linux-kernel - Re: [PATCH 07/41] cpuset: Set up interface for nohz flag

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Date:	Wed, 09 May 2012 06:21:29 +0200
From:	Mike Galbraith <efault@....de>
To:	Christoph Lameter <cl@...ux.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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>,
	Hakan Akkan <hakanakkan@...il.com>,
	Ingo Molnar <mingo@...nel.org>, Kevin Hilman <khilman@...com>,
	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>
Subject: Re: [PATCH 07/41] cpuset: Set up interface for nohz flag

On Tue, 2012-05-08 at 15:45 -0500, Christoph Lameter wrote: 
> On Tue, 8 May 2012, Mike Galbraith wrote:
> 
> > On Tue, 2012-05-08 at 18:16 +0200, Peter Zijlstra wrote:
> > > On Tue, 2012-05-08 at 10:57 -0500, Christoph Lameter wrote:
> >
> > > isolcpus is a very limited hack that adds more pain that its worth. Its
> > > yet another mask to check and its functionality is completely available
> > > through cpusets.
> >
> > Agreed.
> 
> How would that work? By creating cpusets that only have a single cpu in
> them?

No, just turn load balancing off for exclusive set, domains go poof.

> > > You cannot cree multi-cpu partitions using isolcpus, you cannot
> > > dynamically reconfigure it.
> >
> > Big plus for cpusets.
> 
> Why would you want to do anything like it? cpusets are confusing. You can
> have a cpu be part of multiple cpusets. Which nohz setting applies for a
> particular cpu then? If any of the cpusets have nohz set then it applies
> to the cpu? And thus someone in a cpuset that does not has nohz set will
> find that a cpu will have nohz functionality?

nohz has to be at least an exclusive set property.

> Its not a good match for this. You would want a per cpu attribute for
> nohz.

Or per cpuset, which can be the same thing as per cpu if you want.

> > > And on the scheduler side cpusets doesn't add runtime overhead to normal
> > > things, only sched_setaffinity() and a few other rare operations get
> > > slightly more expensive. And it allows to reduce runtime overhead by
> > > making the load-balancer domains smaller.
> >
> > Very big deal if you have a load that doesn't do all the performance 'i'
> > dotting and 't' crossing it maybe could have, but ends up on a big box.
> 
> isolcpus are not part of load balancer domains.

Yup, so if you have an application with an RT component, somewhat
sensitive, needs isolation from rest of a big box, but app also has
SCHED_OTHER components.  isolcpus is a pain, everything has to be static
and nailed to the floor.  Load just works when plugged into a cpuset.

-Mike

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