fs.h: Optimize file struct to prevent false sharing

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 <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

include/linux/fs.h

···
 		index <  ra->start + ra->size);
 }
 
+/*
+ * f_{lock,count,pos_lock} members can be highly contended and share
+ * the same cacheline. f_{lock,mode} are very frequently used together
+ * and so share the same cacheline as well. The read-mostly
+ * f_{path,inode,op} are kept on a separate cacheline.
+ */
 struct file {
 	union {
 		struct llist_node	f_llist;
 		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