linux-kernel - [PATCH v2] sched/fair: fix imbalance due to CPU affinity

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Date:   Mon,  1 Jul 2019 17:47:02 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     linux-kernel@...r.kernel.org, mingo@...hat.com,
        peterz@...radead.org
Cc:     Vincent Guittot <vincent.guittot@...aro.org>
Subject: [PATCH v2] sched/fair: fix imbalance due to CPU affinity

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@...aro.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