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Message-ID: <764e015b-b987-4722-b088-56da22848644@arm.com>
Date:   Wed, 29 Nov 2023 13:16:30 +0100
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Subject: Re: [PATCH v2] sched/fair: Use all little CPUs for CPU-bound workload
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To:     Pierre Gondois <pierre.gondois@arm.com>,
        linux-kernel@vger.kernel.org
Cc:     Qais Yousef <qyousef@layalina.io>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>
References: <20231124153323.3202444-1-pierre.gondois@arm.com>
 <b373fde3-95a1-4b08-a5f0-f66e3463c298@arm.com>
 <980a1a52-80a1-4d3b-8931-5f9a6546bbf5@arm.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
In-Reply-To: <980a1a52-80a1-4d3b-8931-5f9a6546bbf5@arm.com>
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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.

[...]