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Date: Tue, 27 Mar 2012 13:19:45 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Christoph Lameter <cl@linux.com>
Cc: LKML <linux-kernel@vger.kernel.org>, linaro-sched-sig@lists.linaro.org,
        Alessio Igor Bogani <abogani@kernel.org>,
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        Max Krasnyansky <maxk@qualcomm.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Stephen Hemminger <shemminger@vyatta.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Sven-Thorsten Dietrich <thebigcorporation@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>, Zen Lin <zen@openhuawei.org>
Subject: Re: [PATCH 07/32] cpuset: Set up interface for nohz flag
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On Wed, Mar 21, 2012 at 09:50:27AM -0500, Christoph Lameter wrote:
> On Wed, 21 Mar 2012, Frederic Weisbecker wrote:
> 
> > Prepare the interface to implement the nohz cpuset flag.
> > This flag, once set, will tell the system to try to
> > shutdown the periodic timer tick when possible.
> >
> > We use here a per cpu refcounter. As long as a CPU
> > is contained into at least one cpuset that has the
> > nohz flag set, it is part of the set of CPUs that
> > run into adaptive nohz mode.
> 
> What are the drawbacks for nohz?
> 
> If there are none: Can we make nohz default behavior without relying on
> cpusets?

I can't tell for now. I haven't yet covered everything the timer is
handling. Until that happens I can't do measurements.

In theory this sounds like a win in every case. I just would like
to test that in practice. This sets up hooks in kernel entry/exit.
More IPIs here and there. May be this adds overhead on workloads
involving a lot of syscalls or exceptions. I don't know.

Given this is not yet entirely clear, I think it may be better to keep
this interface around until the patchset reaches a version that becomes
mergeable. Then at this point we can get serious testing coverage to take
the decision to drop the interface and make it unconditional on
CONFIG_ADAPTIVE_NO_HZ.