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From 8b9a0af8ca012217bf90d1dc0694f85b49ae09da Mon Sep 17 00:00:00 2001
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From: Noah Goldstein <goldstein.w.n@gmail.com>
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Date: Tue, 18 Jul 2023 10:27:59 -0500
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Subject: [PATCH] [PATCH v1] x86: Use `3/4*sizeof(per-thread-L3)` as low bound
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for NT threshold.
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Content-type: text/plain; charset=UTF-8
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On some machines we end up with incomplete cache information. This can
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make the new calculation of `sizeof(total-L3)/custom-divisor` end up
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lower than intended (and lower than the prior value). So reintroduce
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the old bound as a lower bound to avoid potentially regressing code
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where we don't have complete information to make the decision.
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Reviewed-by: DJ Delorie <dj@redhat.com>
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---
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sysdeps/x86/dl-cacheinfo.h | 15 ++++++++++++---
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1 file changed, 12 insertions(+), 3 deletions(-)
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[DJ - ported to C8S]
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diff -rup b2/sysdeps/x86/cacheinfo.h b3/sysdeps/x86/cacheinfo.h
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--- b2/sysdeps/x86/cacheinfo.h 2023-08-08 13:55:16.474680016 -0400
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+++ b3/sysdeps/x86/cacheinfo.h 2023-08-08 13:59:14.507988958 -0400
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@@ -401,12 +401,20 @@ init_cacheinfo (void)
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provides proper LRU hints so that the maximum thrashing
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capped at 1/associativity. */
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unsigned long int non_temporal_threshold = shared / 4;
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+ /* If the computed non_temporal_threshold <= 3/4 * per-thread L3, we most
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+ likely have incorrect/incomplete cache info in which case, default to
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+ 3/4 * per-thread L3 to avoid regressions. */
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+ unsigned long int non_temporal_threshold_lowbound
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+ = shared_per_thread * 3 / 4;
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+ if (non_temporal_threshold < non_temporal_threshold_lowbound)
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+ non_temporal_threshold = non_temporal_threshold_lowbound;
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+
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/* If no ERMS, we use the per-thread L3 chunking. Normal cacheable stores run
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a higher risk of actually thrashing the cache as they don't have a HW LRU
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hint. As well, their performance in highly parallel situations is
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noticeably worse. */
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if (!CPU_FEATURE_USABLE_P (cpu_features, ERMS))
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- non_temporal_threshold = shared_per_thread * 3 / 4;
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+ non_temporal_threshold = non_temporal_threshold_lowbound;
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__x86_shared_non_temporal_threshold
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= (cpu_features->non_temporal_threshold != 0
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