glibc/glibc-rh2213907-4.patch

From 180897c161a171d8ef0faee1c6c9fd6b57d8b13b Mon Sep 17 00:00:00 2001
From: Noah Goldstein <goldstein.w.n@gmail.com>
Date: Wed, 7 Jun 2023 13:18:03 -0500
Subject: [PATCH] x86: Make the divisor in setting `non_temporal_threshold` cpu
 specific
Content-type: text/plain; charset=UTF-8

Different systems prefer a different divisors.

From benchmarks[1] so far the following divisors have been found:
    ICX     : 2
    SKX     : 2
    BWD     : 8

For Intel, we are generalizing that BWD and older prefers 8 as a
divisor, and SKL and newer prefers 2. This number can be further tuned
as benchmarks are run.

[1]: https://github.com/goldsteinn/memcpy-nt-benchmarks
Reviewed-by: DJ Delorie <dj@redhat.com>
---
 sysdeps/x86/cpu-features.c         | 31 ++++++++++++++++++++---------
 sysdeps/x86/dl-cacheinfo.h         | 32 ++++++++++++++++++------------
 sysdeps/x86/dl-diagnostics-cpu.c   | 11 ++++++----
 sysdeps/x86/include/cpu-features.h |  3 +++
 4 files changed, 51 insertions(+), 26 deletions(-)

[DJ - edited for ABI compatibility]

diff -rup a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
--- a/sysdeps/x86/cpu-features.c	2023-07-26 17:56:19.679300711 -0400
+++ b/sysdeps/x86/cpu-features.c	2023-07-28 15:27:00.336324265 -0400
@@ -35,6 +35,9 @@ extern void TUNABLE_CALLBACK (set_x86_sh
 # endif
 #endif
 
+unsigned long int __rtld_global_ro_cachesize_non_temporal_divisor
+  attribute_hidden;
+
 #if CET_ENABLED
 # include <dl-cet.h>
 #endif
@@ -614,6 +617,7 @@ init_cpu_features (struct cpu_features *
   unsigned int stepping = 0;
   enum cpu_features_kind kind;
 
+  __rtld_global_ro_cachesize_non_temporal_divisor = 4;
 #if !HAS_CPUID
   if (__get_cpuid_max (0, 0) == 0)
     {
@@ -694,13 +698,13 @@ init_cpu_features (struct cpu_features *
 
 	      /* Bigcore/Default Tuning.  */
 	    default:
+	    default_tuning:
 	      /* Unknown family 0x06 processors.  Assuming this is one
 		 of Core i3/i5/i7 processors if AVX is available.  */
 	      if (!CPU_FEATURES_CPU_P (cpu_features, AVX))
 		break;
-	      /* Fall through.  */
-	    case INTEL_BIGCORE_NEHALEM:
-	    case INTEL_BIGCORE_WESTMERE:
+
+	    enable_modern_features:
 	      /* Rep string instructions, unaligned load, unaligned copy,
 		 and pminub are fast on Intel Core i3, i5 and i7.  */
 	      cpu_features->preferred[index_arch_Fast_Rep_String]
@@ -710,12 +714,23 @@ init_cpu_features (struct cpu_features *
 		      | bit_arch_Prefer_PMINUB_for_stringop);
 	      break;
 
-	   /*
-	    Default tuned Bigcore microarch.
+	    case INTEL_BIGCORE_NEHALEM:
+	    case INTEL_BIGCORE_WESTMERE:
+	      /* Older CPUs prefer non-temporal stores at lower threshold.  */
+	      __rtld_global_ro_cachesize_non_temporal_divisor = 8;
+	      goto enable_modern_features;
+
+	      /* Older Bigcore microarch (smaller non-temporal store
+		 threshold).  */
 	    case INTEL_BIGCORE_SANDYBRIDGE:
 	    case INTEL_BIGCORE_IVYBRIDGE:
 	    case INTEL_BIGCORE_HASWELL:
 	    case INTEL_BIGCORE_BROADWELL:
+	      __rtld_global_ro_cachesize_non_temporal_divisor = 8;
+	      goto default_tuning;
+
+	      /* Newer Bigcore microarch (larger non-temporal store
+		 threshold).  */
 	    case INTEL_BIGCORE_SKYLAKE:
 	    case INTEL_BIGCORE_KABYLAKE:
 	    case INTEL_BIGCORE_COMETLAKE:
@@ -731,13 +746,14 @@ init_cpu_features (struct cpu_features *
 	    case INTEL_BIGCORE_SAPPHIRERAPIDS:
 	    case INTEL_BIGCORE_EMERALDRAPIDS:
 	    case INTEL_BIGCORE_GRANITERAPIDS:
-	    */
+	      __rtld_global_ro_cachesize_non_temporal_divisor = 2;
+	      goto default_tuning;
 
-	   /*
-	    Default tuned Mixed (bigcore + atom SOC).
+	      /* Default tuned Mixed (bigcore + atom SOC). */
 	    case INTEL_MIXED_LAKEFIELD:
 	    case INTEL_MIXED_ALDERLAKE:
-	    */
+	      __rtld_global_ro_cachesize_non_temporal_divisor = 2;
+	      goto default_tuning;
 	    }
 
 	      /* Disable TSX on some processors to avoid TSX on kernels that
diff -rup a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
--- a/sysdeps/x86/dl-cacheinfo.h	2023-07-26 17:56:18.662261475 -0400
+++ b/sysdeps/x86/dl-cacheinfo.h	2023-07-26 17:56:20.756342261 -0400
@@ -744,19 +744,25 @@ dl_init_cacheinfo (struct cpu_features *
   cpu_features->level3_cache_linesize = level3_cache_linesize;
   cpu_features->level4_cache_size = level4_cache_size;
 
-  /* The default setting for the non_temporal threshold is 1/4 of size
-     of the chip's cache. For most Intel and AMD processors with an
-     initial release date between 2017 and 2023, a thread's typical
-     share of the cache is from 18-64MB. Using the 1/4 L3 is meant to
-     estimate the point where non-temporal stores begin out-competing
-     REP MOVSB. As well the point where the fact that non-temporal
-     stores are forced back to main memory would already occurred to the
-     majority of the lines in the copy. Note, concerns about the
-     entire L3 cache being evicted by the copy are mostly alleviated
-     by the fact that 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;
+  unsigned long int cachesize_non_temporal_divisor
+      = __rtld_global_ro_cachesize_non_temporal_divisor;
+  if (cachesize_non_temporal_divisor <= 0)
+    cachesize_non_temporal_divisor = 4;
+
+  /* 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
+     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
+     non-temporal stores are forced back to main memory would already occurred
+     to the majority of the lines in the copy. Note, concerns about the entire
+     L3 cache being evicted by the copy are mostly alleviated by the fact that
+     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 / cachesize_non_temporal_divisor;
   /* 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
diff -rup a/sysdeps/x86/dl-diagnostics-cpu.c b/sysdeps/x86/dl-diagnostics-cpu.c
--- a/sysdeps/x86/dl-diagnostics-cpu.c	2021-08-01 21:33:43.000000000 -0400
+++ b/sysdeps/x86/dl-diagnostics-cpu.c	2023-07-26 17:56:20.761342454 -0400
@@ -117,4 +117,6 @@ _dl_diagnostics_cpu (void)
                   + sizeof (cpu_features->level4_cache_size)
                   == sizeof (*cpu_features),
                   "last cpu_features field has been printed");
+  print_cpu_features_value ("cachesize_non_temporal_divisor",
+			    __rtld_global_ro_cachesize_non_temporal_divisor);
 }
diff -rup a/sysdeps/x86/include/cpu-features.h b/sysdeps/x86/include/cpu-features.h
--- a/sysdeps/x86/include/cpu-features.h	2021-08-01 21:33:43.000000000 -0400
+++ b/sysdeps/x86/include/cpu-features.h	2023-07-27 13:51:52.081494751 -0400
@@ -919,6 +919,10 @@ struct cpu_features
   unsigned long int level4_cache_size;
 };
 
+/* When no user non_temporal_threshold is specified. We default to
+   cachesize / cachesize_non_temporal_divisor.  */
+extern unsigned long int __rtld_global_ro_cachesize_non_temporal_divisor;
+
 /* Get a pointer to the CPU features structure.  */
 extern const struct cpu_features *_dl_x86_get_cpu_features (void)
      __attribute__ ((pure));