linux-kernel - Re: [PATCH 06/32] nohz: Move idle ticks stats tracking out of nohz handlers

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Date:	Tue, 6 Sep 2011 02:35:50 +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>,
	Stephen Hemminger <shemminger@...tta.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>,
	Paul Menage <paul@...lmenage.org>
Subject: Re: [PATCH 06/32] nohz: Move idle ticks stats tracking out of nohz
 handlers

On Mon, Aug 29, 2011 at 04:28:08PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> > Idle ticks time tracking is merged into nohz stop/restart
> > handlers. Pull it out into idle entry/exit handlers instead,
> > so that nohz APIs is more idle independant. 
> 
> Are you trying to say:
> 
> Currently idle time tracking is part of the nohz state tracking,
> separate this so that we can disable the tick while we're non-idle?

Right.

> 
> If so, how does idle time tracking work on a !NOHZ kernel, surely such
> things can be idle too..

In that case the tick calls account_process_tick() which delegates
to account_idle_time() if we are idle on tick.

That's also used in NOHZ kernels in fact.

In every tick we call account_process_tick(), but when the tick is stopped
we save its value in ts->idle_jiffies and when it is restarted we account
jiffies - ts->idle_jiffies.
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