linux-kernel - Re: [PATCH] /sched/core: Fix Unixbench spawn test regression

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Message-ID: <8ddd7bb2-3a18-48b5-85c6-23263134308a@arm.com>
Date: Mon, 10 Mar 2025 16:29:10 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>
Cc: 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>,
 Valentin Schneider <vschneid@...hat.com>, Hagar Hemdan
 <hagarhem@...zon.com>, linux-kernel@...r.kernel.org, wuchi.zero@...il.com
Subject: Re: [PATCH] /sched/core: Fix Unixbench spawn test regression

On 10/03/2025 14:59, Vincent Guittot wrote:
> On Thu, 6 Mar 2025 at 17:26, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>
>> Hagar reported a 30% drop in UnixBench spawn test with commit
>> eff6c8ce8d4d ("sched/core: Reduce cost of sched_move_task when config
>> autogroup") on a m6g.xlarge AWS EC2 instance with 4 vCPUs and 16 GiB RAM
>> (aarch64) (single level MC sched domain) [1].
>>
>> There is an early bail from sched_move_task() if p->sched_task_group is
>> equal to p's 'cpu cgroup' (sched_get_task_group()). E.g. both are
>> pointing to taskgroup '/user.slice/user-1000.slice/session-1.scope'
>> (Ubuntu '22.04.5 LTS').
> 
> Isn't this same use case that has been used by commit eff6c8ce8d4d to
> show the benefit of adding the test if ((group ==
> tsk->sched_task_group) ?
> Adding Wuchi who added the condition

IMHO, UnixBench spawn reports a performance number according to how many
tasks could be spawned whereas, IIUC, commit eff6c8ce8d4d was reporting
the time spend in sched_move_task().

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