linux-kernel - Re: [patch 4/7] sched: convert rq->avg_idle to rq->avg

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Date:	Wed, 23 Nov 2011 13:57:10 +0100
From:	Mike Galbraith <mgalbraith@...e.de>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Suresh Siddha <suresh.b.siddha@...el.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, Paul Turner <pjt@...gle.com>
Subject: Re: [patch 4/7] sched: convert rq->avg_idle to rq->avg_event

On Wed, 2011-11-23 at 13:27 +0100, Peter Zijlstra wrote:
> On Wed, 2011-11-23 at 13:09 +0100, Mike Galbraith wrote:
> > On Wed, 2011-11-23 at 12:55 +0100, Peter Zijlstra wrote:
> > > On Tue, 2011-11-22 at 15:22 +0100, Mike Galbraith wrote:
> > > > We update rq->clock only at points of interest to the scheduler.
> > > > Using this distance has the same effect as measuring idle time
> > > > for idle_balance() throttling, and allows other uses as well. 
> > > 
> > > I'm not sure I follow, suppose we're happily context switching away, how
> > > is the avg distance between context switches related to idle time?
> > 
> > Average idle time can't be larger.
> 
> True :-)
> 
> But it can be _much MUCH_ smaller. So the value is a fair upper limit on
> the idle time, but has no relation to the actual idle duration.

Yup, none.

> Now this value seems to be used in 5 to throttle select_idle_sibling(),
> which is again something unrelated to actual idle duration, but also
> unrelated to the avg update_rq_clock() interval.

Yup.  Clock update is a thing we do at sched event-of-interest, so this
seemed like a good spot to create a multi-purpose number.

> In patch 6 we use this value to guestimate if we should enter nohz,
> since its a wild over estimation of the actual idle duration it'll be
> less effective and might not hard much.
> 
> Also, patch 6's use of sched_migration_cost to reflect the nohz
> enter/exit cost is somewhat iffy, but that's another issue.

Yup.

> Now I'm not saying this all isn't worth it, just saying my brain is
> having difficulty seeing how it all makes sense :-)

They make sense only in that one cheap number generator bandaids three
owies.  It's fugly but effective :)

	-Mike

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