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x86: Use 3/4*sizeof(per-thread-L3) as low bound for NT threshold.

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On some machines we end up with incomplete cache information. This can
make the new calculation of `sizeof(total-L3)/custom-divisor` end up
lower than intended (and lower than the prior value). So reintroduce
the old bound as a lower bound to avoid potentially regressing code
where we don't have complete information to make the decision.
Reviewed-by: DJ Delorie <dj@redhat.com>

(cherry picked from commit 8b9a0af8ca)
This commit is contained in:
Noah Goldstein 2023-08-11 18:47:17 -05:00
parent 402324a710
commit 68a2030415
1 changed files with 10 additions and 1 deletions
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11
sysdeps/x86/cacheinfo.c
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@ -797,12 +797,21 @@ init_cacheinfo (void)
modern HW detects streaming patterns and provides proper LRU hints so that
the maximum thrashing capped at 1/associativity. */
unsigned long int non_temporal_threshold = shared / 4;
/* If the computed non_temporal_threshold <= 3/4 * per-thread L3, we most
likely have incorrect/incomplete cache info in which case, default to
3/4 * per-thread L3 to avoid regressions. */
unsigned long int non_temporal_threshold_lowbound
= shared_per_thread * 3 / 4;
if (non_temporal_threshold < non_temporal_threshold_lowbound)
non_temporal_threshold = non_temporal_threshold_lowbound;
/* If no ERMS, we use the per-thread L3 chunking. Normal cacheable stores run
a higher risk of actually thrashing the cache as they don't have a HW LRU
hint. As well, their performance in highly parallel situations is
noticeably worse. */
if (!CPU_FEATURES_CPU_P (cpu_features, ERMS))
non_temporal_threshold = shared_per_thread * 3 / 4;
non_temporal_threshold = non_temporal_threshold_lowbound;
__x86_shared_non_temporal_threshold
= (cpu_features->non_temporal_threshold != 0