Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch

[Jason A. Donenfeld]

On Fri, Jun 15, 2018 at 8:53 PM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>
> On Fri, Jun 15, 2018 at 11:50 AM Dave Hansen
> <dave.hansen@xxxxxxxxxxxxxxx> wrote:
> Even with the modified optimization, kernel_fpu_end() still needs to
> reload the state that was trashed by the kernel FPU use.  If the
> kernel is using something like AVX512 state, then kernel_fpu_end()
> will transfer an enormous amount of data no matter how clever the CPU
> is.  And I think I once measured XSAVEOPT taking a hundred cycles or
> so even when RFBM==0, so it's not exactly super fast.

Indeed the speed up is really significant, especially for the AVX512
case. Here are some numbers from my laptop and a server taken a few
seconds ago:

AVX2 - Intel(R) Xeon(R) CPU E3-1505M v5 @ 2.80GHz
Inside: 684617437
Outside: 547710093
Percent speedup: 24

AVX512 - Intel(R) Xeon(R) Gold 5120 CPU @ 2.20GHz
Inside: 634415672
Outside: 286698960
Percent speedup: 121

This is from this test -- https://xn--4db.cc/F7RF2fhv/c . There are
probably various issues with that test case, and it's possible there
are other effects going on (the avx512 case looks particularly insane)
to make the difference _that_ drastic, but I think there's no doubt
that the optimization here is a meaningful one.