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

[Michael S. Tsirkin]

On Tue, Jan 12, 2016 at 12:59:58PM -0800, Andy Lutomirski wrote:
> On Tue, Jan 12, 2016 at 12:54 PM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> > On Tue, Jan 12, 2016 at 12:30 PM, Andy Lutomirski <luto@xxxxxxxxxx> 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

OK so I'll have to tweak the test to put something
on stack to measure the difference: my test tweaks a
global variable instead.
I'll try that by tomorrow.

I couldn't measure any difference between mfence and lock+addl
except in a micro-benchmark, but hey since we are tweaking this,
let's do the optimal thing.

-- 
MST