2022-02-09 08:31:36

by Jisheng Zhang

Subject: [PATCH] arm64: Add read_mostly declaration/definition to irq stack ptr

Add "read-mostly" qualifier to irq_stack_ptr and
irq_shadow_call_stack_ptr. This is to prevent the false sharing.

Before the patch, I got below percpu layout with one defconfig:
ffffffc008723050 <mde_ref_count>:
ffffffc008723050: 00 00 00 00
....

ffffffc008723054 <kde_ref_count>:
ffffffc008723054: 00 00 00 00
....

ffffffc008723058 <irq_stack_ptr>:
...

ffffffc008723060 <nmi_contexts>:
...

ffffffc008723070 <fpsimd_last_state>:

As can be seen, the irq_stack_ptr sits with the heavy read/write percpu
vars such as fpsimd_last_state etc. at the same cacheline.

After the patch:

ffffffc008723000 <irq_stack_ptr>:
...

ffffffc008723008 <cpu_number>:
...

ffffffc008723010 <arm64_ssbd_callback_required>:
...

ffffffc008723018 <bp_hardening_data>:
...

Now, the irq_stack_ptr sits with read mostly percpu vars such as
cpu_number etc. at the same cacheline.

Signed-off-by: Jisheng Zhang <[email protected]>
---
arch/arm64/include/asm/stacktrace.h | 2 +-
arch/arm64/kernel/irq.c | 7 +++----
2 files changed, 4 insertions(+), 5 deletions(-)

diff --git a/arch/arm64/include/asm/stacktrace.h b/arch/arm64/include/asm/stacktrace.h
index e77cdef9ca29..75c142bfdffe 100644
--- a/arch/arm64/include/asm/stacktrace.h
+++ b/arch/arm64/include/asm/stacktrace.h
@@ -66,7 +66,7 @@ struct stackframe {
extern void dump_backtrace(struct pt_regs *regs, struct task_struct *tsk,
const char *loglvl);

-DECLARE_PER_CPU(unsigned long *, irq_stack_ptr);
+DECLARE_PER_CPU_READ_MOSTLY(unsigned long *, irq_stack_ptr);

static inline bool on_stack(unsigned long sp, unsigned long size,
unsigned long low, unsigned long high,
diff --git a/arch/arm64/kernel/irq.c b/arch/arm64/kernel/irq.c
index bda49430c9ea..d2e75e9bb826 100644
--- a/arch/arm64/kernel/irq.c
+++ b/arch/arm64/kernel/irq.c
@@ -26,13 +26,12 @@
/* Only access this in an NMI enter/exit */
DEFINE_PER_CPU(struct nmi_ctx, nmi_contexts);

-DEFINE_PER_CPU(unsigned long *, irq_stack_ptr);
+DEFINE_PER_CPU_READ_MOSTLY(unsigned long *, irq_stack_ptr);

-
-DECLARE_PER_CPU(unsigned long *, irq_shadow_call_stack_ptr);
+DECLARE_PER_CPU_READ_MOSTLY(unsigned long *, irq_shadow_call_stack_ptr);

#ifdef CONFIG_SHADOW_CALL_STACK
-DEFINE_PER_CPU(unsigned long *, irq_shadow_call_stack_ptr);
+DEFINE_PER_CPU_READ_MOSTLY(unsigned long *, irq_shadow_call_stack_ptr);
#endif

static void init_irq_scs(void)
--
2.34.1

2022-02-16 19:42:49

by Will Deacon

Subject: Re: [PATCH] arm64: Add read_mostly declaration/definition to irq stack ptr

On Mon, Feb 07, 2022 at 04:46:42PM +0800, Jisheng Zhang wrote:
> Add "read-mostly" qualifier to irq_stack_ptr and
> irq_shadow_call_stack_ptr. This is to prevent the false sharing.
>
> Before the patch, I got below percpu layout with one defconfig:
> ffffffc008723050 <mde_ref_count>:
> ffffffc008723050: 00 00 00 00
> ....
>
> ffffffc008723054 <kde_ref_count>:
> ffffffc008723054: 00 00 00 00
> ....
>
> ffffffc008723058 <irq_stack_ptr>:
> ...
>
> ffffffc008723060 <nmi_contexts>:
> ...
>
> ffffffc008723070 <fpsimd_last_state>:
>
> As can be seen, the irq_stack_ptr sits with the heavy read/write percpu
> vars such as fpsimd_last_state etc. at the same cacheline.
>
> After the patch:
>
> ffffffc008723000 <irq_stack_ptr>:
> ...
>
> ffffffc008723008 <cpu_number>:
> ...
>
> ffffffc008723010 <arm64_ssbd_callback_required>:
> ...
>
> ffffffc008723018 <bp_hardening_data>:
> ...
>
> Now, the irq_stack_ptr sits with read mostly percpu vars such as
> cpu_number etc. at the same cacheline.

Were you able to measure any performance difference after this change?

Will