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

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Date:	Wed, 9 May 2012 13:07:18 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mike Galbraith <efault@....de>
Cc:	Christoph Lameter <cl@...ux.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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

(Sorry, pressed sent too quickly)

2012/5/9 Mike Galbraith <efault@....de>:
> 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.

I don't think it's a good idea. It will prevent a set of nohz CPUs to
be used for any
other kind of partition. There is no good reason for that.

Also that doesn't really solve the issue. The root cpuset will still
have the nohz
flag turned off.

May be I should indeed rather use sysfs, it's actually true that
cpusets is confusing
for this kind of thing.
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