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

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Message-ID: <CA+55aFymaQj3ods2ahxj6xv03wL0gtEejWX6T94yGaxEGquSFA@mail.gmail.com>
Date:	Tue, 19 Nov 2013 11:18:07 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Josh Boyer <jwboyer@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	"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 11:09 AM, Ingo Molnar <mingo@...nel.org> wrote:
>
> As you pointed out offstack is definitely nonsensical for
> NR_CPUS <= 64. (on 64-bit CPUs) On-stack is known to crash with
> NR_CPUS >= 1024.
>
> The 128..512 CPUs range is somewhat of an unknown.

Agreed. And we could even decide to ask somewhere in that range.

I think 128 bits is still safely "don't bother with an external
pointer" (it's just two words, it's like a "struct list_head" -
there's no way that should be unsafe on the stack). Once we get to 256
bits I start going "Hmm, that's 32 bytes, maybe an external allocation
makes sense..."

I'd personally put the cut-off point at just keeping it on-stack if
it's smaller than or equal to 256. But I agree that at that point it's
really just a judgement call.

             Linus
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