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

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Message-ID: <20131119191434.GQ16796@laptop.programming.kicks-ass.net>
Date:	Tue, 19 Nov 2013 20:14:34 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...nel.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

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.

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