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@xxxxxxxxxxxxxx> 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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
