Re: [RFC PATCH] fs: Move @f_count to different cacheline with @f_mode

[Shaokun Zhang]

Hi maintainers,

A gentle ping.

Thanks,
Shaokun

On 2020/4/30 11:25, Shaokun Zhang wrote:
> From: Yuqi Jin <jinyuqi@xxxxxxxxxx>
> 
> __fget_files does check the @f_mode with mask variable and will do some
> atomic operations on @f_count while both are on the same cacheline.
> Many CPU cores do file access and it will cause much conflicts on @f_count. 
> If we could make the two members into different cachelines, it shall relax
> the siutations.
> 
> We have tested this on ARM64 and X86, the result is as follows:
> 
> Syscall of unixbench has been run on Huawei Kunpeng920 with this patch:
> 24 x System Call Overhead  1
> 
> System Call Overhead                    3160841.4 lps   (10.0 s, 1 samples)
> 
> System Benchmarks Partial Index              BASELINE       RESULT    INDEX
> System Call Overhead                          15000.0    3160841.4   2107.2
>                                                                    ========
> System Benchmarks Index Score (Partial Only)                         2107.2
> 
> Without this patch:
> 24 x System Call Overhead  1
> 
> System Call Overhead                    2222456.0 lps   (10.0 s, 1 samples)
> 
> System Benchmarks Partial Index              BASELINE       RESULT    INDEX
> System Call Overhead                          15000.0    2222456.0   1481.6
>                                                                    ========
> System Benchmarks Index Score (Partial Only)                         1481.6
> 
> And on Intel 6248 platform with this patch:
> 40 CPUs in system; running 24 parallel copies of tests
> 
> System Call Overhead                        4288509.1 lps   (10.0 s, 1 samples)
> 
> System Benchmarks Partial Index              BASELINE       RESULT    INDEX
> System Call Overhead                          15000.0    4288509.1   2859.0
>                                                                    ========
> System Benchmarks Index Score (Partial Only)                         2859.0
> 
> Without this patch:
> 40 CPUs in system; running 24 parallel copies of tests
> 
> System Call Overhead                        3666313.0 lps   (10.0 s, 1 samples)
> 
> System Benchmarks Partial Index              BASELINE       RESULT    INDEX
> System Call Overhead                          15000.0    3666313.0   2444.2
>                                                                    ========
> System Benchmarks Index Score (Partial Only)                         2444.2
> 
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Yuqi Jin <jinyuqi@xxxxxxxxxx>
> Signed-off-by: Shaokun Zhang <zhangshaokun@xxxxxxxxxxxxx>
> ---
>  include/linux/fs.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 4f6f59b4f22a..90e76283f0fd 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -953,7 +953,6 @@ struct file {
>  	 */
>  	spinlock_t		f_lock;
>  	enum rw_hint		f_write_hint;
> -	atomic_long_t		f_count;
>  	unsigned int 		f_flags;
>  	fmode_t			f_mode;
>  	struct mutex		f_pos_lock;
> @@ -976,6 +975,7 @@ struct file {
>  #endif /* #ifdef CONFIG_EPOLL */
>  	struct address_space	*f_mapping;
>  	errseq_t		f_wb_err;
> +	atomic_long_t		f_count;
>  } __randomize_layout
>    __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */
>  
>