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

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

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?