Message-ID: <b708c5bc-8b06-3c7a-17e9-38e0220cdb32@redhat.com>
Date:   Mon, 30 Jan 2023 14:05:33 -0500
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Subject: Re: [PATCH v3] sched: Store restrict_cpus_allowed_ptr() call state
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To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.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>,
        Will Deacon <will@kernel.org>, Phil Auld <pauld@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
References: <20230127015527.466367-1-longman@redhat.com>
 <Y9eweBuHFeO1jZCH@hirez.programming.kicks-ass.net>
From:   Waiman Long <longman@redhat.com>
In-Reply-To: <Y9eweBuHFeO1jZCH@hirez.programming.kicks-ass.net>
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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