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

[David Nyström]

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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