From ed2f9dc9420c4c61436328778a70459d0a35556a Mon Sep 17 00:00:00 2001
From: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Mon, 8 May 2023 22:10:20 -0500
Subject: [PATCH] x86: Use 64MB as nt-store threshold if no cacheinfo [BZ
#30429]
Content-type: text/plain; charset=UTF-8
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.
Reviewed-by: Florian Weimer <fweimer@redhat.com>
---
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 ec88945b39..877e73d700 100644
--- a/sysdeps/x86/dl-cacheinfo.h
+++ b/sysdeps/x86/dl-cacheinfo.h
@@ -749,8 +749,16 @@ 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;
--
2.39.3