linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
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

  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 \
    --subject='Re: [PATCH v2] sched/cfs: make util/load_avg more stable' \
    /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

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).