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Subject: Re: [PATCH 12/32] nohz: Try not to give the timekeeping duty to a
 cpuset nohz cpu
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Frederic Weisbecker <fweisbec@gmail.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Anton Blanchard <anton@au1.ibm.com>, Avi Kivity <avi@redhat.com>,
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        "Paul E . McKenney" <paulmck@linux.vnet.ibm.com>,
        Stephen Hemminger <shemminger@vyatta.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Tim Pepper <lnxninja@linux.vnet.ibm.com>,
        Dimitri Sivanich <sivanich@sgi.com>, Paul Menage <paul@paulmenage.org>
Date: Tue, 30 Aug 2011 17:37:28 +0200
In-Reply-To: <20110830151704.GV9748@somewhere.redhat.com>
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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@sgi.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.