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    SubjectRe: [RFC PATCH 5/6] sched/fair: Select an energy-efficient CPU on task wake-up
    On Friday 23 Mar 2018 at 16:00:59 (+0000), Morten Rasmussen wrote:
    > On Thu, Mar 22, 2018 at 09:27:43AM -0700, Joel Fernandes wrote:
    > > Hi,
    > >
    > > On Tue, Mar 20, 2018 at 2:43 AM, Dietmar Eggemann
    > > <dietmar.eggemann@arm.com> wrote:

    [...]

    > > Is it possible that before the wakeup, the task's affinity is changed
    > > so that p->cpus_allowed no longer contains prev_cpu ? In that case
    > > prev_energy wouldn't matter since previous CPU is no longer an option?
    >
    > It is possible to wake-up with a disallowed prev_cpu. In fact
    > select_idle_sibling() may happily return a disallowed cpu in that case.
    > The mistake gets fixed in select_task_rq() which uses
    > select_fallback_rq() to find an allowed cpu instead.
    >
    > Could we fix the issue in find_energy_efficient_cpu() by a simple test
    > like below
    >
    > if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed))
    > prev_energy = best_energy = compute_energy(p, prev_cpu);
    > else
    > prev_energy = best_energy = ULONG_MAX;

    Right, that should work. I'll change this in v2.

    Thanks,
    Quentin

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