Re: [PATCH 3/4] x86,asm: Re-work smp_store_mb()

[Davidlohr Bueso]

On Mon, 02 Nov 2015, Linus Torvalds wrote:

> On Mon, Nov 2, 2015 at 12:15 PM, Davidlohr Bueso <dave@xxxxxxxxxxxx> wrote:
> > So I ran some experiments on an IvyBridge (2.8GHz) and the cost of XCHG is
> > constantly cheaper (by at least half the latency) than MFENCE. While there
> > was a decent amount of variation, this difference remained rather constant.
>
> Mind testing "lock addq $0,0(%rsp)" instead of mfence? That's what we
> use on old cpu's without one (ie 32-bit).

I'm getting results very close to xchg.

> I'm not actually convinced that mfence is necessarily a good idea. I
> could easily see it being microcode, for example.

Interesting.

> At least on my Haswell, the "lock addq" is pretty much exactly half
> the cost of "mfence".

Ok, his coincides with my results on IvB.
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