linux-kernel - [PATCH RESEND] fs: Move @f_count to different cacheline with @f

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Date:   Wed, 24 Jun 2020 16:32:28 +0800
From:   Shaokun Zhang <zhangshaokun@...ilicon.com>
To:     <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>
CC:     Shaokun Zhang <zhangshaokun@...ilicon.com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Boqun Feng <boqun.feng@...il.com>,
        Yuqi Jin <jinyuqi@...wei.com>
Subject: [PATCH RESEND] fs: Move @f_count to different cacheline with @f_mode

get_file_rcu_many, which is called by __fget_files, has used
atomic_try_cmpxchg now and it can reduce the access number of the global
variable to improve the performance of atomic instruction compared with
atomic_cmpxchg. 

__fget_files does check the @f_mode with mask variable and will do some
atomic operations on @f_count, but 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: Will Deacon <will@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Boqun Feng <boqun.feng@...il.com>
Signed-off-by: Yuqi Jin <jinyuqi@...wei.com>
Signed-off-by: Shaokun Zhang <zhangshaokun@...ilicon.com>
---
 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 3f881a892ea7..0faeab5622fb 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -955,7 +955,6 @@ 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;
@@ -979,6 +978,7 @@ struct file {
 	struct address_space	*f_mapping;
 	errseq_t		f_wb_err;
 	errseq_t		f_sb_err; /* for syncfs */
+	atomic_long_t		f_count;
 } __randomize_layout
   __attribute__((aligned(4)));	/* lest something weird decides that 2 is OK */
 
-- 
2.7.4