linux-kernel - Re: [PATCH 07/32] cpuset: Set up interface for nohz flag

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Date:	Tue, 27 Mar 2012 13:19:45 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	linaro-sched-sig@...ts.linaro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Ingo Molnar <mingo@...nel.org>,
	Max Krasnyansky <maxk@...lcomm.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Stephen Hemminger <shemminger@...tta.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Zen Lin <zen@...nhuawei.org>
Subject: Re: [PATCH 07/32] cpuset: Set up interface for nohz flag

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.
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