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SubjectRe: [PATCH] sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity
On Wed, 24 Nov 2021 at 15:15, Vincent Donnefort
<vincent.donnefort@arm.com> wrote:
>
> A shortcut has been introduced in select_idle_sibling() to return prev_cpu
> if the wakee is woken up by a per-CPU kthread. This is an issue for
> asymmetric CPU capacity systems where the wakee might not fit prev_cpu
> anymore. Evaluate asym_fits_capacity() for prev_cpu before using that
> shortcut.
>
> Fixes: 52262ee567ad ("sched/fair: Allow a per-CPU kthread waking a task to stack on the same CPU, to fix XFS performance regression")
> Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6291876a9d32..b90dc6fd86ca 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6410,7 +6410,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
> */
> if (is_per_cpu_kthread(current) &&
> prev == smp_processor_id() &&
> - this_rq()->nr_running <= 1) {
> + this_rq()->nr_running <= 1 &&
> + asym_fits_capacity(task_util, prev)) {

With this patch, i don't think that
20211124154239.3191366-1-vincent.donnefort@arm.com is needed anymore

> return prev;
> }
>
> --
> 2.25.1
>

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