2022-04-02 20:42:17

by Tianchen Ding

Subject: [RFC PATCH] sched: avoid unnecessary atomic_read when sync_core_before_usermode() is empty

On archs except x86, CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE is not
defined. We found membarrier_mm_sync_core_before_usermode() looks like
this when compiled by gcc10:

if (current->mm != mm)
return;
atomic_read(&mm->membarrier_state);

This memory access is unnecessary. Remove it to improve performance.

Signed-off-by: Tianchen Ding <[email protected]>
---
include/linux/sched/mm.h | 6 ++++++
1 file changed, 6 insertions(+)

diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
index a80356e9dc69..3ded68d9f913 100644
--- a/include/linux/sched/mm.h
+++ b/include/linux/sched/mm.h
@@ -401,6 +401,7 @@ enum {
#include <asm/membarrier.h>
#endif

+#ifdef CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
{
if (current->mm != mm)
@@ -410,6 +411,11 @@ static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
return;
sync_core_before_usermode();
}
+#else
+static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
+{
+}
+#endif

extern void membarrier_exec_mmap(struct mm_struct *mm);

--
2.33.0

2022-04-06 15:57:20

by Tianchen Ding

Subject: Re: [RFC PATCH] sched: avoid unnecessary atomic_read when sync_core_before_usermode() is empty

We've run schbench and found wakeup latency on some arm64 machines worse
than others. Perf shows there's a hotspot on
atomic_read(&mm->membarrier_state);

We're still working for the real reason behind it (maybe cache or sth
hardware related), and we do see remove this function can help improve
performance.

Thanks.

On 2022/4/2 11:08, Tianchen Ding wrote:
> On archs except x86, CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE is not
> defined. We found membarrier_mm_sync_core_before_usermode() looks like
> this when compiled by gcc10:
>
> if (current->mm != mm)
> return;
> atomic_read(&mm->membarrier_state);
>
> This memory access is unnecessary. Remove it to improve performance.
>
> Signed-off-by: Tianchen Ding <[email protected]>
> ---
> include/linux/sched/mm.h | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index a80356e9dc69..3ded68d9f913 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -401,6 +401,7 @@ enum {
> #include <asm/membarrier.h>
> #endif
>
> +#ifdef CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
> {
> if (current->mm != mm)
> @@ -410,6 +411,11 @@ static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
> return;
> sync_core_before_usermode();
> }
> +#else
> +static inline void membarrier_mm_sync_core_before_usermode(struct mm_struct *mm)
> +{
> +}
> +#endif
>
> extern void membarrier_exec_mmap(struct mm_struct *mm);
>