linux-kernel - Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks

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Date:	Wed, 10 Aug 2016 07:19:55 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Dietmar Eggemann <dietmar.eggemann@....com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mike Galbraith <umgwanakikbuti@...il.com>,
	Benjamin Segall <bsegall@...gle.com>,
	Paul Turner <pjt@...gle.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Matt Fleming <matt@...eblueprint.co.uk>
Subject: Re: [PATCH 4/4] sched,fair: Fix PELT integrity for new tasks

On Fri, Jun 24, 2016 at 03:03:24PM +0200, Peter Zijlstra wrote:
> > Task's last_update_time means this task is detached from fair queue. This
> > (re)definition is by all means much better than migrating. No?
> 
> I would maybe redefine it as an up-to-date marker for a migration across
> a clock discontinuity. Both CPU migration and group movement suffer from
> this, albeit for different reasons.
> 
> In the CPU migration case we simply cannot tell time by our refusal to
> acquire the old rq lock. So we age to the last time we 'know' and then
> mark it up-to-date.
> 
> For the cgroup move the timelines simply _are_ discontinuous. So we have
> to mark it up-to-date after we update it to the instant of detach, such
> that when we attach it to the new group we don't try to age it across
> the time difference.

Got it. I'm glad to see you worked out all these.
 
It's like I am replying to an email in the last century, sorry, but I was
distracted for many other things.