[PATCH v2 2/2] x86: Add seperate non-temporal tunable for memset

[Noah Goldstein]

On Fri, Jun 14, 2024 at 5:41 AM Borislav Petkov <bp@alien8.de> wrote:
>
> Hi,
>
> I'm not subscribed to the glibc list - pls CC me directly on replies.
>
> On Wed, May 29, 2024 at 03:53:20PM -0700, H.J. Lu wrote:
> > On Fri, May 24, 2024 at 10:39?AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > >
> > > The tuning for non-temporal stores for memset vs memcpy is not always
> > > the same. This includes both the exact value and whether non-temporal
> > > stores are profitable at all for a given arch.
> > >
> > > This patch add `x86_memset_non_temporal_threshold`. Currently we
> > > disable non-temporal stores for non Intel vendors as the only
> > > benchmarks showing its benefit have been on Intel hardware.
> > > ---
> > >  manual/tunables.texi                             | 16 +++++++++++++++-
> > >  sysdeps/x86/cacheinfo.h                          |  8 +++++++-
> > >  sysdeps/x86/dl-cacheinfo.h                       | 16 ++++++++++++++++
> > >  sysdeps/x86/dl-diagnostics-cpu.c                 |  2 ++
> > >  sysdeps/x86/dl-tunables.list                     |  3 +++
> > >  sysdeps/x86/include/cpu-features.h               |  4 +++-
> > >  .../x86_64/multiarch/memset-vec-unaligned-erms.S |  6 +++---
> > >  7 files changed, 49 insertions(+), 6 deletions(-)
>
> ...
>
> > > +  /* Non-temporal stores in memset have only been tested on Intel hardware.
> > > +     Until we benchmark data on other x86 processor, disable non-temporal
> > > +     stores in memset. */
>
> Well, something's fishy here:
>
> $ ./elf/ld.so --list-tunables | grep threshold
> glibc.malloc.mmap_threshold: 0x0 (min: 0x0, max: 0xffffffffffffffff)
> glibc.cpu.x86_rep_movsb_threshold: 0x600000 (min: 0x100, max: 0xffffffffffffffff)
> glibc.cpu.x86_non_temporal_threshold: 0x600000 (min: 0x4040, max: 0xfffffffffffffff)
> glibc.malloc.trim_threshold: 0x0 (min: 0x0, max: 0xffffffffffffffff)
> glibc.cpu.x86_rep_stosb_threshold: 0xffffffffffffffff (min: 0x1, max: 0xffffffffffffffff)
> glibc.cpu.x86_memset_non_temporal_threshold: 0x0 (min: 0x0, max: 0xffffffffffffffff)
>                                             ^^^^^^^^^
>
> on glibc-2.39.9000-300-g54c1efdac55b from git.
>
> That's on a AMD Zen1 so I'd expect that memset NT threshold to be
> 0xffffffffffffffff by default...
>
> Thx.
>

Thanks for bringing this up, looking into it.
> --
> Regards/Gruss,
>     Boris.
>
> https://people.kernel.org/tglx/notes-about-netiquette