linux-kernel - Re: [PATCH 08/32] nohz: Try not to give the timekeeping duty to an adaptive tickless cpu

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Date:	Tue, 27 Mar 2012 20:12:58 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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>,
	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>,
	Dimitri Sivanich <sivanich@....com>
Subject: Re: [PATCH 08/32] nohz: Try not to give the timekeeping duty to an
 adaptive tickless cpu

On Tue, 27 Mar 2012, Peter Zijlstra wrote:

> On Tue, 2012-03-27 at 11:08 -0500, Christoph Lameter wrote:
> >
> > I wish you would disentangle the nohz work from the cpusets. Cpusets is
> > aged and being replaced by cgroups. And the cgroup work is something that
> > is not suitable for many loads given the VM overhead added.
>
> What VM overhead? Are you talking about the memcg nonsense? That's
> entirely optional, you don't need to either build that or enable it.

cgroups in general cause a much more complex VM processing with multiple
LRUs and additional checks in various places.

Even just adding cpusets enables the group scheduler functionality f.e.
which creates significantly larger scheduling latencies. Also complicates
key allocation VM paths etc etc.

> And if we ever get rid of that multiple hierarchy nonsense I don't see a
> reason to get rid of cpuset at all. The only reason to want to replace
> it is to avoid the dis-joint-ness it has with the cpu controller (and
> possible the memcg one).

I like cpusets much more than cgroups. I agree with you.

But I am not sure that cpusets are needed for nohz. We already have an
isolcpu set and it sounds to me that nohz is generally useful.

It would seem that the nohz patches would be much simpler if it would not
require cpusets to administer. The only thing that would be needed is to
have one cpu that is not subject to nohz. The logical choice is a
timekeeper cpu (which is usually cpu 0). Having that configurable would be
an extra bonus.

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