Date: Tue, 11 Apr 2017 11:12:19 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Vincent Guittot <vincent.guittot@linaro.org>
Cc: mingo@kernel.org, linux-kernel@vger.kernel.org,
        dietmar.eggemann@arm.com, Morten.Rasmussen@arm.com,
        yuyang.du@intel.com, pjt@google.com, bsegall@google.com
Subject: Re: [PATCH v2] sched/fair: update scale invariance of PELT
Message-ID: <20170411091219.z26r7sa7hpz7gvix@hirez.programming.kicks-ass.net>
References: <1491815909-13345-1-git-send-email-vincent.guittot@linaro.org>
 <20170410173802.orygigjbcpefqtdv@hirez.programming.kicks-ass.net>
 <20170411075221.GA30421@linaro.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20170411075221.GA30421@linaro.org>
User-Agent: NeoMutt/20170113 (1.7.2)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 859
Lines: 20

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.