for:
>>> - one little CPU which is left idle.
>>> - one big CPU which has 2 tasks affine.
>>> After the 100ms (step 2), remove the cpumask affinity.
>>>
>>> Before patch:
>>> During step 2, the load balancer running from the idle CPU tags sched
>>> domains as:
>>> - little CPUs: 'group_has_spare'. Indeed, 3 CPU-bound tasks run on a
>>>    4 CPUs sched-domain, and the idle CPU provides enough spare
>>>    capacity.
>>
>> What is meant by 'idle CPU provides enough spare capacity? I thought the
>> task (util_avg ~ 512_ does not fit on the sched group [1,3-5] when we
>> consider util_avg/capacity (383)
> 
> Right, I meant that when evaluating the 'group_type', there is enough spare
> capacity when summing the utilization of CPUs in the the MC sched domain:
> 
> ---
> group_has_capacity()
> {
>     [...]
>     if ((sgs->group_capacity * 100) >
>             (sgs->group_util * imbalance_pct))
>         return true;
>     [...]
> }

I see. But doesn't group_has_capacity() already return true because of
`if (sgs->sum_nr_running < sgs->group_weight)` in this case (3 < 4)?

Maybe you can make this clearer when sending the v3 with the Reviewed-By
tags?

> ---
> 
>>
>> The calculated imbalance of ~350 is too small for the task-size and
>> that's why we need the 'shr_bound(util, env->sd->nr_balance_failed)' to
>> let the task load-balance if nr_balance_failed = 2?
> 
> Yes exact, the tasks are too big and cannot fit this imbalance value
> (representing the available spare capacity in the little CPUs in this
> case).
> 
> 'shr_bound(...)' allows to progressively reduce the size of the tasks and
> allow migrations after having tried to balance 'nr_balance_failed' times.

[...]