linux-kernel - Re: [PATCH v3] sched: Store restrict_cpus_allowed

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Message-ID: <b708c5bc-8b06-3c7a-17e9-38e0220cdb32@redhat.com>
Date:   Mon, 30 Jan 2023 14:05:33 -0500
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....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>,
        Will Deacon <will@...nel.org>, Phil Auld <pauld@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] sched: Store restrict_cpus_allowed_ptr() call state


On 1/30/23 06:56, Peter Zijlstra wrote:
> On Thu, Jan 26, 2023 at 08:55:27PM -0500, Waiman Long wrote:
>> The user_cpus_ptr field was originally added by commit b90ca8badbd1
>> ("sched: Introduce task_struct::user_cpus_ptr to track requested
>> affinity"). It was used only by arm64 arch due to possible asymmetric
>> CPU setup.
>>
>> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
>> cpumask"), task_struct::user_cpus_ptr is repurposed to store user
>> requested cpu affinity specified in the sched_setaffinity().
>>
>> This results in a slight performance regression on an arm64
>> system when booted with "allow_mismatched_32bit_el0"
> Dude, how can you still call this a slight performance regression after
> Will told you time and time again that's not the problem.
>
> It clearly is a behavioural problem.

I am trying to figure out if this behavioral problem is a result of my 
scheduler patch or just as a result of cgroup v1 current behavior as I 
don't see how my patch will cause this behavioral change if it is not an 
existing problem.

Cheers,
Longman