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

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Message-ID: <52B05B09.5090807@arm.com>
Date:	Tue, 17 Dec 2013 14:09:13 +0000
From:	Chris Redpath <chris.redpath@....com>
To:	Peter Zijlstra <peterz@...radead.org>
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" <bsegall@...gle.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Frederic Weisbecker <fweisbec@...il.com>
Subject: Re: [PATCH 2/2] sched: update runqueue clock before migrations away

On 12/12/13 18:24, Peter Zijlstra wrote:
> Would pre_schedule_idle() -> rq_last_tick_reset() -> rq->last_sched_tick
> be useful?
>
> I suppose we could easily lift that to NO_HZ_COMMON.
>

Many thanks for the tip Peter, I have tried this out and it does provide 
enough information to be able to correct the problem. The new version 
doesn't update the rq, just carries the extra unaccounted time 
(estimated from the jiffies) over to be processed during enqueue.

However before I send a new patch set I have a question about the 
existing behavior. Ben, you may already know the answer to this?

During a wake migration we call __synchronize_entity_decay in 
migrate_task_rq_fair, which will decay avg.runnable_avg_sum. We also 
record the amount of periods we decayed for as a negative number in 
avg.decay_count.

We then enqueue the task on its target runqueue, and again we decay the 
load by the number of periods it has been off-rq.

if (unlikely(se->avg.decay_count <= 0)) {
	se->avg.last_runnable_update = rq_clock_task(rq_of(cfs_rq));
	if (se->avg.decay_count) {
		se->avg.last_runnable_update -= (-se->avg.decay_count)
							<< 20;
 >>>		update_entity_load_avg(se, 0);

Am I misunderstanding how this is supposed to work or have we been 
always double-accounting sleep time for wake migrations?

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