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SubjectRe: [RFC PATCH] sched: avoid unnecessary atomic_read when sync_core_before_usermode() is empty
From
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 <dtcccc@linux.alibaba.com>
> ---
> 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);
>

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