Messages in this thread Patch in this message | | | From | Vincent Guittot <> | Subject | [PATCH v2] sched/fair: fix imbalance due to CPU affinity | Date | Mon, 1 Jul 2019 17:47:02 +0200 |
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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> ---
Sorry, I sent the patch before rebasing it on top of sched-tip and it might conlfict when applying because it was on top of my ongoing rework of load_balance
This version is rebased on top of latest shced-tip/sched/core
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 b798fe7..fff5632 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8992,9 +8992,10 @@ 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) -- 2.7.4
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