[PATCH 2/3] sched/fair: Correctly insert cfs_rq's to list on unthrottle

From: Odin Ugedal
Date: Tue May 18 2021 - 08:55:08 EST


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.

This fix change this behavior to save what cfs_rq's was removed from the
list, and readds them properly on unthrottle.

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>
---
kernel/sched/fair.c | 11 ++++++-----
kernel/sched/sched.h | 1 +
2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ceda53c2a87a..e7423d658389 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -376,7 +376,8 @@ static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
return false;
}

-static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
+/* Returns 1 if cfs_rq was present in the list and removed */
+static inline bool list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)
{
if (cfs_rq->on_list) {
struct rq *rq = rq_of(cfs_rq);
@@ -393,7 +394,9 @@ static inline void list_del_leaf_cfs_rq(struct cfs_rq *cfs_rq)

list_del_rcu(&cfs_rq->leaf_cfs_rq_list);
cfs_rq->on_list = 0;
+ return 1;
}
+ return 0;
}

static inline void assert_list_leaf_cfs_rq(struct rq *rq)
@@ -4742,9 +4745,7 @@ static int tg_unthrottle_up(struct task_group *tg, void *data)
if (!cfs_rq->throttle_count) {
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)
+ if (cfs_rq->insert_on_unthrottle)
list_add_leaf_cfs_rq(cfs_rq);
}

@@ -4759,7 +4760,7 @@ static int tg_throttle_down(struct task_group *tg, void *data)
/* group is entering throttled state, stop time */
if (!cfs_rq->throttle_count) {
cfs_rq->throttled_clock_task = rq_clock_task(rq);
- list_del_leaf_cfs_rq(cfs_rq);
+ cfs_rq->insert_on_unthrottle = list_del_leaf_cfs_rq(cfs_rq);
}
cfs_rq->throttle_count++;

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index a189bec13729..12a707d99ee6 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -602,6 +602,7 @@ struct cfs_rq {
u64 throttled_clock_task_time;
int throttled;
int throttle_count;
+ int insert_on_unthrottle;
struct list_head throttled_list;
#endif /* CONFIG_CFS_BANDWIDTH */
#endif /* CONFIG_FAIR_GROUP_SCHED */
--
2.31.1