linux-kernel - Re: [PATCH 04/32] nohz: Separate idle sleeping time accounting from nohz switching

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Date:	Mon, 29 Aug 2011 18:32:14 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Avi Kivity <avi@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Paul Menage <menage@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [PATCH 04/32] nohz: Separate idle sleeping time accounting from
 nohz switching

On Mon, Aug 29, 2011 at 04:23:19PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> > To prepare for having nohz mode switching independant from idle,
> > pull the idle sleeping time accounting out of the tick stop API.
> > 
> > This implies to implement some new API to call when we
> > enter/exit idle. 
> 
> I mean, I really love brevity, but you seem to just not state all the
> important bits ;-)
> 
> So the goal is to disable the tick more often (say when running 1
> userbound task), why does that need new hooks? If we already had the
> tick disabled, the tick_nohz_stop_sched_tick() call on going idle will
> simply not do anything.
> 
> If we go from idle to running something we want to enable the tick
> initially because doing the task wakeup involves RCU etc.. Once we find
> the task is indeed userbound and we've finished all our state we can
> disable the thing again.

That's because we are going to have two different sources of stop/restarting
the tick: either idle or a random task. In the case of idle we have very
specific things to handle like idle time accounting, idle stats, rcu, ...

I could do these things conditionally using a some idle_cpu() checks but
the end result would not be very proper.
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