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    Subject[PATCH 5.11 147/342] sched/pelt: Fix task util_est update filtering
    Date
    From: Vincent Donnefort <vincent.donnefort@arm.com>

    [ Upstream commit b89997aa88f0b07d8a6414c908af75062103b8c9 ]

    Being called for each dequeue, util_est reduces the number of its updates
    by filtering out when the EWMA signal is different from the task util_avg
    by less than 1%. It is a problem for a sudden util_avg ramp-up. Due to the
    decay from a previous high util_avg, EWMA might now be close enough to
    the new util_avg. No update would then happen while it would leave
    ue.enqueued with an out-of-date value.

    Taking into consideration the two util_est members, EWMA and enqueued for
    the filtering, ensures, for both, an up-to-date value.

    This is for now an issue only for the trace probe that might return the
    stale value. Functional-wise, it isn't a problem, as the value is always
    accessed through max(enqueued, ewma).

    This problem has been observed using LISA's UtilConvergence:test_means on
    the sd845c board.

    No regression observed with Hackbench on sd845c and Perf-bench sched pipe
    on hikey/hikey960.

    Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
    Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
    Link: https://lkml.kernel.org/r/20210225165820.1377125-1-vincent.donnefort@arm.com
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/fair.c | 15 ++++++++++++---
    1 file changed, 12 insertions(+), 3 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index bbc78794224a..dfb65140eb2d 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -3959,6 +3959,8 @@ static inline void util_est_dequeue(struct cfs_rq *cfs_rq,
    trace_sched_util_est_cfs_tp(cfs_rq);
    }

    +#define UTIL_EST_MARGIN (SCHED_CAPACITY_SCALE / 100)
    +
    /*
    * Check if a (signed) value is within a specified (unsigned) margin,
    * based on the observation that:
    @@ -3976,7 +3978,7 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
    struct task_struct *p,
    bool task_sleep)
    {
    - long last_ewma_diff;
    + long last_ewma_diff, last_enqueued_diff;
    struct util_est ue;

    if (!sched_feat(UTIL_EST))
    @@ -3997,6 +3999,8 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
    if (ue.enqueued & UTIL_AVG_UNCHANGED)
    return;

    + last_enqueued_diff = ue.enqueued;
    +
    /*
    * Reset EWMA on utilization increases, the moving average is used only
    * to smooth utilization decreases.
    @@ -4010,12 +4014,17 @@ static inline void util_est_update(struct cfs_rq *cfs_rq,
    }

    /*
    - * Skip update of task's estimated utilization when its EWMA is
    + * Skip update of task's estimated utilization when its members are
    * already ~1% close to its last activation value.
    */
    last_ewma_diff = ue.enqueued - ue.ewma;
    - if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100)))
    + last_enqueued_diff -= ue.enqueued;
    + if (within_margin(last_ewma_diff, UTIL_EST_MARGIN)) {
    + if (!within_margin(last_enqueued_diff, UTIL_EST_MARGIN))
    + goto done;
    +
    return;
    + }

    /*
    * To avoid overestimation of actual task utilization, skip updates if
    --
    2.30.2


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