Re: For your amusement: slightly faster syscalls

From: H. Peter Anvin
Date: Thu Jun 18 2015 - 05:05:01 EST

Well... with UP we don't even need GS in the kernel...

On June 18, 2015 1:01:06 AM PDT, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>* Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> On Mon, Jun 15, 2015 at 2:42 PM, H. Peter Anvin <hpa@xxxxxxxxx>
>> > On 06/15/2015 02:30 PM, Linus Torvalds wrote:
>> >>
>> >> On Jun 12, 2015 2:09 PM, "Andy Lutomirski" <luto@xxxxxxxxxxxxxx
>> >> <mailto:luto@xxxxxxxxxxxxxx>> 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
>> >> (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
>> >
>> > 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
>> > 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
> doing a long refresh ... )
>> I think this isn't actually true. If we were going to do a per-cpu
>> 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
>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 ;-)
> Ingo

Sent from my mobile phone. Please pardon brevity and lack of formatting.
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