linux-kernel - [PATCH AUTOSEL 5.13 182/189] sched/fair: Ensure _sum and

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Date:   Tue,  6 Jul 2021 07:14:02 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Odin Ugedal <odin@...d.al>,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Sasha Levin <sashal@...nel.org>
Subject: [PATCH AUTOSEL 5.13 182/189] sched/fair: Ensure _sum and _avg values stay consistent

From: Odin Ugedal <odin@...d.al>

[ Upstream commit 1c35b07e6d3986474e5635be566e7bc79d97c64d ]

The _sum and _avg values are in general sync together with the PELT
divider. They are however not always completely in perfect sync,
resulting in situations where _sum gets to zero while _avg stays
positive. Such situations are undesirable.

This comes from the fact that PELT will increase period_contrib, also
increasing the PELT divider, without updating _sum and _avg values to
stay in perfect sync where (_sum == _avg * divider). However, such PELT
change will never lower _sum, making it impossible to end up in a
situation where _sum is zero and _avg is not.

Therefore, we need to ensure that when subtracting load outside PELT,
that when _sum is zero, _avg is also set to zero. This occurs when
(_sum < _avg * divider), and the subtracted (_avg * divider) is bigger
or equal to the current _sum, while the subtracted _avg is smaller than
the current _avg.

Reported-by: Sachin Sant <sachinp@...ux.vnet.ibm.com>
Reported-by: Naresh Kamboju <naresh.kamboju@...aro.org>
Signed-off-by: Odin Ugedal <odin@...d.al>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
Tested-by: Sachin Sant <sachinp@...ux.vnet.ibm.com>
Link: https://lore.kernel.org/r/20210624111815.57937-1-odin@uged.al
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 kernel/sched/fair.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 23663318fb81..1dd1ed98171b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3716,15 +3716,15 @@ update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)

 		r = removed_load;
 		sub_positive(&sa->load_avg, r);
-		sub_positive(&sa->load_sum, r * divider);
+		sa->load_sum = sa->load_avg * divider;

 		r = removed_util;
 		sub_positive(&sa->util_avg, r);
-		sub_positive(&sa->util_sum, r * divider);
+		sa->util_sum = sa->util_avg * divider;

 		r = removed_runnable;
 		sub_positive(&sa->runnable_avg, r);
-		sub_positive(&sa->runnable_sum, r * divider);
+		sa->runnable_sum = sa->runnable_avg * divider;

 		/*
 		 * removed_runnable is the unweighted version of removed_load so we
-- 
2.30.2