This fixes an issue where fairness is decreased since cfs_rq's can
end up not being decayed properly. For two sibling control groups with
the same priority, this can often lead to a load ratio of 99/1 (!!).
This happen because when a cfs_rq is throttled, all the descendant cfs_rq's
will be removed from the leaf list. When they initial cfs_rq is
unthrottled, it will currently only re add descendant cfs_rq's if they
have one or more entities enqueued. This is not a perfect heuristic.
Insted, we insert all cfs_rq's that contain one or more enqueued
entities, or contributes to the load of the task group.
Can often lead to sutiations like this for equally weighted control
groups:
$ ps u -C stress
USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
root 10009 88.8 0.0 3676 100 pts/1 R+ 11:04 0:13 stress --cpu 1
root 10023 3.0 0.0 3676 104 pts/1 R+ 11:04 0:00 stress --cpu 1
Fixes: 31bc6aeaab1d ("sched/fair: Optimize update_blocked_averages()")
Signed-off-by: Odin Ugedal <odin@xxxxxxx>
---
Original thread: https://lore.kernel.org/lkml/20210518125202.78658-3-odin@xxxxxxx/
Changes since v1:
- Replaced cfs_rq field with using tg_load_avg_contrib
- Went from 3 to 1 pathces; one is merged and one is replaced
by a new patchset.
kernel/sched/fair.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 794c2cb945f8..0f1b39ca5ca8 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4719,8 +4719,11 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
cfs_rq->throttled_clock_task_time += rq_clock_task(rq) -
cfs_rq->throttled_clock_task;
- /* Add cfs_rq with already running entity in the list */
- if (cfs_rq->nr_running >= 1)
+ /*
+ * Add cfs_rq with tg load avg contribution or one or more
+ * already running entities to the list
+ */
+ if (cfs_rq->tg_load_avg_contrib || cfs_rq->nr_running)
list_add_leaf_cfs_rq(cfs_rq);
}
--
2.31.1
