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    Subject[PATCH 5.15 199/846] sched/fair: Fix detection of per-CPU kthreads waking a task
    Date
    From: Vincent Donnefort <vincent.donnefort@arm.com>

    [ Upstream commit 8b4e74ccb582797f6f0b0a50372ebd9fd2372a27 ]

    select_idle_sibling() has a special case for tasks woken up by a per-CPU
    kthread, where the selected CPU is the previous one. However, the current
    condition for this exit path is incomplete. A task can wake up from an
    interrupt context (e.g. hrtimer), while a per-CPU kthread is running. A
    such scenario would spuriously trigger the special case described above.
    Also, a recent change made the idle task like a regular per-CPU kthread,
    hence making that situation more likely to happen
    (is_per_cpu_kthread(swapper) being true now).

    Checking for task context makes sure select_idle_sibling() will not
    interpret a wake up from any other context as a wake up by a per-CPU
    kthread.

    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>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
    Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
    Link: https://lore.kernel.org/r/20211201143450.479472-1-vincent.donnefort@arm.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/fair.c | 1 +
    1 file changed, 1 insertion(+)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 6f16dfb742462..c6cb8832796b5 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -6429,6 +6429,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
    * pattern is IO completions.
    */
    if (is_per_cpu_kthread(current) &&
    + in_task() &&
    prev == smp_processor_id() &&
    this_rq()->nr_running <= 1) {
    return prev;
    --
    2.34.1


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