From: Vincent Guittot <vincent.guittot@linaro.org>
To: peterz@infradead.org, mingo@kernel.org, linux-kernel@vger.kernel.org
Cc: dietmar.eggemann@arm.com, Morten.Rasmussen@arm.com,
yuyang.du@intel.com, pjt@google.com, bsegall@google.com,
Vincent Guittot <vincent.guittot@linaro.org>
Subject: [PATCH v2] sched/cfs: make util/load_avg more stable
Date: Wed, 19 Apr 2017 18:54:04 +0200 [thread overview]
Message-ID: <1492620844-30979-1-git-send-email-vincent.guittot@linaro.org> (raw)
In-Reply-To: <1492619370-29246-1-git-send-email-vincent.guittot@linaro.org>
In the current implementation of load/util_avg, we assume that the ongoing
time segment has fully elapsed, and util/load_sum is divided by LOAD_AVG_MAX,
even if part of the time segment still remains to run. As a consequence, this
remaining part is considered as idle time and generates unexpected variations
of util_avg of a busy CPU in the range ]1002..1024[ whereas util_avg should
stay at 1023.
In order to keep the metric stable, we should not consider the ongoing time
segment when computing load/util_avg but only the segments that have already
fully elapsed. Bu to not consider the current time segment adds unwanted
latency in the load/util_avg responsivness especially when the time is scaled
instead of the contribution. Instead of waiting for the current time segment
to have fully elapsed before accounting it in load/util_avg, we can already
account the elapsed part but change the range used to compute load/util_avg
accordingly.
At the very beginning of a new time segment, the past segments have been
decayed and the max value is MAX_LOAD_AVG*y. At the very end of the current
time segment, the max value becomes 1024(us) + MAX_LOAD_AVG*y which is equal
to MAX_LOAD_AVG. In fact, the max value is
sa->period_contrib + MAX_LOAD_AVG*y at any time in the time segment.
Taking advantage of the fact that MAX_LOAD_AVG*y == MAX_LOAD_AVG-1024, the
range becomes [0..MAX_LOAD_AVG-1024+sa->period_contrib].
As the elapsed part is already accounted in load/util_sum, we update the max
value according to the current position in the time segment instead of
removing its contribution.
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
---
Fold both patches in one
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 3f83a35..c3b8f0f 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3017,12 +3017,12 @@ ___update_load_avg(u64 now, int cpu, struct sched_avg *sa,
/*
* Step 2: update *_avg.
*/
- sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX);
+ sa->load_avg = div_u64(sa->load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib);
if (cfs_rq) {
cfs_rq->runnable_load_avg =
- div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX);
+ div_u64(cfs_rq->runnable_load_sum, LOAD_AVG_MAX - 1024 + sa->period_contrib);
}
- sa->util_avg = sa->util_sum / LOAD_AVG_MAX;
+ sa->util_avg = sa->util_sum / (LOAD_AVG_MAX - 1024 + sa->period_contrib);
return 1;
}
--
2.7.4
next prev parent reply other threads:[~2017-04-19 16:54 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-19 16:29 [PATCH 0/2] sched/cfs: make util/load_avg stable Vincent Guittot
2017-04-19 16:29 ` [PATCH 1/2] sched/cfs: make util/load_avg more stable Vincent Guittot
2017-04-19 16:29 ` [PATCH 2/2] sched/cfs: take into account current time segment Vincent Guittot
2017-04-19 16:44 ` [PATCH 0/2] sched/cfs: make util/load_avg stable Vincent Guittot
2017-04-19 16:44 ` [PATCH 1/2] sched/cfs: make util/load_avg more stable Vincent Guittot
2017-04-19 16:44 ` [PATCH 2/2] sched/cfs: take into account current time segment Vincent Guittot
2017-04-19 16:54 ` Vincent Guittot [this message]
2017-04-25 11:05 ` [PATCH v2] sched/cfs: make util/load_avg more stable Dietmar Eggemann
2017-04-25 12:40 ` Vincent Guittot
2017-04-25 14:53 ` Dietmar Eggemann
2017-04-25 15:17 ` Vincent Guittot
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=1492620844-30979-1-git-send-email-vincent.guittot@linaro.org \
--to=vincent.guittot@linaro.org \
--cc=Morten.Rasmussen@arm.com \
--cc=bsegall@google.com \
--cc=dietmar.eggemann@arm.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@kernel.org \
--cc=peterz@infradead.org \
--cc=pjt@google.com \
--cc=yuyang.du@intel.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).