linux-kernel - Re: [PATCH v2] sched/fair: update scale invariance of PELT

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Date:   Tue, 11 Apr 2017 11:12:19 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        dietmar.eggemann@....com, Morten.Rasmussen@....com,
        yuyang.du@...el.com, pjt@...gle.com, bsegall@...gle.com
Subject: Re: [PATCH v2] sched/fair: update scale invariance of PELT

On Tue, Apr 11, 2017 at 09:52:21AM +0200, Vincent Guittot wrote:

> > > +	} else if (!weight) {
> > > +		if (sa->util_sum < (LOAD_AVG_MAX * 1000)) {
> > 
> > But here I'm completely lost. WTF just happened ;-)
> > 
> > Firstly, I think we want a comment on why we care about the !weight
> > case. Why isn't !running sufficient?
> 
> We track the time when the task is "really" idle but not the time that
> the task spent to wait for running on the CPU. Running is used to
> detect when the task is really running and how much idle time has been
> lost while weight is used to detect when the task is back to sleep
> state and when we have account the lost idle time.

Huh? You're redefining what 'idle' means wrt. util_sum.

util used to consider anything !running as idle. So this is the main
trickery? I feel that deserves a comment of exceptional clarity.