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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 12, 2016 at 01:37:38PM -0800, Linus Torvalds wrote:
> On Tue, Jan 12, 2016 at 12:59 PM, Andy Lutomirski <luto@amacapital.net> wrote:
> >
> > Here's an article with numbers:
> >
> > http://shipilev.net/blog/2014/on-the-fence-with-dependencies/
> 
> Well, that's with the busy loop and one set of code generation. It
> doesn't show the "oops, deeper stack isn't even in the cache any more
> due to call chains" issue.
> 
> But yes:
> 
> > 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.
> 
> I think a negative offset might work very well. Partly exactly
> *because* we have the redzone disabled: we know that inside the
> kernel, we'll never have any live stack frame accesses under the stack
> pointer, so "-4(%rsp)" sounds good to me. There should never be any
> pending writes in the write buffer, because even if it *was* live, it
> would have been read off first.
> 
> Yeah, it potentially does extend the stack cache footprint by another
> 4 bytes, but that sounds very benign.
> 
> So perhaps it might be worth trying to switch the "mfence" to "lock ;
> addl $0,-4(%rsp)" in the kernel for x86-64, and remove the alternate
> for x86-32.
> 
> I'd still want to see somebody try to benchmark it. I doubt it's
> noticeable, but making changes because you think it might save a few
> cycles without then even measuring it is just wrong.
> 
>                  Linus

Oops, I posted v2 with just offset 0 before reading
the rest of this thread.
I did try with offset 0 and didn't measure any
change on any perf bench test, or on kernel build.
I wonder which benchmark stresses smp_mb the most.
I'll look into using a negative offset.

-- 
MST