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    Subject[PATCH 5.4 103/340] sched/eas: Dont update misfit status if the task is pinned
    Date
    From: Qais Yousef <qais.yousef@arm.com>

    [ Upstream commit 0ae78eec8aa64e645866e75005162603a77a0f49 ]

    If the task is pinned to a cpu, setting the misfit status means that
    we'll unnecessarily continuously attempt to migrate the task but fail.

    This continuous failure will cause the balance_interval to increase to
    a high value, and eventually cause unnecessary significant delays in
    balancing the system when real imbalance happens.

    Caught while testing uclamp where rt-app calibration loop was pinned to
    cpu 0, shortly after which we spawn another task with high util_clamp
    value. The task was failing to migrate after over 40ms of runtime due to
    balance_interval unnecessary expanded to a very high value from the
    calibration loop.

    Not done here, but it could be useful to extend the check for pinning to
    verify that the affinity of the task has a cpu that fits. We could end
    up in a similar situation otherwise.

    Fixes: 3b1baa6496e6 ("sched/fair: Add 'group_misfit_task' load-balance type")
    Signed-off-by: Qais Yousef <qais.yousef@arm.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Reviewed-by: Quentin Perret <qperret@google.com>
    Acked-by: Valentin Schneider <valentin.schneider@arm.com>
    Link: https://lkml.kernel.org/r/20210119120755.2425264-1-qais.yousef@arm.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/fair.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 3dd7c10d6a582..611adca1e6d0c 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -3814,7 +3814,7 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
    if (!static_branch_unlikely(&sched_asym_cpucapacity))
    return;

    - if (!p) {
    + if (!p || p->nr_cpus_allowed == 1) {
    rq->misfit_task_load = 0;
    return;
    }
    --
    2.27.0


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