From f8ce8234aa7247c8f3a4317f5671b5fd40ffb8c8 Mon Sep 17 00:00:00 2001
From: Ian Moffett <ian@osmora.org>
Date: Fri, 7 Jun 2024 21:25:10 -0400
Subject: kernel: Add __cacheline_aligned macro

This commit introduces a "__cacheline_aligned" macro which aligns data
by the cache line size (COHERENCY_UNIT bytes). This is useful for
heavily contended locks.

Signed-off-by: Ian Moffett <ian@osmora.org>
---
 sys/arch/amd64/conf/link.ld |  8 +++++++-
 sys/include/sys/cdefs.h     | 17 +++++++++++++++++
 sys/include/sys/param.h     |  5 +++++
 3 files changed, 29 insertions(+), 1 deletion(-)

(limited to 'sys')

diff --git a/sys/arch/amd64/conf/link.ld b/sys/arch/amd64/conf/link.ld
index b073054..9c47a81 100644
--- a/sys/arch/amd64/conf/link.ld
+++ b/sys/arch/amd64/conf/link.ld
@@ -32,7 +32,7 @@ SECTIONS
     . += CONSTANT(MAXPAGESIZE);
 
     .data : {
-        *(.data .data.*)
+        *(.data)
     } :data
 
     .bss : {
@@ -40,6 +40,12 @@ SECTIONS
         *(.bss .bss.*)
     } :data
 
+    /* -- Cache line alignment -- */
+    . = ALIGN(64);
+    .data.cacheline_aligned : {
+        *(.data.cacheline_aligned)
+    }
+
     /DISCARD/ : {
         *(.eh_frame)
         *(.note .note.*)
diff --git a/sys/include/sys/cdefs.h b/sys/include/sys/cdefs.h
index fc21750..655a023 100644
--- a/sys/include/sys/cdefs.h
+++ b/sys/include/sys/cdefs.h
@@ -30,10 +30,27 @@
 #ifndef _SYS_CDEFS_H_
 #define _SYS_CDEFS_H_
 
+#include <sys/param.h>
+
 #define __ASMV          __asm__ __volatile__
 #define __always_inline __attribute__((__always_inline__))
 #define __packed        __attribute__((__packed__))
 #define __likely(exp)   __builtin_expect(((exp) != 0), 1)
 #define __unlikely(exp) __builtin_expect(((exp) != 0), 0)
 
+#if defined(_KERNEL)
+/*
+ *  Align data on a cache line boundary. This is
+ *  mostly useful for certain locks to ensure they
+ *  have their own cache line to reduce contention.
+ *
+ */
+#ifndef __cacheline_aligned
+#define __cacheline_aligned                             \
+    __attribute__((__aligned__(COHERENCY_UNIT),         \
+                __section__(".data.cacheline_aligned")))
+
+#endif  /* __cacheline_aligned */
+#endif  /* _KERNEL */
+
 #endif  /* !_SYS_CDEFS_H_ */
diff --git a/sys/include/sys/param.h b/sys/include/sys/param.h
index a35a094..483dc15 100644
--- a/sys/include/sys/param.h
+++ b/sys/include/sys/param.h
@@ -30,6 +30,11 @@
 #ifndef _SYS_PARAM_H_
 #define _SYS_PARAM_H_
 
+/* Assumed cache line size */
+#ifndef COHERENCY_UNIT
+#define COHERENCY_UNIT 64
+#endif
+
 /* Bit related macros */
 #define ISSET(v, f)  ((v) & (f))
 #define BIT(n) (1 << (n))
-- 
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