Well... with UP we don't even need GS in the kernel... On June 18, 2015 1:01:06 AM PDT, Ingo Molnar <[email protected]> wrote: > >* Andy Lutomirski <[email protected]> wrote: > >> On Mon, Jun 15, 2015 at 2:42 PM, H. Peter Anvin <[email protected]> >wrote: >> > On 06/15/2015 02:30 PM, Linus Torvalds wrote: >> >> >> >> On Jun 12, 2015 2:09 PM, "Andy Lutomirski" <[email protected] >> >> <mailto:[email protected]>> wrote: >> >>> >> >>> Caveat emptor: it also disables SMP. >> >> >> >> OK, I don't think it's interesting in that form. >> >> >> >> For small cpu counts, I guess we could have per-cpu syscall entry >points >> >> (unless the syscall entry msr is shared across hyperthreading? >Some msr's are >> >> per thread, others per core, AFAIK), and it could actually work >that way. >> >> >> >> But I'm not sure the three cycles is worth the worry and the >complexity. >> > >> > We discussed the per-cpu syscall entry point, and the issue at hand >is that it >> > is very hard to do that without with fairly high probability touch >another >> > cache line and quite possibly another page (and hence a TLB entry.) > >( So apparently I wasn't Cc:ed, or gmail ate the mail - so I can only >guess from >the surrounding discussion what this patch does, as my lkml folder is >still > doing a long refresh ... ) > >> >> I think this isn't actually true. If we were going to do a per-cpu >syscall >> entry point, then we might as well duplicate all of the entry code >per cpu >> instead of just a short trampoline. That would avoid extra TLB >misses and (L1) >> cache misses, I think. >> >> I still think this is far too complicated for three cycles. I was >hoping for >> more. > >The other problem with duplicating entry code is that with per CPU >entry code we >split its cache footprint in higher level caches (such as the L2 but >also L3 >cache). > >The interesting number would be to check cache cold entry performance, >not cache >hot one: the NUMA latency advantage of having per node copies of the >entry code >might be worth it. > >... and that's why UP is the least interesting case ;-) > >Thanks, > > Ingo
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