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    Subject[PATCH 3/4] sched/fair: minimize concurrent LBs between domain level
    Date
    sched domains tend to trigger simultaneously the load balance loop but
    the larger domains often need more time to collect statistics. This
    slowness makes the larger domain trying to detach tasks from a rq whereas
    tasks already migrated somewhere else at a sub-domain level. This is not
    a real problem for idle LB because the period of smaller domains will
    increase with its CPUs being busy and this will let time for higher ones
    to pulled tasks. But this becomes a problem when all CPUs are already busy
    because all domains stay synced when they trigger their LB.

    A simple way to minimize simultaneous LB of all domains is to decrement the
    the busy interval by 1 jiffies. Because of the busy_factor, the interval of
    larger domain will not be a multiple of smaller ones anymore.

    Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
    ---
    kernel/sched/fair.c | 3 +++
    1 file changed, 3 insertions(+)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 765be8273292..7d7eefd8e2d4 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -9780,6 +9780,9 @@ get_sd_balance_interval(struct sched_domain *sd, int cpu_busy)

    /* scale ms to jiffies */
    interval = msecs_to_jiffies(interval);
    + if (cpu_busy)
    + interval -= 1;
    +
    interval = clamp(interval, 1UL, max_load_balance_interval);

    return interval;
    --
    2.17.1
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