Re: [PATCH v1] x86: Use 64MB as nt-store threshold if no cacheinfo [BZ #30429]

[Noah Goldstein <goldstein.w.n@gmail.com>]

On Mon, Jun 5, 2023 at 12:46 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Mon, Jun 5, 2023 at 10:15 AM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > On Mon, May 8, 2023 at 10:10 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > >
> > > If `non_temporal_threshold` is below `minimum_non_temporal_threshold`,
> > > it almost certainly means we failed to read the systems cache info.
> > >
> > > In this case, rather than defaulting the minimum correct value, we
> > > should default to a value that gets at least reasonable
> > > performance. 64MB is chosen conservatively to be at the very high
> > > end. This should never cause non-temporal stores when, if we had read
> > > cache info, we wouldn't have otherwise.
> > > ---
> > >  sysdeps/x86/dl-cacheinfo.h | 10 +++++++++-
> > >  1 file changed, 9 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
> > > index 864b00a521..6225c852f6 100644
> > > --- a/sysdeps/x86/dl-cacheinfo.h
> > > +++ b/sysdeps/x86/dl-cacheinfo.h
> > > @@ -771,8 +771,16 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
> > >       reflected in the manual.  */
> > >    unsigned long int maximum_non_temporal_threshold = SIZE_MAX >> 4;
> > >    unsigned long int minimum_non_temporal_threshold = 0x4040;
> > > +
> > > +  /* If `non_temporal_threshold` less than `minimum_non_temporal_threshold`
> > > +     it most likely means we failed to detect the cache info. We don't want
> > > +     to default to `minimum_non_temporal_threshold` as such a small value,
> > > +     while correct, has bad performance. We default to 64MB as reasonable
> > > +     default bound. 64MB is likely conservative in that most/all systems would
> > > +     choose a lower value so it should never forcing non-temporal stores when
> > > +     they otherwise wouldn't be used.  */
> > >    if (non_temporal_threshold < minimum_non_temporal_threshold)
> > > -    non_temporal_threshold = minimum_non_temporal_threshold;
> > > +    non_temporal_threshold = 64 * 1024 * 1024;
> > >    else if (non_temporal_threshold > maximum_non_temporal_threshold)
> > >      non_temporal_threshold = maximum_non_temporal_threshold;
> > >
> > > --
> > > 2.34.1
> > >
> >
> > I want to backport down to 2.28.
> > Thoughts?
>
> Who will use such backport?

People using systems where we miscalculate cache size.
>
> --
> H.J.