linux-kernel - Re: [PATCH 2/4] sched/fair: Drop out incomplete current period when sched averages accrue

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Date:	Thu, 14 Apr 2016 13:52:43 +0100
From:	Dietmar Eggemann <dietmar.eggemann@....com>
To:	Yuyang Du <yuyang.du@...el.com>,
	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Benjamin Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Juri Lelli <juri.lelli@....com>
Subject: Re: [PATCH 2/4] sched/fair: Drop out incomplete current period when
 sched averages accrue

On 13/04/16 19:44, Yuyang Du wrote:
> On Wed, Apr 13, 2016 at 05:28:18PM +0200, Vincent Guittot wrote:

[...]

> By "bailing out", you mean return without update because the delta is less
> than 1ms?

yes.

> 
>>> Examples of 1 periodic task pinned to a cpu on an ARM64 system, HZ=250
>>> in steady state:
>>>
>>> (1) task runtime = 100us period = 200us
>>>
>>>   pelt          load/util signal
>>>
>>>   1us:          488-491
>>>
>>>   1ms:          483-534
> 
> 100us/200us = 50%, so the util should center around 512, it seems in this
> regard, it is better, but the variance is undesirable.

I see. You mentioned the over-decay thing in the patch header. Is this
also why you change the contribution of the most recent period from 1002
(1024*y) to 1024?

This variance gets worse if the ratio runtime/period is further reduced
(e.g. 25us/1000us).

You can even create tasks which go stealth mode (e.g. 25us/1048us). It
shows periods of 0 load/util (~1.55s) and than massive spikes (~700 for
~300ms). The short runtime and the task period synced to 1024*1024ns
allow that we hit consecutive enqueues or dequeues for a long time even
the task might drift relative to the pelt window.

[...]