sched/fair: handle case of task_h_load() returning 0sched/backport-h_load-v4.4.230

[ Upstream commit a61fa2799ef9bf6c4f54cf7295036577cececc72 ]

task_h_load() can return 0 in some situations like running stress-ng
mmapfork, which forks thousands of threads, in a sched group on a 224 cores
system. The load balance doesn't handle this correctly because
env->imbalance never decreases and it will stop pulling tasks only after
reaching loop_max, which can be equal to the number of running tasks of
the cfs. Make sure that imbalance will be decreased by at least 1.

We can't simply ensure that task_h_load() returns at least one because it
would imply to handle underflow in other places.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: <stable@vger.kernel.org> # v5.4
Link: https://lkml.kernel.org/r/20200710152426.16981-1-vincent.guittot@linaro.org

1 files changed, 9 insertions, 1 deletions

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 971e31e47bfd..15952d0e340b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5939,7 +5939,15 @@ static int detach_tasks(struct lb_env *env)
 		if (!can_migrate_task(p, env))
 			goto next;
 
-		load = task_h_load(p);
+		/*
+		 * Depending of the number of CPUs and tasks and the
+		 * cgroup hierarchy, task_h_load() can return a null
+		 * value. Make sure that env->imbalance decreases
+		 * otherwise detach_tasks() will stop only after
+		 * detaching up to loop_max tasks.
+		 */
+		load = max_t(unsigned long, task_h_load(p), 1);
+
 
 		if (sched_feat(LB_MIN) && load < 16 && !env->sd->nr_balance_failed)
 			goto next;