linux-kernel - Re: [PATCH 00/32] [RFC] nohz/cpuset: Start discussions on nohz CPUs

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Date:	Fri, 02 Nov 2012 15:50:37 +0100
From:	David Nyström <david.c.nystrom@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Christoph Lameter <cl@...ux.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Clark Williams <clark.williams@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Li Zefan <lizf@...fujitsu.com>, Ingo Molnar <mingo@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Mike Galbraith <efault@....de>
Subject: Re: [PATCH 00/32] [RFC] nohz/cpuset: Start discussions on nohz CPUs

On 11/02/2012 03:37 PM, Steven Rostedt wrote:
> On Fri, 2012-11-02 at 14:23 +0000, Christoph Lameter wrote:
>> On Mon, 29 Oct 2012, Steven Rostedt wrote:
>>
>>> A while ago Frederic posted a series of patches to get an idea on
>>> how to implement nohz cpusets. Where you can add a task to a cpuset
>>> and mark the set to be 'nohz'. When the task runs on a CPU and is
>>> the only task scheduled (nr_running == 1), the tick will stop.
>>> The idea is to give the task the least amount of kernel interference
>>> as possible. If the task doesn't do any system calls (and possibly
>>> even if it does), no timer interrupt will bother it. By using
>>> isocpus and nohz cpuset, a task would be able to achieve true cpu
>>> isolation.
>>

One other aspect that this patch probably needs to address is the cache 
localization of irq spinlocks.

At least in 3.6, with !CONFIG_SPARSE_IRQ
--
struct  irq_desc irq_desc[NR_IRQS] __cacheline_aligned_in_smp = {
	[0 ... NR_IRQS-1] = {
		.handle_irq	= handle_bad_irq,
		.depth		= 1,
		.lock		= __RAW_SPIN_LOCK_UNLOCKED(irq_desc->lock),
	}
};
--

You are likely to get a cache miss in the top half of your low latency 
CPU anytime some other CPU has taken a spinlock which lies within the 
same cache line.

Or is my understanding of the __cacheline_aligned_in_smp declaration wrong ?

Br,
David

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