linux-kernel - Re: [RFC] Documentation/scheduler/schedutil.txt

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Date:   Fri, 20 Nov 2020 09:56:53 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        dietmar.eggemann@....com, patrick.bellasi@...bug.net,
        lenb@...nel.org, linux-kernel@...r.kernel.org,
        valentin.schneider@....com, ionela.voinescu@....com,
        qperret@...gle.com, viresh.kumar@...aro.org
Subject: Re: [RFC] Documentation/scheduler/schedutil.txt

On Fri, Nov 20, 2020 at 08:55:27AM +0100, Peter Zijlstra wrote:
>  - In saturated scenarios task movement will cause some transient dips,
>    suppose we have a CPU saturated with 4 tasks, then when we migrate a task
>    to an idle CPU, the old CPU will have a 'running' value of 0.75 while the
>    new CPU will gain 0.25. This is inevitable and time progression will
>    correct this. XXX do we still guarantee f_max due to no idle-time?

Do we want something like this? Is the 1.5 threshold sane? (it's been too
long since I looked at actual numbers here)

---

diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index 68d369cba9e4..f0bed8902c40 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -90,3 +90,4 @@ SCHED_FEAT(WA_BIAS, true)
  */
 SCHED_FEAT(UTIL_EST, true)
 SCHED_FEAT(UTIL_EST_FASTUP, true)
+SCHED_FEAT(UTIL_SAT, true)
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 590e6f27068c..bf70e5ed8ba6 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2593,10 +2593,17 @@ static inline unsigned long cpu_util_dl(struct rq *rq)
 	return READ_ONCE(rq->avg_dl.util_avg);
 }
 
+#define RUNNABLE_SAT (SCHED_CAPACITY_SCALE + SCHED_CAPACITY_SCALE/2)
+
 static inline unsigned long cpu_util_cfs(struct rq *rq)
 {
 	unsigned long util = READ_ONCE(rq->cfs.avg.util_avg);
 
+	if (sched_feat(UTIL_SAT)) {
+		if (READ_ONCE(rq->cfs.avg.runnable_avg) > RUNNABLE_SAT)
+			return SCHED_CAPACITY_SCALE;
+	}
+
 	if (sched_feat(UTIL_EST)) {
 		util = max_t(unsigned long, util,
 			     READ_ONCE(rq->cfs.avg.util_est.enqueued));