[tip: x86/core] x86: Put hot per CPU variables into a struct

tip-bot2 for Thomas Gleixner
2022-10-17T14:57:15Z

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

Commit-ID: e57ef2ed97c1d078973298658a8096644a1e9e09
Gitweb: https://git.kernel.org/tip/e57ef2ed97c1d078973298658a8096644a1e9e09
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Thu, 15 Sep 2022 13:11:01 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Mon, 17 Oct 2022 16:41:03 +02:00

x86: Put hot per CPU variables into a struct

The layout of per-cpu variables is at the mercy of the compiler. This
can lead to random performance fluctuations from build to build.

Create a structure to hold some of the hottest per-cpu variables,
starting with current_task.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111145.179707194@infradead.org
---
arch/x86/include/asm/current.h | 19 ++++++++++++++++---
arch/x86/kernel/cpu/common.c | 14 +++++---------
arch/x86/kernel/process_32.c | 2 +-
arch/x86/kernel/process_64.c | 2 +-
arch/x86/kernel/smpboot.c | 2 +-
5 files changed, 24 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/current.h b/arch/x86/include/asm/current.h
index 3e204e6..63c42ac 100644
--- a/arch/x86/include/asm/current.h
+++ b/arch/x86/include/asm/current.h
@@ -3,16 +3,29 @@
#define _ASM_X86_CURRENT_H

#include <linux/compiler.h>
-#include <asm/percpu.h>

#ifndef __ASSEMBLY__
+
+#include <linux/cache.h>
+#include <asm/percpu.h>
+
struct task_struct;

-DECLARE_PER_CPU(struct task_struct *, current_task);
+struct pcpu_hot {
+ union {
+ struct {
+ struct task_struct *current_task;
+ };
+ u8 pad[64];
+ };
+};
+static_assert(sizeof(struct pcpu_hot) == 64);
+
+DECLARE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot);

static __always_inline struct task_struct *get_current(void)
{
- return this_cpu_read_stable(current_task);
+ return this_cpu_read_stable(pcpu_hot.current_task);
}

#define current get_current()
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 8e87318..5207153 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2012,18 +2012,16 @@ static __init int setup_clearcpuid(char *arg)
}
__setup("clearcpuid=", setup_clearcpuid);

+DEFINE_PER_CPU_ALIGNED(struct pcpu_hot, pcpu_hot) = {
+ .current_task = &init_task,
+};
+EXPORT_PER_CPU_SYMBOL(pcpu_hot);
+
#ifdef CONFIG_X86_64
DEFINE_PER_CPU_FIRST(struct fixed_percpu_data,
fixed_percpu_data) __aligned(PAGE_SIZE) __visible;
EXPORT_PER_CPU_SYMBOL_GPL(fixed_percpu_data);

-/*
- * The following percpu variables are hot. Align current_task to
- * cacheline size such that they fall in the same cacheline.
- */
-DEFINE_PER_CPU(struct task_struct *, current_task) ____cacheline_aligned =
- &init_task;
-EXPORT_PER_CPU_SYMBOL(current_task);

DEFINE_PER_CPU(void *, hardirq_stack_ptr);
DEFINE_PER_CPU(bool, hardirq_stack_inuse);
@@ -2083,8 +2081,6 @@ void syscall_init(void)

#else /* CONFIG_X86_64 */

-DEFINE_PER_CPU(struct task_struct *, current_task) = &init_task;
-EXPORT_PER_CPU_SYMBOL(current_task);
DEFINE_PER_CPU(int, __preempt_count) = INIT_PREEMPT_COUNT;
EXPORT_PER_CPU_SYMBOL(__preempt_count);

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 2f314b1..807da45 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -207,7 +207,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
if (prev->gs | next->gs)
loadsegment(gs, next->gs);

- this_cpu_write(current_task, next_p);
+ raw_cpu_write(pcpu_hot.current_task, next_p);

switch_fpu_finish();

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 6b3418b..c4f6cac 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -617,7 +617,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
/*
* Switch the PDA and FPU contexts.
*/
- this_cpu_write(current_task, next_p);
+ raw_cpu_write(pcpu_hot.current_task, next_p);
this_cpu_write(cpu_current_top_of_stack, task_top_of_stack(next_p));

switch_fpu_finish();
diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index ce8728d..05f3157 100644
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -1046,7 +1046,7 @@ int common_cpu_up(unsigned int cpu, struct task_struct *idle)
/* Just in case we booted with a single CPU. */
alternatives_enable_smp();

- per_cpu(current_task, cpu) = idle;
+ per_cpu(pcpu_hot.current_task, cpu) = idle;
cpu_init_stack_canary(cpu, idle);

/* Initialize the interrupt stack(s) */