Re: [patch 3/3] x86/fpu: Make FPU protection more robust

[Jason A. Donenfeld]

Hi Thomas,

On Thu, May 05, 2022 at 02:55:58AM +0200, Thomas Gleixner wrote:
> > So if truly the only user of this is random.c as of 5.18 (is it? I'm
> > assuming from a not very thorough survey...), and if the performance
> > boost doesn't even exist, then yeah, I think it'd make sense to just get
> > rid of it, and have kernel_fpu_usable() return false in those cases.
> >
> > I'll run some benchmarks on a little bit more hardware in representative
> > cases and see.
> 
> Find below a combo patch which makes use of strict softirq serialization
> for the price of not supporting the hardirq FPU usage. 

Thanks, I'll give it a shot in the morning (3am) when trying to do a
more realistic benchmark. But just as a synthetic thing, I ran the
numbers in kBench900 and am getting:

     generic:    430 cycles per call
       ssse3:    315 cycles per call
      avx512:    277 cycles per call

for a single call to the compression function, which is the most any of
those mix_pool_bytes() calls do from add_{input,disk}_randomness(), on
Tiger Lake, using RDPMC from kernel space.

This _doesn't_ take into account the price of calling kernel_fpu_begin().
That's a little hard to bench synthetically by running it in a loop and
taking medians because of the lazy restoration. But that's an indication
anyway that I should be looking at the cost of the actual function as
its running in random.c, rather than the synthetic test. Will keep this
thread updated.

Jason