linux-kernel - [PATCH] sched/fair: fix initial util

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Message-Id: <20240308202209.2452-1-daweilics@gmail.com>
Date: Fri,  8 Mar 2024 12:22:08 -0800
From: Dawei Li <daweilics@...il.com>
To: 
Cc: Dawei Li <daweilics@...il.com>,
	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>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: [PATCH] sched/fair: fix initial util_avg calculation

According to the comment for post_init_entity_util_avg(), it seems that
we are assuming se->load.weight has the same scale/unit as that of
cfs_rq->avg.load_avg.

As far as I understand, se->load.weight is the scaled-up load, instead
of the true weight (as mapped directly from the nice value) of a task.
When CONFIG_32BIT is set, we have load == weight; when CONFIG_64BIT is
set, we have load == weight * 1024. However, cfs_rq->avg.load_avg is
the sum of true weights of tasks, as se->avg.load_avg corresponds to
the true weight of a task.

Based on how sa->util_avg is calculated in the code, we could be
inflating sa->util_avg by 1024 times? Could this be the reason why
"However, in many cases, the above util_avg does not give a desired
value. ... "?

I'm not entirely sure about it. Posting this for clarification and
comments.

Signed-off-by: Dawei Li <daweilics@...il.com>
---
 kernel/sched/fair.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 6a16129f9a5c..0d13e52e1a92 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1031,7 +1031,8 @@ void init_entity_runnable_average(struct sched_entity *se)
  * With new tasks being created, their initial util_avgs are extrapolated
  * based on the cfs_rq's current util_avg:
  *
- *   util_avg = cfs_rq->util_avg / (cfs_rq->load_avg + 1) * se.load.weight
+ *   util_avg = cfs_rq->avg.util_avg / (cfs_rq->avg.load_avg + 1)
+ *		* se_weight(se)
  *
  * However, in many cases, the above util_avg does not give a desired
  * value. Moreover, the sum of the util_avgs may be divergent, such
@@ -1078,7 +1079,7 @@ void post_init_entity_util_avg(struct task_struct *p)

 	if (cap > 0) {
 		if (cfs_rq->avg.util_avg != 0) {
-			sa->util_avg  = cfs_rq->avg.util_avg * se->load.weight;
+			sa->util_avg  = cfs_rq->avg.util_avg * se_weight(se);
 			sa->util_avg /= (cfs_rq->avg.load_avg + 1);

 			if (sa->util_avg > cap)
-- 
2.40.1