Dave Hansen <[email protected]> wrote: > On 01/31/2018 01:09 PM, Nadav Amit wrote: >>> 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. > > I'm certainly not nak'ing this. I think your patch is likely a good > idea. But, could you please take ten or twenty minutes to go see if > practice matches your assumptions? I'd really appreciate it. If you > can't measure it, then no biggie.
[CC’ing the mailing list] Per your request, I measured it (which perhaps I should have done before). I caused a misalignment intentionally by adding some padding to flush_tlb_info and compared it with an aligned version. I used ftrace to measure the execution time of flush_tlb_func_remote() on a 2-socket Haswell machine, using a microbenchmark I wrote for some research project. It turns out that your skepticism may be correct - In both cases the function execution time is roughly 400ns (2% improvement on the aligned case which is probably noise). So it is up to you whether you want to discard the patch. Regards, Nadav
