linux-kernel - Re: [PATCH v2 00/12] sched: Address schbench regression

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Message-ID: <8c6b7753-02ec-44d5-9b32-5c8fbfa2e7da@meta.com>
Date: Tue, 22 Jul 2025 13:20:13 -0700
From: Chris Mason <clm@...a.com>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc: linux-kernel@...r.kernel.org, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        vschneid@...hat.com
Subject: Re: [PATCH v2 00/12] sched: Address schbench regression

On 7/21/25 12:37 PM, Shrikanth Hegde wrote:

> *** issues/observations in schbench.
> 
> Chris,
> 
> When one does -W auto or -M auto i think code is meant to run, n message
> threads on first n CPUs and worker threads
> on remaining CPUs?
> I don't see that happening.  above behavior can be achieved only with -M
> <cpus> -W <cpus>
> 
>         int i = 0;
>         CPU_ZERO(m_cpus);
>         for (int i = 0; i < m_threads; ++i) {
>                 CPU_SET(i, m_cpus);
>                 CPU_CLR(i, w_cpus);
>         }
>         for (; i < CPU_SETSIZE; i++) {             << here i refers to
> the one in scope. which is 0. Hence w_cpus is set for all cpus.
>                                                       And hence workers
> end up running on all CPUs even with -W auto
>                 CPU_SET(i, w_cpus);
>         }

Oh, you're exactly right.  Fixing this up, thanks.  I'll do some runs to
see if this changes things on my test boxes as well.

> 
> 
> Another issue, is that if CPU0 if offline, then auto pinning fails.
> Maybe no one cares about that case?

The auto pinning is pretty simple right now, I'm planning on making it
numa/ccx aware.  Are CPUs offline enough on test systems that we want to
worry about that?

-chris