linux-kernel - Re: [PATCH 1/1] sched: Consider CPU contention in frequency & load-balance busiest CPU selection

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Date:   Wed, 3 May 2023 19:13:17 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Qais Yousef <qyousef@...alina.io>,
        Kajetan Puchalski <kajetan.puchalski@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Donnefort <vdonnefort@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] sched: Consider CPU contention in frequency &
 load-balance busiest CPU selection

On 29/04/2023 16:58, Peter Zijlstra wrote:
> On Thu, Apr 06, 2023 at 05:50:30PM +0200, Dietmar Eggemann wrote:
>> Use new cpu_boosted_util_cfs() instead of cpu_util_cfs().
>>
>> The former returns max(util_avg, runnable_avg) capped by max CPU
>> capacity. CPU contention is thereby considered through runnable_avg.
>>
>> The change in load-balance only affects migration type `migrate_util`.
> 
> But why, and how does it affect? That is, isn't this Changelog a wee bit
> sparse?

Absolutely. 

I have compelling test data based on JankbenchX on Pixel6 for 
sugov_get_util() case I will share with v2.

But for the find_busiest_queue() (lb migration_type = migrate_util) case 
it is tricky to create a test env.

`migrate_util` only operates in DIE or NUMA SD (!SD_SHARE_PKG_RESOURCES) 
and the system should not be overloaded (spare capacity on the local 
group).

perf bench sched messaging with a small number of tasks compared to CPU 
number shows some improvement.

E.g. Ampere Altra with 160 CPUs, SDs = {MC, DIE, NUMA} and 1 group = 40 
tasks shows some improvement:

perf stat --null --repeat 10 -- perf bench sched messaging -t -g 1 -l 2000

0.4869 +- 0.0173 seconds time elapsed  ( +-  3.55% ) -> 0.4377 +- 0.0147 seconds time elapsed  ( +-  3.36% )

If I put more tasks onto the machine, the conditions to go into 
`migrate_util` lb vanish so there is no difference.

Also if I test on an 8 CPUs system, SDs = {MC, DIE} and 1 group = 40 
tasks the conditions to do migrate_util lb are only true for a short
moment of the beginning of the test so it does not have much implication
on the score.

[...]