Andy Lutomirski <l...@kernel.org> wrote: > On Wed, Jan 31, 2018 at 1:09 PM, Nadav Amit <nadav.a...@gmail.com> wrote: >> Dave Hansen <dave.han...@linux.intel.com> wrote: >> >>> On 01/31/2018 12:11 PM, Nadav Amit wrote: >>>> The TLB invalidation info is allocated on the stack, which might cause >>>> it to be unaligned. Since this information may be transferred to >>>> different cores for TLB shootdown, this might result in an additional >>>> cache-line bouncing between the cores. >>>> >>>> GCC provides a way to deal with it by using >>>> __builtin_alloca_with_align(). Use it to avoid the bouncing cache lines. >>> >>> It doesn't really *bounce*, though, does it? I don't see any writes on >>> the remote side. The remote use seems entirely read-only. >>> >>> You also don't have to exhaustively test this, but I'd love to see at >>> least a sanity check with a microbenchmark (or something) that, yes, >>> this does help *something*. Maybe it makes the remote >>> flush_tlb_func_common() run faster because it's pulling in fewer lines, >>> or maybe you can even detect fewer misses in there. >> >> I agree that with the whole Meltdown/Spectre entry-cost it might not even be >> measurable, at least on small ( < 2 sockets) machines. But I do not think it >> worth profiling. Basically, AFAIK, all the data structures that are used for >> inter-processor communication by the kernel are aligned, and this is an >> exception. > > This is only going to be measurable at all on NUMA, I suspect.
Yes, I meant <= 2 ...
