linux-kernel - [PATCH v2] sched/fair: Correctly insert cfs

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Date:   Thu,  3 Jun 2021 13:38:47 +0200
From:   Odin Ugedal <odin@...d.al>
To:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>
Cc:     cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        Odin Ugedal <odin@...d.al>
Subject: [PATCH v2] sched/fair: Correctly insert cfs_rq's to list on unthrottle

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@...d.al>
---

Original thread: https://lore.kernel.org/lkml/20210518125202.78658-3-odin@uged.al/
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