linux-kernel - Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT

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Date:   Wed, 28 Nov 2018 11:53:36 +0000
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <Morten.Rasmussen@....com>,
        Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>,
        Thara Gopinath <thara.gopinath@...aro.org>,
        pkondeti@...eaurora.org, Quentin Perret <quentin.perret@....com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: Re: [PATCH v7 2/2] sched/fair: update scale invariance of PELT

On 28-Nov 11:02, Peter Zijlstra wrote:
> On Wed, Nov 28, 2018 at 10:54:13AM +0100, Vincent Guittot wrote:
> 
> > Is there anything else that I should do for these patches ?
> 
> IIRC, Morten mention they break util_est; Patrick was going to explain.

I guess the problem is that, once we cross the current capacity,
strictly speaking util_avg does not represent anymore a utilization.

With the new signal this could happen and we end up storing estimated
utilization samples which will overestimate the task requirements.

We will have a spike in estimated utilization at next wakeup, since we
use MAX(util_avg@...ueue_time, ewma). Potentially we also inflate the EWMA in
case we collect multiple samples above the current capacity.

So, a possible fix could be to avoid storing util_est samples if we
end up with a utilization above the current capacity.

Something like:

----8<---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ac855b2f4774..93e0cf5d8a76 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3661,6 +3661,10 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
 	if (!task_sleep)
 		return;
 
+	/* Skip samples which do not represent an actual utilization */
+	if (unlikely(task_util(p) > capacity_of(task_cpu(p))))
+		return;
+
 	/*
 	 * If the PELT values haven't changed since enqueue time,
 	 * skip the util_est update.
---8<---

Could that work ?

Maybe using a new utility function to wrap the new check.

-- 
#include <best/regards.h>

Patrick Bellasi