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

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Message-ID: <CAFTL4hzrTXsVucJDHrirDTxM7NwxD5DKuGjRW3fkiWmHyHUOpg@mail.gmail.com>
Date:	Wed, 9 May 2012 13:02:30 +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

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.

I don't think it's

>> > > 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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