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Date:   Thu, 1 Jun 2023 12:06:30 +0200
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Subject: Re: [PATCH] fs.h: Optimize file struct to prevent false sharing
To:     chenzhiyin <zhiyin.chen@intel.com>, viro@zeniv.linux.org.uk,
        brauner@kernel.org
Cc:     linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        nanhai.zou@intel.com
References: <20230531-wahlkabine-unantastbar-9f73a13262c0@brauner>
 <20230601092400.27162-1-zhiyin.chen@intel.com>
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From:   Bernd Schubert <bernd.schubert@fastmail.fm>
In-Reply-To: <20230601092400.27162-1-zhiyin.chen@intel.com>
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On 6/1/23 11:24, chenzhiyin wrote:
> In the syscall test of UnixBench, performance regression occurred due
> to false sharing.
> 
> The lock and atomic members, including file::f_lock, file::f_count and
> file::f_pos_lock are highly contended and frequently updated in the
> high-concurrency test scenarios. perf c2c indentified one affected
> read access, file::f_op.
> To prevent false sharing, the layout of file struct is changed as
> following
> (A) f_lock, f_count and f_pos_lock are put together to share the same
> cache line.
> (B) The read mostly members, including f_path, f_inode, f_op are put
> into a separate cache line.
> (C) f_mode is put together with f_count, since they are used frequently
>   at the same time.
> Due to '__randomize_layout' attribute of file struct, the updated layout
> only can be effective when CONFIG_RANDSTRUCT_NONE is 'y'.
> 
> The optimization has been validated in the syscall test of UnixBench.
> performance gain is 30~50%. Furthermore, to confirm the optimization
> effectiveness on the other codes path, the results of fsdisk, fsbuffer
> and fstime are also shown.
> 
> Here are the detailed test results of unixbench.
> 
> Command: numactl -C 3-18 ./Run -c 16 syscall fsbuffer fstime fsdisk
> 
> Without Patch
> ------------------------------------------------------------------------
> File Copy 1024 bufsize 2000 maxblocks   875052.1 KBps  (30.0 s, 2 samples)
> File Copy 256 bufsize 500 maxblocks     235484.0 KBps  (30.0 s, 2 samples)
> File Copy 4096 bufsize 8000 maxblocks  2815153.5 KBps  (30.0 s, 2 samples)
> System Call Overhead                   5772268.3 lps   (10.0 s, 7 samples)
> 
> System Benchmarks Partial Index         BASELINE       RESULT    INDEX
> File Copy 1024 bufsize 2000 maxblocks     3960.0     875052.1   2209.7
> File Copy 256 bufsize 500 maxblocks       1655.0     235484.0   1422.9
> File Copy 4096 bufsize 8000 maxblocks     5800.0    2815153.5   4853.7
> System Call Overhead                     15000.0    5772268.3   3848.2
>                                                                ========
> System Benchmarks Index Score (Partial Only)                    2768.3
> 
> With Patch
> ------------------------------------------------------------------------
> File Copy 1024 bufsize 2000 maxblocks  1009977.2 KBps  (30.0 s, 2 samples)
> File Copy 256 bufsize 500 maxblocks     264765.9 KBps  (30.0 s, 2 samples)
> File Copy 4096 bufsize 8000 maxblocks  3052236.0 KBps  (30.0 s, 2 samples)
> System Call Overhead                   8237404.4 lps   (10.0 s, 7 samples)
> 
> System Benchmarks Partial Index         BASELINE       RESULT    INDEX
> File Copy 1024 bufsize 2000 maxblocks     3960.0    1009977.2   2550.4
> File Copy 256 bufsize 500 maxblocks       1655.0     264765.9   1599.8
> File Copy 4096 bufsize 8000 maxblocks     5800.0    3052236.0   5262.5
> System Call Overhead                     15000.0    8237404.4   5491.6
>                                                                ========
> System Benchmarks Index Score (Partial Only)                    3295.3
> 
> Signed-off-by: chenzhiyin <zhiyin.chen@intel.com>
> ---
>   include/linux/fs.h | 10 +++++-----
>   1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 133f0640fb24..cf1388e4dad0 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -962,23 +962,23 @@ struct file {
>   		struct rcu_head 	f_rcuhead;
>   		unsigned int 		f_iocb_flags;
>   	};
> -	struct path		f_path;
> -	struct inode		*f_inode;	/* cached value */
> -	const struct file_operations	*f_op;
>   
>   	/*
>   	 * Protects f_ep, f_flags.
>   	 * Must not be taken from IRQ context.
>   	 */
>   	spinlock_t		f_lock;
> -	atomic_long_t		f_count;
> -	unsigned int 		f_flags;
>   	fmode_t			f_mode;
> +	atomic_long_t		f_count;
>   	struct mutex		f_pos_lock;
>   	loff_t			f_pos;
> +	unsigned int		f_flags;
>   	struct fown_struct	f_owner;
>   	const struct cred	*f_cred;
>   	struct file_ra_state	f_ra;
> +	struct path		f_path;
> +	struct inode		*f_inode;	/* cached value */
> +	const struct file_operations	*f_op;
>   
>   	u64			f_version;
>   #ifdef CONFIG_SECURITY

Maybe add a comment for the struct that values are cache line optimized? 
I.e. any change in the structure that does not check for cache lines 
might/will invalidate your optimization - your patch adds maintenance 
overhead, without giving a hint about that.


Thanks,
Bernd