linux-kernel - Re: [PATCH 3/4] x86,asm: Re-work smp_store

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Date:	Tue, 12 Jan 2016 12:54:02 -0800
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Andy Lutomirski <luto@...nel.org>
Cc:	"Michael S. Tsirkin" <mst@...hat.com>,
	Davidlohr Bueso <dave@...olabs.net>,
	Davidlohr Bueso <dbueso@...e.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	virtualization <virtualization@...ts.linux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()

On Tue, Jan 12, 2016 at 12:30 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> I recall reading somewhere that lock addl $0, 32(%rsp) or so (maybe even 64)
> was better because it avoided stomping on very-likely-to-be-hot write
> buffers.

I suspect it could go either way. You want a small constant (for the
isntruction size), but any small constant is likely to be within the
current stack frame anyway. I don't think 0(%rsp) is particularly
likely to have a spill on it right then and there, but who knows..

And 64(%rsp) is  possibly going to be cold in the L1 cache, especially
if it's just after a deep function call. Which it might be. So it
might work the other way.

So my guess would be that you wouldn't be able to measure the
difference. It might be there, but probably too small to really see in
any noise.

But numbers talk, bullshit walks. It would be interesting to be proven wrong.

                 Linus