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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <b9ef3469-1042-43b8-884d-57064b79bbe7@meta.com>
Date: Thu, 24 Jul 2025 14:23:45 -0400
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 7/22/25 1:20 PM, Chris Mason wrote:
> 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.

Fixing this makes it substantially slower (5.2M RPS -> 3.8M RPS), with
more time spent in select_task_rq().  I need to trace a bit to
understand if the message thread CPUs are actually getting used that
often for workers, or if the exclusion makes our idle CPU hunt slower
somehow.

-chris