Commit 99c228a9 authored Oct 18, 2018 by Amir Goldstein Committed by Jan Kara Oct 19, 2018

fs: group frequently accessed fields of struct super_block together

Kernel test robot reported [1] a 6% performance regression in a
concurrent unlink(2) workload on commit 60f7ed8c ("fsnotify: send
path type events to group with super block marks").

The performance test was run with no fsnotify marks at all on the
data set, so the only extra instructions added by the offending
commit are tests of the super_block fields s_fsnotify_{marks,mask}
and these tests happen on almost every single inode access.

When adding those fields to the super_block struct, we did not give much
thought of placing them on a hot cache lines (we just placed them at the
end of the struct).

Re-organize struct super_block to try and keep some frequently accessed
fields on the same cache line.

Move the frequently accessed fields s_fsnotify_{marks,mask} near the
frequently accessed fields s_fs_info,s_time_gran, while filling a 64bit
alignment hole after s_time_gran.

Move the seldom accessed fields s_id,s_uuid,s_max_links,s_mode near the
seldom accessed fields s_vfs_rename_mutex,s_subtype.

Rong Chen confirmed that this patch solved the reported problem.

[1] https://lkml.org/lkml/2018/9/30/206



Reported-by: kernel test robot <rong.a.chen@intel.com>
Tested-by: kernel test robot <rong.a.chen@intel.com>
Fixes: 1e6cb723 ("fsnotify: add super block object type")
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>

parent d0a6a87e

include/linux/fs.h

+14 −10

Original line number	Diff line number	Diff line
		@@ -1393,17 +1393,26 @@ struct super_block {

		struct sb_writers s_writers;

		/*
		* Keep s_fs_info, s_time_gran, s_fsnotify_mask, and
		* s_fsnotify_marks together for cache efficiency. They are frequently
		* accessed and rarely modified.
		*/
		void s_fs_info; / Filesystem private info */

		/* Granularity of c/m/atime in ns (cannot be worse than a second) */
		u32 s_time_gran;
		#ifdef CONFIG_FSNOTIFY
		__u32 s_fsnotify_mask;
		struct fsnotify_mark_connector __rcu *s_fsnotify_marks;
		#endif

		char s_id[32]; /* Informational name */
		uuid_t s_uuid; /* UUID */

		void s_fs_info; / Filesystem private info */
		unsigned int s_max_links;
		fmode_t s_mode;

		/* Granularity of c/m/atime in ns.
		Cannot be worse than a second */
		u32 s_time_gran;

		/*
		* The next field is for VFS only. No filesystems have any business
		* even looking at it. You had been warned.
		@@ -1464,11 +1473,6 @@ struct super_block {

		spinlock_t s_inode_wblist_lock;
		struct list_head s_inodes_wb; /* writeback inodes */

		#ifdef CONFIG_FSNOTIFY
		__u32 s_fsnotify_mask;
		struct fsnotify_mark_connector __rcu *s_fsnotify_marks;
		#endif
		} __randomize_layout;

		/* Helper functions so that in most cases filesystems will

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