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    Subject[PATCH 5.16 0231/1039] sched/fair: Fix per-CPU kthread and wakee stacking for asym CPU capacity
    Date
    From: Vincent Donnefort <vincent.donnefort@arm.com>

    [ Upstream commit 014ba44e8184e1acf93e0cbb7089ee847802f8f0 ]

    select_idle_sibling() has a special case for tasks woken up by a per-CPU
    kthread where the selected CPU is the previous one. For asymmetric CPU
    capacity systems, the assumption was that the wakee couldn't have a
    bigger utilization during task placement than it used to have during the
    last activation. That was not considering uclamp.min which can completely
    change between two task activations and as a consequence mandates the
    fitness criterion asym_fits_capacity(), even for the exit path described
    above.

    Fixes: b4c9c9f15649 ("sched/fair: Prefer prev cpu in asymmetric wakeup path")
    Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
    Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
    Link: https://lkml.kernel.org/r/20211129173115.4006346-1-vincent.donnefort@arm.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/fair.c | 3 ++-
    1 file changed, 2 insertions(+), 1 deletion(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 97d89516fbea2..f2cf047b25e56 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -6400,7 +6400,8 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
    if (is_per_cpu_kthread(current) &&
    in_task() &&
    prev == smp_processor_id() &&
    - this_rq()->nr_running <= 1) {
    + this_rq()->nr_running <= 1 &&
    + asym_fits_capacity(task_util, prev)) {
    return prev;
    }

    --
    2.34.1


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