linux-kernel - [PATCH v2] sched/fair: util_est: fast ramp-up EWMA on utilization increases

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-Id: <20191023205630.14469-1-patrick.bellasi@matbug.net>
Date:   Wed, 23 Oct 2019 21:56:30 +0100
From:   Patrick Bellasi <patrick.bellasi@...bug.net>
To:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Douglas Raillard <douglas.raillard@....com>,
        Quentin Perret <qperret@...gle.com>,
        Patrick Bellasi <patrick.bellasi@...bug.com>
Subject: [PATCH v2] sched/fair: util_est: fast ramp-up EWMA on utilization increases

The estimated utilization for a task:

   util_est = max(util_avg, est.enqueue, est.ewma)

is defined based on:
 - util_avg: the PELT defined utilization
 - est.enqueued: the util_avg at the end of the last activation
 - est.ewma:     a exponential moving average on the est.enqueued
                 samples

According to this definition, when a task suddenly change its bandwidth
requirements from small to big, the EWMA will need to collect multiple
samples before converging up to track the new big utilization.

This slow convergence towards bigger utilization values is not
aligned to the default scheduler behavior, which is to optimize for
performance. Moreover, the est.ewma component fails to compensate for
temporarely utilization drops which spans just few est.enqueued samples.

To let util_est do a better job in the scenario depicted above, change
its definition by making util_est directly follow upward motion and
only decay the est.ewma on downward.

Signed-off-by: Patrick Bellasi <patrick.bellasi@...bug.com>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Peter Zijlstra <peterz@...radead.org>
---
 kernel/sched/fair.c     | 14 +++++++++++++-
 kernel/sched/features.h |  1 +
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a81c36472822..a14487462b6c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3768,11 +3768,22 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
 	if (ue.enqueued & UTIL_AVG_UNCHANGED)
 		return;
 
+	/*
+	 * Reset EWMA on utilization increases, the moving average is used only
+	 * to smooth utilization decreases.
+	 */
+	ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED);
+	if (sched_feat(UTIL_EST_FASTUP)) {
+		if (ue.ewma < ue.enqueued) {
+			ue.ewma = ue.enqueued;
+			goto done;
+		}
+	}
+
 	/*
 	 * Skip update of task's estimated utilization when its EWMA is
 	 * already ~1% close to its last activation value.
 	 */
-	ue.enqueued = (task_util(p) | UTIL_AVG_UNCHANGED);
 	last_ewma_diff = ue.enqueued - ue.ewma;
 	if (within_margin(last_ewma_diff, (SCHED_CAPACITY_SCALE / 100)))
 		return;
@@ -3805,6 +3816,7 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
 	ue.ewma <<= UTIL_EST_WEIGHT_SHIFT;
 	ue.ewma  += last_ewma_diff;
 	ue.ewma >>= UTIL_EST_WEIGHT_SHIFT;
+done:
 	WRITE_ONCE(p->se.avg.util_est, ue);
 }
 
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 2410db5e9a35..7481cd96f391 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -89,3 +89,4 @@ SCHED_FEAT(WA_BIAS, true)
  * UtilEstimation. Use estimated CPU utilization.
  */
 SCHED_FEAT(UTIL_EST, true)
+SCHED_FEAT(UTIL_EST_FASTUP, true)
-- 
2.17.1