Re: For your amusement: slightly faster syscalls

[H. Peter Anvin]

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>
>wrote:
>> > 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
>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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