linux-kernel - has_idle

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Message-ID: <alpine.DEB.2.22.394.2506152216150.3463@hadrien>
Date: Sun, 15 Jun 2025 22:26:17 +0200 (CEST)
From: Julia Lawall <julia.lawall@...ia.fr>
To: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>, 
    Vincent Guittot <vincent.guittot@...aro.org>, 
    Dietmar Eggemann <dietmar.eggemann@....com>, linux-kernel@...r.kernel.org
Subject: has_idle_core

Hello,

I wonder about the following code in select_idle_cpu:

        for_each_cpu_wrap(cpu, cpus, target + 1) {
                if (has_idle_core) {
                        i = select_idle_core(p, cpu, cpus, &idle_cpu);
                        if ((unsigned int)i < nr_cpumask_bits)
                                return i;

                } else {
                        if (--nr <= 0)
                                return -1;
                        idle_cpu = __select_idle_cpu(cpu, p);
                        if ((unsigned int)idle_cpu < nr_cpumask_bits)
                                break;
                }
        }

        if (has_idle_core)
                set_idle_cores(target, false);

set/test_idle_cores works at the LLC level.  But the mask cpus in
for_each_cpu_wrap depends on the set of cores on which the task to be
placed can run.  So can't the placement of one task that is pinned to a
small set of busy cores prevent the search for an idle core for subsequent
less restricted waking tasks, until the idle cores flag is reset?

For the moment, this is a purely theoretical question.  I don't have a
concrete example that illustrates the problem.

thanks,
julia