[PATCH 2/3] sched/fair: Correctly insert cfs_rq's to list on unthrottle
From: Odin Ugedal <hidden>
Date: 2021-05-18 12:55:08
Also in:
lkml
Subsystem:
scheduler, the rest · Maintainers:
Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot, Linus Torvalds
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 <redacted>
---
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