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

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Date:	Fri, 15 Apr 2016 04:05:23 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	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

Hi Dietmar,

On Thu, Apr 14, 2016 at 01:52:43PM +0100, Dietmar Eggemann wrote:
> 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?

Yes, it is because that (most recent) period is the "current" period.
 
> 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).

En... this is a good case to beat it.

> 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.

But whenever we pass 1ms, we will update. And I am curious, how does the
current 1us works in this case? Anyway, I will reproduce it myself.