sched/fair: Fix enqueue_task_fair warning

When a cfs rq is throttled, the latter and its child are removed from the
leaf list but their nr_running is not changed which includes staying higher
than 1. When a task is enqueued in this throttled branch, the cfs rqs must
be added back in order to ensure correct ordering in the list but this can
only happens if nr_running == 1.
When cfs bandwidth is used, we call unconditionnaly list_add_leaf_cfs_rq()
when enqueuing an entity to make sure that the complete branch will be
added.

Similarly unthrottle_cfs_rq() can stop adding cfs in the list when a parent
is throttled. Iterate the remaining entity to ensure that the complete
branch will be added in the list.

Reported-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: stable@vger.kernel.org
Cc: stable@vger.kernel.org #v5.1+
Link: https://lkml.kernel.org/r/20200306135257.25044-1-vincent.guittot@linaro.org

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1dea8554ead06374440891d6d5ae116a01a30b57..c7aaae2b1030d10c460a62478b90b321d74ac136 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4136,6 +4136,7 @@ static inline void check_schedstat_required(void)
 #endif
 }
 
+static inline bool cfs_bandwidth_used(void);
 
 /*
  * MIGRATION
@@ -4214,10 +4215,16 @@ enqueue_entity(struct cfs_rq *cfs_rq, struct sched_entity *se, int flags)
                __enqueue_entity(cfs_rq, se);
        se->on_rq = 1;
 
-       if (cfs_rq->nr_running == 1) {
+       /*
+        * When bandwidth control is enabled, cfs might have been removed
+        * because of a parent been throttled but cfs->nr_running > 1. Try to
+        * add it unconditionnally.
+        */
+       if (cfs_rq->nr_running == 1 || cfs_bandwidth_used())
                list_add_leaf_cfs_rq(cfs_rq);
+
+       if (cfs_rq->nr_running == 1)
                check_enqueue_throttle(cfs_rq);
-       }
 }
 
 static void __clear_buddies_last(struct sched_entity *se)
@@ -4808,11 +4815,22 @@ void unthrottle_cfs_rq(struct cfs_rq *cfs_rq)
                        break;
        }
 
-       assert_list_leaf_cfs_rq(rq);
-
        if (!se)
                add_nr_running(rq, task_delta);
 
+       /*
+        * The cfs_rq_throttled() breaks in the above iteration can result in
+        * incomplete leaf list maintenance, resulting in triggering the
+        * assertion below.
+        */
+       for_each_sched_entity(se) {
+               cfs_rq = cfs_rq_of(se);
+
+               list_add_leaf_cfs_rq(cfs_rq);
+       }
+
+       assert_list_leaf_cfs_rq(rq);
+
        /* Determine whether we need to wake up potentially idle CPU: */
        if (rq->curr == rq->idle && rq->cfs.nr_running)
                resched_curr(rq);