[PATCH 1/1] sched/pelt: Refine the enqueue_load_avg calculate method

From: Kuyo Chang <kuyo.chang@mediatek.com>
To: Ingo Molnar <mingo@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, "Mel Gorman" <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Matthias Brugger <matthias.bgg@gmail.com>
Cc: <wsd_upstream@mediatek.com>, kuyo chang <kuyo.chang@mediatek.com>,
	<linux-kernel@vger.kernel.org>,
	<linux-arm-kernel@lists.infradead.org>,
	<linux-mediatek@lists.infradead.org>
Subject: [PATCH 1/1] sched/pelt: Refine the enqueue_load_avg calculate method
Date: Mon, 11 Apr 2022 14:16:56 +0800	[thread overview]
Message-ID: <20220411061702.22978-1-kuyo.chang@mediatek.com> (raw)

From: kuyo chang <kuyo.chang@mediatek.com>

I meet the warning message at cfs_rq_is_decayed at below code.

SCHED_WARN_ON(cfs_rq->avg.load_avg ||
		    cfs_rq->avg.util_avg ||
		    cfs_rq->avg.runnable_avg)

Following is the calltrace.

Call trace:
__update_blocked_fair
update_blocked_averages
newidle_balance
pick_next_task_fair
__schedule
schedule
pipe_read
vfs_read
ksys_read

After code analyzing and some debug messages, I found it exits a corner
case at attach_entity_load_avg which will cause load_sum is zero and
load_avg is not.
Consider se_weight is 88761 according by sched_prio_to_weight table.
And assume the get_pelt_divider() is 47742, se->avg.load_avg is 1.
By the calculating for se->avg.load_sum as following will become zero
as following.
se->avg.load_sum =
	div_u64(se->avg.load_avg * se->avg.load_sum, se_weight(se));
se->avg.load_sum = 1*47742/88761 = 0.

After enqueue_load_avg code as below.
cfs_rq->avg.load_avg += se->avg.load_avg;
cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;

Then the load_sum for cfs_rq will be 1 while the load_sum for cfs_rq is 0.
So it will hit the warning message.

After all, I refer the following commit patch to do the similar thing at
enqueue_load_avg.
sched/pelt: Relax the sync of load_sum with load_avg

After long time testing, the kernel warning was gone and the system runs
as well as before.

Signed-off-by: kuyo chang <kuyo.chang@mediatek.com>
---
 kernel/sched/fair.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d4bd299d67ab..30d8b6dba249 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3074,8 +3074,10 @@ account_entity_dequeue(struct cfs_rq *cfs_rq, struct sched_entity *se)
 static inline void
 enqueue_load_avg(struct cfs_rq *cfs_rq, struct sched_entity *se)
 {
-	cfs_rq->avg.load_avg += se->avg.load_avg;
-	cfs_rq->avg.load_sum += se_weight(se) * se->avg.load_sum;
+	add_positive(&cfs_rq->avg.load_avg, se->avg.load_avg);
+	add_positive(&cfs_rq->avg.load_sum, se_weight(se) * se->avg.load_sum);
+	cfs_rq->avg.load_sum = max_t(u32, cfs_rq->avg.load_sum,
+					  cfs_rq->avg.load_avg * PELT_MIN_DIVIDER);
 }
 
 static inline void
-- 
2.18.0


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