linux-kernel - Re: [GIT PULL] core kernel update

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Date:	Tue, 19 Nov 2013 20:18:06 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Josh Boyer <jwboyer@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Josh Boyer <jwboyer@...oraproject.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [GIT PULL] core kernel update


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Nov 19, 2013 at 08:09:04PM +0100, Ingo Molnar wrote:
> > The actual value of the limit - here's the on-stack cpumask sizes of 
> > the candidate range:
> > 
> > 	128 CPUs: 16 byte cpumasks
> > 	256 CPUs: 32 byte cpumasks
> > 	512 CPUs: 64 byte cpumasks
> 
> So 512 / 64bytes is a single cacheline and feels like a nice cut-off
> before requiring an extra indirection and more cachelines.
> 
> 64 bytes also doesn't sound _that_ big to have on-stack.

The cacheline size itself isn't necessarily super meaningful for 
on-stack variables: they are rarely cacheline aligned so they will 
take part in two cachelines.

> So I'd go for having the cut-off on >512, unless of course theres 
> evidence 64bytes is already too much.

I'm fine with that in any case, for the other reason I outlined: it's 
the highest one and we can iterate down if it proves to be bad. If we 
start out too low we'll probably never know it was too low.

Thanks,

	Ingo
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