[tip:sched/core] sched/fair: Fix imbalance due to CPU affinity

From: tip-bot for Vincent Guittot
Date: Thu Jul 25 2019 - 14:08:11 EST

Commit-ID: f6cad8df6b30a5d2bbbd2e698f74b4cafb9fb82b
Gitweb: https://git.kernel.org/tip/f6cad8df6b30a5d2bbbd2e698f74b4cafb9fb82b
Author: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
AuthorDate: Mon, 1 Jul 2019 17:47:02 +0200
Committer: Ingo Molnar <mingo@xxxxxxxxxx>
CommitDate: Thu, 25 Jul 2019 15:51:52 +0200

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

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@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/1561996022-28829-1-git-send-email-vincent.guittot@xxxxxxxxxx
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
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 b5546a15206c..9be36ffb5689 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9048,9 +9048,10 @@ more_balance:
* 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)