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

[Shaokun Zhang]

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 */
 
-- 
2.7.4