linux-kernel - Re: [PATCH 2/2] sched: Rewrite per entity runnable load average tracking

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Message-ID: <20140709233049.GA12024@intel.com>
Date:	Thu, 10 Jul 2014 07:30:49 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	bsegall@...gle.com, mingo@...hat.com, linux-kernel@...r.kernel.org,
	rafael.j.wysocki@...el.com, arjan.van.de.ven@...el.com,
	len.brown@...el.com, alan.cox@...el.com, mark.gross@...el.com,
	pjt@...gle.com, fengguang.wu@...el.com
Subject: Re: [PATCH 2/2] sched: Rewrite per entity runnable load average
 tracking

Thanks, Peter.

On Wed, Jul 09, 2014 at 08:45:43PM +0200, Peter Zijlstra wrote:

> Nope :-).. we got rid of that lock for a good reason.
> 
> Also, this is one area where I feel performance really trumps
> correctness, we can fudge the blocked load a little. So the
> sched_clock_cpu() difference is a strict upper bound on the
> rq_clock_task() difference (and under 'normal' circumstances shouldn't
> be much off).

Strictly, migrating wakee task on remote CPU entails two steps:

(1) Catch up with task's queue's last_update_time, and then substract

(2) Cache up with "current" time of remote CPU (for comparable matter), and then
    on new CPU, change to the new timing source (when enqueue)

So I will try sched_clock_cpu(remote_cpu) for step (2). For step (2), maybe we
should not use cfs_rq_clock_task anyway, since the task is about to going
to another CPU/queue. Is this right?

I made another mistake. Should not only track task entity load, group entity
(as an entity) is also needed. Otherwise, task_h_load can't be done correctly...
Sorry for the messup. But this won't make much change in the codes.

Thanks,
Yuyang
 
> So we could simply use a timestamps from dequeue and one from enqueue,
> and use that.
> 
> As to the remote subtraction, a RMW on another cacheline than the
> rq->lock one should be good; esp since we don't actually observe the
> per-rq total often (once per tick or so) I think, no?
> 
> The thing is, we do not want to disturb scheduling on whatever cpu the
> task last ran on if we wake it to another cpu. Taking rq->lock wrecks
> that for sure. 


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