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

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Message-ID: <Y9eweBuHFeO1jZCH@hirez.programming.kicks-ass.net>
Date:   Mon, 30 Jan 2023 12:56:40 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Waiman Long <longman@...hat.com>
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 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.