[PATCH] fs: place f_ref to 3rd cache line in struct file to resolve false sharing

[Pan Deng <pan.deng@intel.com>]

When running syscall pread in a high core count system, f_ref contends
with the reading of f_mode, f_op, f_mapping, f_inode, f_flags in the
same cache line.

This change places f_ref to the 3rd cache line where fields are not
updated as frequently as the 1st cache line, and the contention is
grealy reduced according to tests. In addition, the size of file
object is kept in 3 cache lines.

This change has been tested with rocksdb benchmark readwhilewriting case
in 1 socket 64 physical core 128 logical core baremetal machine, with
build config CONFIG_RANDSTRUCT_NONE=y
Command:
./db_bench --benchmarks="readwhilewriting" --threads $cnt --duration 60
The throughput(ops/s) is improved up to ~21%.
=====
thread		baseline	compare
16		 100%		 +1.3%
32		 100%		 +2.2%
64		 100%		 +7.2%
128		 100%		 +20.9%

It was also tested with UnixBench: syscall, fsbuffer, fstime,
fsdisk cases that has been used for file struct layout tuning, no
regression was observed.

Signed-off-by: Pan Deng <pan.deng@intel.com>
Reviewed-by: Tianyou Li <tianyou.li@intel.com>
Reviewed-by: Tim Chen <tim.c.chen@linux.intel.com>
Tested-by: Lipeng Zhu <lipeng.zhu@intel.com>
---
 include/linux/fs.h | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index 2c3b2f8a621f..63b402cc9219 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1067,7 +1067,6 @@ static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
 
 /**
  * struct file - Represents a file
- * @f_ref: reference count
  * @f_lock: Protects f_ep, f_flags. Must not be taken from IRQ context.
  * @f_mode: FMODE_* flags often used in hotpaths
  * @f_op: file operations
@@ -1090,9 +1089,9 @@ static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
  * @f_llist: work queue entrypoint
  * @f_ra: file's readahead state
  * @f_freeptr: Pointer used by SLAB_TYPESAFE_BY_RCU file cache (don't touch.)
+ * @f_ref: reference count
  */
 struct file {
-	file_ref_t			f_ref;
 	spinlock_t			f_lock;
 	fmode_t				f_mode;
 	const struct file_operations	*f_op;
@@ -1102,6 +1101,7 @@ struct file {
 	unsigned int			f_flags;
 	unsigned int			f_iocb_flags;
 	const struct cred		*f_cred;
+	u8				padding[8];
 	/* --- cacheline 1 boundary (64 bytes) --- */
 	struct path			f_path;
 	union {
@@ -1127,6 +1127,7 @@ struct file {
 		struct file_ra_state	f_ra;
 		freeptr_t		f_freeptr;
 	};
+	file_ref_t			f_ref;
 	/* --- cacheline 3 boundary (192 bytes) --- */
 } __randomize_layout
   __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */
-- 
2.43.5