linux-kernel - Re: [PATCH 02/32 RESEND] nohz: Drop ts->idle

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Message-ID: <20110829161508.GA8649@somewhere>
Date:	Mon, 29 Aug 2011 18:15:11 +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 02/32 RESEND] nohz: Drop ts->idle_active

On Mon, Aug 29, 2011 at 04:23:12PM +0200, Peter Zijlstra wrote:
> On Mon, 2011-08-15 at 17:51 +0200, Frederic Weisbecker wrote:
> > ts->idle_active is used to know if we want to account the idle sleep
> > time. But ts->inidle is enough to check that.
> > 
> While possibly true, its not immediately obvious and no hints are
> supplied. For example: tick_check_nohz() would disable ->idle_active..
> where is this mirrored in the ->inidle state.

Hmm, you're right. By the time we call tick_check_nohz() (irq_enter())
and tick_nohz_stop_sched_tick() (irq_exit()) there may be a softirq
and then another hard irq that could update the idle time spuriously.

So the mapping inidle - idle_active is wrong.

Let's drop that patch.

> 
> Also, tick_nohz_stop_sched_tick() has this comment:
> 
> 	/*
> 	 * Set ts->inidle unconditionally. Even if the system did not
> 	 * switch to NOHZ mode the cpu frequency governers rely on the
> 	 * update of the idle time accounting in tick_nohz_start_idle().
> 	 */
> 	ts->inidle = 1;
> 
> Which suggest the ->inidle state doesn't accurately reflect things.
> 
> This is all rather hairy code, such changes really want more in terms of
> explanation.
> 
> 
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