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Subject[PATCH v3] sched/fair : fix reordering of enqueue/dequeue_task_fair
Date
Even when a cgroup is throttled, the group se of a child cgroup can still
be enqueued and its gse->on_rq stays true. When a task is enqueued on such
child, we still have to update the load_avg and increase
h_nr_running of the throttled cfs. Nevertheless, the 1st
for_each_sched_entity loop is skipped because of gse->on_rq == true and the
2nd loop because the cfs is throttled whereas we have to update both
load_avg with the old h_nr_running and increase h_nr_running in such case.

The same sequence can happen during dequeue when se moves to parent before
breaking in the 1st loop.

Note that the update of load_avg will effectively happen only once in order
to sync up to the throttled time. Next call for updating load_avg will stop
early because the clock stays unchanged.

Fixes: 6d4d22468dae ("sched/fair: Reorder enqueue/dequeue_task_fair path")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---

Changes since v2:
- added similar changes into dequeue_task_fair as reported by Ben

kernel/sched/fair.c | 17 +++++++++--------
1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index fcc968669aea..ea2748a132a2 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5431,16 +5431,16 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
for_each_sched_entity(se) {
cfs_rq = cfs_rq_of(se);

- /* end evaluation on encountering a throttled cfs_rq */
- if (cfs_rq_throttled(cfs_rq))
- goto enqueue_throttle;
-
update_load_avg(cfs_rq, se, UPDATE_TG);
se_update_runnable(se);
update_cfs_group(se);

cfs_rq->h_nr_running++;
cfs_rq->idle_h_nr_running += idle_h_nr_running;
+
+ /* end evaluation on encountering a throttled cfs_rq */
+ if (cfs_rq_throttled(cfs_rq))
+ goto enqueue_throttle;
}

enqueue_throttle:
@@ -5529,16 +5529,17 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
for_each_sched_entity(se) {
cfs_rq = cfs_rq_of(se);

- /* end evaluation on encountering a throttled cfs_rq */
- if (cfs_rq_throttled(cfs_rq))
- goto dequeue_throttle;
-
update_load_avg(cfs_rq, se, UPDATE_TG);
se_update_runnable(se);
update_cfs_group(se);

cfs_rq->h_nr_running--;
cfs_rq->idle_h_nr_running -= idle_h_nr_running;
+
+ /* end evaluation on encountering a throttled cfs_rq */
+ if (cfs_rq_throttled(cfs_rq))
+ goto dequeue_throttle;
+
}

dequeue_throttle:
--
2.17.1
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