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Date:   Tue, 15 Nov 2022 16:02:00 +0000
From:   Joel Fernandes <joel@joelfernandes.org>
To:     Connor O'Brien <connoro@google.com>
Cc:     linux-kernel@vger.kernel.org, kernel-team@android.com,
        John Stultz <jstultz@google.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        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>,
        Waiman Long <longman@redhat.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "Paul E . McKenney" <paulmck@kernel.org>
Subject: Re: [RFC PATCH 10/11] torture: support randomized shuffling for
 proxy exec testing
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On Mon, Nov 14, 2022 at 12:44:21PM -0800, Connor O'Brien wrote:
> > Instead of doing it this way, maybe another approach is to randomize the
> > sleep interval in:
> >
> >  */
> > static int torture_shuffle(void *arg)
> > {
> >         VERBOSE_TOROUT_STRING("torture_shuffle task started");
> >         do {
> >                 schedule_timeout_interruptible(shuffle_interval);
> >                 torture_shuffle_tasks();
> >                 ...
> >         } while (...)
> >         ...
> > }
> >
> > Right now with this patch you still wakeup the shuffle thread when skipping
> > the affinity set operation.
> >
> > thanks,
> >
> >  - Joel
> >
> 
> Wouldn't the affinities of all the tasks still change in lockstep
> then? The intent with this patch is to get into situations where the
> tasks have different affinity masks, which I think requires changing
> the behavior of torture_shuffle_tasks() rather than how often it's
> called.

Correct me if I'm wrong, but you are still changing the affinities of all the
tasks at the same time (shuffle_task_list still has all the threads being set
to the same affinity). The difference is with your patch, you occasionally
skip punching a consecutive hole into shuffle_tmp_mask.

I was thinking how you could make this patch more upstreamable, you are right
calling less often is not what you are specifically looking for. However,
would a better approach be to:
	a) randomize the shuffle duration.
	b) Instead of skipping the set_cpus_allowed_ptr(), why not randomize
	the number of times you call cpumask_next() to pick a random hole.

These are just some ideas.

thanks,

 - Joel