linux-kernel - [PATCH] sched/fair: Ensure _sum and

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Date:   Thu, 24 Jun 2021 13:18:15 +0200
From:   Odin Ugedal <odin@...d.al>
To:     Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Sachin Sant <sachinp@...ux.vnet.ibm.com>,
        Naresh Kamboju <naresh.kamboju@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Juri Lelli <juri.lelli@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org, Odin Ugedal <odin@...d.al>
Subject: [PATCH] sched/fair: Ensure _sum and _avg values stay consistent

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>
---

Reports and discussion can be found here:

https://lore.kernel.org/lkml/2ED1BDF5-BC0C-47CD-8F33-9A46C738F8CF@linux.vnet.ibm.com/
https://lore.kernel.org/lkml/CA+G9fYsMXELmjGUzw4SY1bghTYz_PeR2diM6dRp2J37bBZzMSA@mail.gmail.com/

 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 bfaa6e1f6067..def48bc2e90b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3688,15 +3688,15 @@ 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
-- 
2.32.0