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    SubjectRe: [PATCH] sched/fair: Fix detection of per-CPU kthreads waking a task
    On Wed, 24 Nov 2021 at 16:42, Vincent Donnefort
    <vincent.donnefort@arm.com> wrote:
    >
    > select_idle_sibling() will return prev_cpu for the case where the task is
    > woken up by a per-CPU kthread. However, the idle task has been recently
    > modified and is now identified by is_per_cpu_kthread(), breaking the
    > behaviour described above. Using !is_idle_task() ensures we do not
    > spuriously trigger that select_idle_sibling() exit path.
    >
    > Fixes: 00b89fe0197f ("sched: Make the idle task quack like a per-CPU kthread")
    > Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
    >
    > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    > index 945d987246c5..8bf95b0e368d 100644
    > --- a/kernel/sched/fair.c
    > +++ b/kernel/sched/fair.c
    > @@ -6399,6 +6399,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
    > * pattern is IO completions.
    > */
    > if (is_per_cpu_kthread(current) &&
    > + !is_idle_task(current) &&
    > prev == smp_processor_id() &&
    > this_rq()->nr_running <= 1) {
    > return prev;

    AFAICT, this can't be possible for a symmetric system because it would
    have been already returned by other conditions.
    Only an asymmetric system can face such a situation if the task
    doesn't fit which is the subject of your other patch.
    so this patch seems irrelevant outside the other one


    > --
    > 2.25.1
    >

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