From e45f52b11e23d7d18828591be0c769596ab00966 Mon Sep 17 00:00:00 2001
From: Peter Zijlstra <peterz@infradead.org>
Date: Thu, 11 May 2017 18:16:06 +0200
Subject: [PATCH] sched/fair: Cure calc_cfs_shares() vs. reweight_entity()

References: bnc#1066110 Scheduler utilisation tracking
Patch-mainline: v4.15-rc1
Git-commit: 3d4b60d3e3dde6ea24e439000eb3b71078da81f1

Vincent reported that when running in a cgroup, his root
cfs_rq->avg.load_avg dropped to 0 on task idle.

This is because reweight_entity() will now immediately propagate the
weight change of the group entity to its cfs_rq, and as it happens,
our approxmation (5) for calc_cfs_shares() results in 0 when the group
is idle.

Avoid this by using the correct (3) as a lower bound on (5). This way
the empty cgroup will slowly decay instead of instantly drop to 0.

Reported-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Mel Gorman <mgorman@suse.de>
---
 kernel/sched/fair.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7d8aa227f005..e88bfd029ec0 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -2776,11 +2776,10 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq)
 	tg_shares = READ_ONCE(tg->shares);
 
 	/*
-	 * This really should be: cfs_rq->avg.load_avg, but instead we use
-	 * cfs_rq->load.weight, which is its upper bound. This helps ramp up
-	 * the shares for small weight interactive tasks.
+	 * Because (5) drops to 0 when the cfs_rq is idle, we need to use (3)
+	 * as a lower bound.
 	 */
-	load = scale_load_down(cfs_rq->load.weight);
+	load = max(scale_load_down(cfs_rq->load.weight), cfs_rq->avg.load_avg);
 
 	tg_weight = atomic_long_read(&tg->load_avg);