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 Tested-by: Christian Borntraeger borntraeger@de.ibm.com Reviewed-by: Dietmar Eggemann dietmar.eggemann@arm.com Tested-by: Dietmar Eggemann dietmar.eggemann@arm.com Cc: stable@vger.kernel.org #v5.1+ Signed-off-by: Vincent Guittot vincent.guittot@linaro.org ---
Change since v1: - added the iteration at the end of unthrottle_cfs_rq() as Dietmar reported use case which triggered assert_list_leaf_cfs_rq()
kernel/sched/fair.c | 26 ++++++++++++++++++++++---- 1 file changed, 22 insertions(+), 4 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index fcc968669aea..ae50a09cbead 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -4117,6 +4117,7 @@ static inline void check_schedstat_required(void) #endif }
+static inline bool cfs_bandwidth_used(void);
/* * MIGRATION @@ -4195,10 +4196,16 @@ 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) @@ -4779,11 +4786,22 @@ 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);