linux-kernel - Re: [PATCH 2/2] sched: update runqueue clock before migrations away

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Message-ID: <20131210151428.GH12849@twins.programming.kicks-ass.net>
Date:	Tue, 10 Dec 2013 16:14:28 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Chris Redpath <chris.redpath@....com>
Cc:	"pjt@...gle.com" <pjt@...gle.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"alex.shi@...aro.org" <alex.shi@...aro.org>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Dietmar Eggemann <Dietmar.Eggemann@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	bsegall@...gle.com
Subject: Re: [PATCH 2/2] sched: update runqueue clock before migrations away

On Tue, Dec 10, 2013 at 01:24:21PM +0000, Chris Redpath wrote:
> What happens is that if you have a task which sleeps for a while and wakes
> on a different CPU and the previous CPU hasn't had a tick for a while, then
> that sleep time is lost.

/me more confused now.

Where does update_cfs_rq_blocked_load() account sleep time? Its only
concerned with the blocked decay.

Or is it the decay_count <= 0 case in enqueue_entity_load_avg() that's
screwing you over?

That's guestimating the last_runnable_update based on decay_count, and
per the previous the decay count can get slightly out of sync.

If so, we should look to cure the issue enqueue_entity_load_avg() is
trying to work around, namely the fact that we cannot assume synced
clocks, nor can we read remote clocks atomically.

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