linux-kernel - Re: [PATCH 12/32] nohz: Try not to give the timekeeping duty to a cpuset nohz cpu

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Message-ID: <20110830224404.GE15953@somewhere.redhat.com>
Date:	Wed, 31 Aug 2011 00:44:06 +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>,
	Dimitri Sivanich <sivanich@....com>,
	Paul Menage <paul@...lmenage.org>
Subject: Re: [PATCH 12/32] nohz: Try not to give the timekeeping duty to a
 cpuset nohz cpu

On Tue, Aug 30, 2011 at 05:37:28PM +0200, Peter Zijlstra wrote:
> On Tue, 2011-08-30 at 17:17 +0200, Frederic Weisbecker wrote:
> > On Mon, Aug 29, 2011 at 04:55:45PM +0200, Peter Zijlstra wrote:
> > > On Mon, 2011-08-15 at 17:52 +0200, Frederic Weisbecker wrote:
> > > > Try to give the timekeeing duty to a CPU that doesn't belong
> > > > to any nohz cpuset when possible, so that we increase the chance
> > > > for these nohz cpusets to run their CPUs out of periodic tick
> > > > mode. 
> > > 
> > > You and Dmitiri might want to get together:
> > > 
> > > lkml.kernel.org/r/20110823195628.GB4533@....com
> > 
> > Right!
> > 
> > There is another missing piece in my patchset. If every non adaptive-nohz
> > CPUs are sleeping, then none is handling the do_timer duty and adaptive nohz
> > CPUs run with a stale jiffies and walltime.
> 
> Doesn't nohz already deal with the case of all cpus being idle? In that
> case the cpu that wakes up first gets to play catch up on irq_enter() or
> so.

Sure and that works for the nohz idle case. But that's not enough anymore
in the case of adaptive nohz CPUs. They can run for a while without the tick
and if nobody else maintains a tick either then jiffies and walltime are
not maintained anymore.
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