linux-kernel - [tip: x86/core] percpu: Introduce percpu hot section

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Message-ID: <174107687023.14745.3503430539442439687.tip-bot2@tip-bot2>
Date: Tue, 04 Mar 2025 08:27:50 -0000
From: "tip-bot2 for Brian Gerst" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Brian Gerst <brgerst@...il.com>, Ingo Molnar <mingo@...nel.org>,
 Uros Bizjak <ubizjak@...il.com>,
 Linus Torvalds <torvalds@...ux-foundation.org>,
 Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org
Subject: [tip: x86/core] percpu: Introduce percpu hot section

The following commit has been merged into the x86/core branch of tip:

Commit-ID:     56f91085a914d807477fc90b414998dcd7a0d0c3
Gitweb:        https://git.kernel.org/tip/56f91085a914d807477fc90b414998dcd7a0d0c3
Author:        Brian Gerst <brgerst@...il.com>
AuthorDate:    Mon, 03 Mar 2025 11:52:36 -05:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Mon, 03 Mar 2025 21:07:16 +01:00

percpu: Introduce percpu hot section

Add a subsection to the percpu data for frequently accessed variables
that should remain cached on each processor.  These varables should not
be accessed from other processors to avoid cacheline bouncing.

This will replace the pcpu_hot struct on x86, and open up similar
functionality to other architectures and the kernel core.

Signed-off-by: Brian Gerst <brgerst@...il.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Acked-by: Uros Bizjak <ubizjak@...il.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Link: https://lore.kernel.org/r/20250303165246.2175811-2-brgerst@gmail.com
---
 include/asm-generic/vmlinux.lds.h | 11 +++++++++++
 include/linux/percpu-defs.h       | 13 +++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index b32e453..c4e8fac 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -385,6 +385,11 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 	. = ALIGN(PAGE_SIZE);						\
 	__nosave_end = .;
 
+#define CACHE_HOT_DATA(align)						\
+	. = ALIGN(align);						\
+	*(SORT_BY_ALIGNMENT(.data..hot.*))				\
+	. = ALIGN(align);
+
 #define PAGE_ALIGNED_DATA(page_align)					\
 	. = ALIGN(page_align);						\
 	*(.data..page_aligned)						\
@@ -1065,6 +1070,11 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 	. = ALIGN(PAGE_SIZE);						\
 	*(.data..percpu..page_aligned)					\
 	. = ALIGN(cacheline);						\
+	__per_cpu_hot_start = .;					\
+	*(SORT_BY_ALIGNMENT(.data..percpu..hot.*))			\
+	__per_cpu_hot_pad = .;						\
+	. = ALIGN(cacheline);						\
+	__per_cpu_hot_end = .;						\
 	*(.data..percpu..read_mostly)					\
 	. = ALIGN(cacheline);						\
 	*(.data..percpu)						\
@@ -1112,6 +1122,7 @@ defined(CONFIG_AUTOFDO_CLANG) || defined(CONFIG_PROPELLER_CLANG)
 		INIT_TASK_DATA(inittask)				\
 		NOSAVE_DATA						\
 		PAGE_ALIGNED_DATA(pagealigned)				\
+		CACHE_HOT_DATA(cacheline)				\
 		CACHELINE_ALIGNED_DATA(cacheline)			\
 		READ_MOSTLY_DATA(cacheline)				\
 		DATA_DATA						\
diff --git a/include/linux/percpu-defs.h b/include/linux/percpu-defs.h
index 40d34e0..0fcacb9 100644
--- a/include/linux/percpu-defs.h
+++ b/include/linux/percpu-defs.h
@@ -113,6 +113,19 @@
 	DEFINE_PER_CPU_SECTION(type, name, "")
 
 /*
+ * Declaration/definition used for per-CPU variables that are frequently
+ * accessed and should be in a single cacheline.
+ *
+ * For use only by architecture and core code.  Only use scalar or pointer
+ * types to maximize density.
+ */
+#define DECLARE_PER_CPU_CACHE_HOT(type, name)				\
+	DECLARE_PER_CPU_SECTION(type, name, "..hot.." #name)
+
+#define DEFINE_PER_CPU_CACHE_HOT(type, name)				\
+	DEFINE_PER_CPU_SECTION(type, name, "..hot.." #name)
+
+/*
  * Declaration/definition used for per-CPU variables that must be cacheline
  * aligned under SMP conditions so that, whilst a particular instance of the
  * data corresponds to a particular CPU, inefficiencies due to direct access by