glibc/SOURCES/glibc-rh2213907-6.patch

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2023-09-21 18:38:09 +00:00
From 8b9a0af8ca012217bf90d1dc0694f85b49ae09da Mon Sep 17 00:00:00 2001
From: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Tue, 18 Jul 2023 10:27:59 -0500
Subject: [PATCH] [PATCH v1] x86: Use `3/4*sizeof(per-thread-L3)` as low bound
for NT threshold.
Content-type: text/plain; charset=UTF-8
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>
---
sysdeps/x86/dl-cacheinfo.h | 15 ++++++++++++---
1 file changed, 12 insertions(+), 3 deletions(-)
[diff rebased by DJ]
diff -rup a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
--- a/sysdeps/x86/dl-cacheinfo.h 2023-07-25 00:38:43.343986368 -0400
+++ b/sysdeps/x86/dl-cacheinfo.h 2023-07-25 00:38:44.336025100 -0400
@@ -751,8 +751,8 @@ dl_init_cacheinfo (struct cpu_features *
/* The default setting for the non_temporal threshold is [1/8, 1/2] of size
of the chip's cache (depending on `cachesize_non_temporal_divisor` which
- is microarch specific. The defeault is 1/4). For most Intel and AMD
- processors with an initial release date between 2017 and 2023, a thread's
+ is microarch specific. The default is 1/4). For most Intel processors
+ with an initial release date between 2017 and 2023, a thread's
typical share of the cache is from 18-64MB. Using a reasonable size
fraction of L3 is meant to estimate the point where non-temporal stores
begin out-competing REP MOVSB. As well the point where the fact that
@@ -763,12 +763,21 @@ dl_init_cacheinfo (struct cpu_features *
the maximum thrashing capped at 1/associativity. */
unsigned long int non_temporal_threshold
= shared / cachesize_non_temporal_divisor;
+
+ /* 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_FEATURE_USABLE_P (cpu_features, ERMS))
- non_temporal_threshold = shared_per_thread * 3 / 4;
+ non_temporal_threshold = non_temporal_threshold_lowbound;
/* SIZE_MAX >> 4 because memmove-vec-unaligned-erms right-shifts the value of
'x86_non_temporal_threshold' by `LOG_4X_MEMCPY_THRESH` (4) and it is best
if that operation cannot overflow. Minimum of 0x4040 (16448) because the