linux-kernel - Re: [RFC PATCH 04/15] nohz_task: Stop the tick when the nohz task runs alone

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Message-ID: <20101221132226.GB1750@nowhere>
Date:	Tue, 21 Dec 2010 14:22:28 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 04/15] nohz_task: Stop the tick when the nohz task
 runs alone

On Tue, Dec 21, 2010 at 08:35:58AM +0100, Peter Zijlstra wrote:
> On Tue, 2010-12-21 at 00:37 +0100, Frederic Weisbecker wrote:
> > On Mon, Dec 20, 2010 at 04:51:39PM +0100, Peter Zijlstra wrote:
> > > On Mon, 2010-12-20 at 16:24 +0100, Frederic Weisbecker wrote:
> > > > Check from the timer interrupt that we are a nohz task running
> > > > alone in the CPU and stop the tick if this is the case.
> > > > 
> > > Does this verify that the tick has no other work to do?
> > > 
> > > I see no list of things the tick does and a checklist that everything it
> > > does is indeed superfluous.
> > 
> > In a subsequent patch we check if rcu also needs the tick.
> > For the rest, tick_nohz_stop_sched_tick() knows what to
> > do: keep the next tick or switch to nohz.
> > 
> > Hm?
> 
> No, and that worries me, you don't even seem to know what the tick does
> and yet you're working on stopping it.

Yeah, I've been focusing a lot on preemption and rcu because they seemed
to be the trickiest part of the thing, and I in the meantime I just forgot
the rest or tagged it too quickly as details to deal with later. And I
finally forgot them :)

Bah it's a first take, it can only improve ;)
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