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    Subject[PATCH AUTOSEL 4.19 015/128] sched/fair: Fix imbalance due to CPU affinity
    Date
    From: Vincent Guittot <vincent.guittot@linaro.org>

    [ Upstream commit f6cad8df6b30a5d2bbbd2e698f74b4cafb9fb82b ]

    The load_balance() has a dedicated mecanism to detect when an imbalance
    is due to CPU affinity and must be handled at parent level. In this case,
    the imbalance field of the parent's sched_group is set.

    The description of sg_imbalanced() gives a typical example of two groups
    of 4 CPUs each and 4 tasks each with a cpumask covering 1 CPU of the first
    group and 3 CPUs of the second group. Something like:

    { 0 1 2 3 } { 4 5 6 7 }
    * * * *

    But the load_balance fails to fix this UC on my octo cores system
    made of 2 clusters of quad cores.

    Whereas the load_balance is able to detect that the imbalanced is due to
    CPU affinity, it fails to fix it because the imbalance field is cleared
    before letting parent level a chance to run. In fact, when the imbalance is
    detected, the load_balance reruns without the CPU with pinned tasks. But
    there is no other running tasks in the situation described above and
    everything looks balanced this time so the imbalance field is immediately
    cleared.

    The imbalance field should not be cleared if there is no other task to move
    when the imbalance is detected.

    Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Link: https://lkml.kernel.org/r/1561996022-28829-1-git-send-email-vincent.guittot@linaro.org
    Signed-off-by: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    kernel/sched/fair.c | 5 +++--
    1 file changed, 3 insertions(+), 2 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 49ed38914669b..ad78a15bd5677 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -8863,9 +8863,10 @@ static int load_balance(int this_cpu, struct rq *this_rq,
    out_balanced:
    /*
    * We reach balance although we may have faced some affinity
    - * constraints. Clear the imbalance flag if it was set.
    + * constraints. Clear the imbalance flag only if other tasks got
    + * a chance to move and fix the imbalance.
    */
    - if (sd_parent) {
    + if (sd_parent && !(env.flags & LBF_ALL_PINNED)) {
    int *group_imbalance = &sd_parent->groups->sgc->imbalance;

    if (*group_imbalance)
    --
    2.20.1
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