Messages in this thread | | | From | Noah Goldstein <> | Date | Wed, 27 Oct 2021 12:37:16 -0500 | Subject | Re: [PATCH v3 2/2] x86/xstate: Make AVX512 status tracking more accurate |
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On Wed, Oct 27, 2021 at 12:11 PM Borislav Petkov <bp@alien8.de> wrote: > > On Wed, Oct 27, 2021 at 11:26:15AM -0500, Noah Goldstein wrote: > > diff --git a/arch/x86/include/asm/fpu/types.h b/arch/x86/include/asm/fpu/types.h > > index f5a38a5f3ae1..cb10909fa3da 100644 > > --- a/arch/x86/include/asm/fpu/types.h > > +++ b/arch/x86/include/asm/fpu/types.h > > @@ -330,11 +330,21 @@ struct fpu { > > unsigned int last_cpu; > > > > /* > > - * @avx512_timestamp: > > + * @avx512_ZMM_Hi256_timestamp: > > * > > - * Records the timestamp of AVX512 use during last context switch. > > + * Records the timestamp of AVX512 use in the ZMM_Hi256 xfeature > > + * set. This include zmm0...zmm15. > > */ > > - unsigned long avx512_timestamp; > > + unsigned long avx512_ZMM_Hi256_timestamp; > > + > > + /* > > + * @avx512_Hi16_ZMM_timestamp: > > + * > > + * Records the timestamp of AVX512 use in the Hi16_ZMM xfeature > > + * set. This includes usage of any of the hi16 xmm, ymm, or zmm > > + * registers. > > + */ > > + unsigned long avx512_Hi16_ZMM_timestamp; > > No, not more of this but less. > > That was a bad idea to begin with as exposing this to userspace would > cause exactly this: but but, I need to track my special use case more > precisely. > > But the feature mask can't give you that precision so it'll be only an > approximation no matter what you do. > > And I'm being told future cores won't have this "problem" so on them > that file becomes actively misleading.
What I've heard is it's a lot better on Rocket Lake (only extra downclocking on multicore usage) and TBD for Saphire Rapids. > > If you really wanna track performance drop precisely or AVX use or > whatnot, there's performance counters for that which can give you > exactly what you wanna know. > > So I'll take a simple patch carving out that into a function and which > removes the opmask and otherwise let that thing die. And on future cores > which are not affected, that thing will report only 0 anyway.
What about just splitting off a field for 'AVX512_ZMM_Hi256'? That field seems like it can give meaningful information.
I think mostly like 'AVX512_Hi16_ZMM' will almost always be set because GLIBC's ifunc uses EVEX implementations for all string/memory functions on CPUs with avx512.
But if you insist will do. > > Thx. > > -- > Regards/Gruss, > Boris. > > https://people.kernel.org/tglx/notes-about-netiquette
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