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

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Date:	Tue, 12 Jan 2016 12:59:58 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andy Lutomirski <luto@...nel.org>,
	"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 12:54 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> 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.

Here's an article with numbers:

http://shipilev.net/blog/2014/on-the-fence-with-dependencies/

I think they're suggesting using a negative offset, which is safe as
long as it doesn't page fault, even though we have the redzone
disabled.

--Andy