linux-kernel - [PATCH 1/3] sched/fair: Add tg_load_contrib cfs

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Date:   Tue, 18 May 2021 14:52:00 +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 1/3] sched/fair: Add tg_load_contrib cfs_rq decay checking

Make sure cfs_rq does not contribute to task group load avg when
checking if it is decayed. Due to how the pelt tracking works,
the divider can result in a situation where:

cfs_rq->avg.load_sum = 0
cfs_rq->avg.load_avg = 4
cfs_rq->avg.tg_load_avg_contrib = 4

If pelt tracking in this case does not cross a period, there is no
"change" in load_sum, and therefore load_avg is not recalculated, and
keeps its value.

If this cfs_rq is then removed from the leaf list, it results in a
situation where the load is never removed from the tg. If that happen,
the fiarness is permanently skewed.

Fixes: 039ae8bcf7a5 ("sched/fair: Fix O(nr_cgroups) in the load balancing path")
Signed-off-by: Odin Ugedal <odin@...d.al>
---
 kernel/sched/fair.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3248e24a90b0..ceda53c2a87a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8004,6 +8004,9 @@ static inline bool cfs_rq_is_decayed(struct cfs_rq *cfs_rq)
 	if (cfs_rq->avg.runnable_sum)
 		return false;
 
+	if (cfs_rq->tg_load_avg_contrib)
+		return false;
+
 	return true;
 }
 
-- 
2.31.1