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

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 8b9a0af8ca012217bf90d1dc0694f85b49ae09da)

1 files changed, 10 insertions, 1 deletions

diff --git a/sysdeps/x86/cacheinfo.c b/sysdeps/x86/cacheinfo.c
index 62986a7327..0080e8dad5 100644
--- a/sysdeps/x86/cacheinfo.c
+++ b/sysdeps/x86/cacheinfo.c
@@ -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