linux-kernel - Re: [PATCH RFC] cpumask: Randomly distribute the tasks within affinity mask

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Date:   Wed, 11 Oct 2023 16:55:35 -0700
From:   Josh Don <joshdon@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ankit Jain <ankitja@...are.com>, yury.norov@...il.com,
        andriy.shevchenko@...ux.intel.com, linux@...musvillemoes.dk,
        qyousef@...alina.io, pjt@...gle.com, bristot@...hat.com,
        vschneid@...hat.com, linux-kernel@...r.kernel.org,
        namit@...are.com, amakhalov@...are.com, srinidhir@...are.com,
        vsirnapalli@...are.com, vbrahmajosyula@...are.com,
        akaher@...are.com, srivatsa@...il.mit.edu
Subject: Re: [PATCH RFC] cpumask: Randomly distribute the tasks within
 affinity mask

Hey Peter,

> +static struct cpumask *root_domain_allowed(struct cpumask *newmask,
> +                                          struct cpumask *scratch,
> +                                          struct cpumask *valid)
> +{
> +       struct root_domain *rd;
> +       struct cpumask *mask;
> +       struct rq *rq;
> +
> +       int first = cpumask_first_and(newmask, valid);
> +       if (first >= nr_cpu_ids)
> +               return NULL;

This picks the first arbitrarily, but isn't it possible for the mask
to span both isolated and non-isolated cpus? In which case, the rest
of this just boils down to chance (ie. whatever the span happens to be
for the first cpu here)?