linux-kernel - Re: [RFC PATCH 01/15] nohz_task: New mask for cpus having nohz task

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Message-ID: <20101224122903.GA1879@nowhere>
Date:	Fri, 24 Dec 2010 13:29:14 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Dario Faggioli <raistlin@...ux.it>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Anton Blanchard <anton@....ibm.com>,
	Tim Pepper <lnxninja@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 01/15] nohz_task: New mask for cpus having nohz task

On Fri, Dec 24, 2010 at 09:19:56AM +0100, Dario Faggioli wrote:
> On Fri, 2010-12-24 at 16:00 +0800, Lai Jiangshan wrote: 
> > On 12/20/2010 11:24 PM, Frederic Weisbecker wrote:
> > >  
> > > +config NO_HZ_TASK
> > > +       bool "Tickless task"
> > > +       depends on HAVE_NO_HZ_TASK && NO_HZ && SMP && HIGH_RES_TIMERS
> > > +       help
> > > +         When a task runs alone on a CPU and switches into this mode,
> > > +         the timer interrupt will only trigger when it is strictly
> > > +         needed.
> > 
> > Why it depends on SMP?
> > 
> I guess that's because you need at least a ticking CPU for timekeeping
> and stuff... Right?
> 
> Regards,
> Dario


Exactly.

In the next version I'll probably drop that requirement. On UP a single task
must still be able to be in nohz mode when it is in userspace, as timekeeping
doesn't matter there until we reenter the kernel.

Dunno, I'll see what I can do.
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