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

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Message-ID: <20160621084119.GN30154@twins.programming.kicks-ass.net>
Date:	Tue, 21 Jun 2016 10:41:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	Vincent Guittot <vincent.guittot@...aro.org>,
	Yuyang Du <yuyang.du@...el.com>,
	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 Mon, Jun 20, 2016 at 03:49:34PM +0100, Dietmar Eggemann wrote:
> On 20/06/16 13:35, Vincent Guittot wrote:

> > It will go through wake_up_new_task and post_init_entity_util_avg
> > during its fork which is enough to set last_update_time. Then, it will
> > use the switched_to_fair if the task becomes a fair one
> 
> Oh I see. We want to make sure that every task (even when forked as
> !fair) has a last_update_time value != 0, when becoming fair one day.

Right, see 2 below. I need to write a bunch of comments explaining PELT
proper, as well as document these things.

The things we ran into with these patches were that:

 1) You need to update the cfs_rq _before_ any entity attach/detach
    (and might need to update_tg_load_avg when update_cfs_rq_load_avg()
    returns true).

 2) (fair) entities are always attached, switched_from/to deal with !fair.

 3) cpu migration is the only exception and uses the last_update_time=0
    thing -- because refusal to take second rq->lock.

Which is why I dislike Yuyang's patches, they create more exceptions
instead of applying existing rules (albeit undocumented).

Esp. 1 is important, because while for mathematically consistency you
don't actually need to do this, you only need the entities to be
up-to-date with the cfs rq when you attach/detach, but that forgets the
temporal aspect of _when_ you do this.