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

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Message-ID: <alpine.DEB.2.00.1205081054310.27713@router.home>
Date:	Tue, 8 May 2012 10:57:21 -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>,
	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, 8 May 2012, Peter Zijlstra wrote:

> > For some reason this seems to work here. What is broken with isolcpus?
>
> It mostly still works I think, but iirc there were a few places that
> ignored the cpuisol mask.

Yes there is still superfluous stuff going on on isolated processors.

> But really the moment we get proper means of flushing cpu state
> (currently achievable by unplug-replug) isolcpu gets depricated and
> eventually removed.

Not sure what that means and how that is relevant. Scheduler?

> cpusets can do what isolcpu can and more (provided this flush thing).

cpusets is a pretty heavy handed thing and causes inefficiencies in the
allocators if compiled into the kernel because checks will have to be done
in hot allocation paths.

> > > Furthermore there is no other partitioning scheme, cpusets is it.
> >
> > One can partition the system anyway one wants by setting cpu affinities
> > and memory policies etc. No need for cpusets/cgroups.
>
> Not so, the load-balancer will still try to move the tasks and
> subsequently fail. Partitioning means it won't even try to move tasks
> across the partition boundary.

Ok so the scheduler is inefficient on this. Maybe that can be improved?

Setting affinities should not cause overhead in the scheduler.

> By proper partitioning you can split load balance domains (or completely
> disable the load-balancer by giving it a single cpu domain).

I thought that was the point of isolcpus?

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