[PATCH v4] pipe: use __pipe_{lock,unlock} instead of spinlock

Hongchen Zhang
2023-01-29T06:05:16Z

Use spinlock in pipe_{read,write} cost too much time,IMO
pipe->{head,tail} can be protected by __pipe_{lock,unlock}.
On the other hand, we can use __pipe_{lock,unlock} to protect
the pipe->{head,tail} in pipe_resize_ring and
post_one_notification.

Reminded by Matthew, I tested this patch using UnixBench's pipe
test case on a x86_64 machine,and get the following data:
1) before this patch
System Benchmarks Partial Index BASELINE RESULT INDEX
Pipe Throughput 12440.0 493023.3 396.3
========
System Benchmarks Index Score (Partial Only) 396.3

2) after this patch
System Benchmarks Partial Index BASELINE RESULT INDEX
Pipe Throughput 12440.0 507551.4 408.0
========
System Benchmarks Index Score (Partial Only) 408.0

so we get ~3% speedup.

Reminded by Andrew, I tested this patch with the test code in
Linus's 0ddad21d3e99 and get following result:
1) before this patch
13,136.54 msec task-clock # 3.870 CPUs utilized
1,186,779 context-switches # 90.342 K/sec
668,867 cpu-migrations # 50.917 K/sec
895 page-faults # 68.131 /sec
29,875,711,543 cycles # 2.274 GHz
12,372,397,462 instructions # 0.41 insn per cycle
2,480,235,723 branches # 188.804 M/sec
47,191,943 branch-misses # 1.90% of all branches

3.394806886 seconds time elapsed

0.037869000 seconds user
0.189346000 seconds sys

2) after this patch

12,395.63 msec task-clock # 4.138 CPUs utilized
1,193,381 context-switches # 96.274 K/sec
585,543 cpu-migrations # 47.238 K/sec
1,063 page-faults # 85.756 /sec
27,691,587,226 cycles # 2.234 GHz
11,738,307,999 instructions # 0.42 insn per cycle
2,351,299,522 branches # 189.688 M/sec
45,404,526 branch-misses # 1.93% of all branches

2.995280878 seconds time elapsed

0.010615000 seconds user
0.206999000 seconds sys
After adding this patch, the time used on this test program becomes less.

Signed-off-by: Hongchen Zhang <zhanghongchen@loongson.cn>

v4:
- fixes a typo in changelog when reviewed by Sedat.
v3:
- fixes the error reported by kernel test robot <oliver.sang@intel.com>
Link: https://lore.kernel.org/oe-lkp/202301061340.c954d61f-oliver.sang@intel.com
- add perf stat data for the test code in Linus's 0ddad21d3e99 in
commit message.
v2:
- add UnixBench test data in commit message
- fixes the test error reported by kernel test robot <lkp@intel.com>
by adding the missing fs.h header file.
---
fs/pipe.c | 22 +---------------------
include/linux/pipe_fs_i.h | 12 ++++++++++++
kernel/watch_queue.c | 8 ++++----
3 files changed, 17 insertions(+), 25 deletions(-)

diff --git a/fs/pipe.c b/fs/pipe.c
index 42c7ff41c2db..4355ee5f754e 100644
--- a/fs/pipe.c
+++ b/fs/pipe.c
@@ -98,16 +98,6 @@ void pipe_unlock(struct pipe_inode_info *pipe)
}
EXPORT_SYMBOL(pipe_unlock);

-static inline void __pipe_lock(struct pipe_inode_info *pipe)
-{
- mutex_lock_nested(&pipe->mutex, I_MUTEX_PARENT);
-}
-
-static inline void __pipe_unlock(struct pipe_inode_info *pipe)
-{
- mutex_unlock(&pipe->mutex);
-}
-
void pipe_double_lock(struct pipe_inode_info *pipe1,
struct pipe_inode_info *pipe2)
{
@@ -253,8 +243,7 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)
*/
was_full = pipe_full(pipe->head, pipe->tail, pipe->max_usage);
for (;;) {
- /* Read ->head with a barrier vs post_one_notification() */
- unsigned int head = smp_load_acquire(&pipe->head);
+ unsigned int head = pipe->head;
unsigned int tail = pipe->tail;
unsigned int mask = pipe->ring_size - 1;

@@ -322,14 +311,12 @@ pipe_read(struct kiocb *iocb, struct iov_iter *to)

if (!buf->len) {
pipe_buf_release(pipe, buf);
- spin_lock_irq(&pipe->rd_wait.lock);
#ifdef CONFIG_WATCH_QUEUE
if (buf->flags & PIPE_BUF_FLAG_LOSS)
pipe->note_loss = true;
#endif
tail++;
pipe->tail = tail;
- spin_unlock_irq(&pipe->rd_wait.lock);
}
total_len -= chars;
if (!total_len)
@@ -506,16 +493,13 @@ pipe_write(struct kiocb *iocb, struct iov_iter *from)
* it, either the reader will consume it or it'll still
* be there for the next write.
*/
- spin_lock_irq(&pipe->rd_wait.lock);

head = pipe->head;
if (pipe_full(head, pipe->tail, pipe->max_usage)) {
- spin_unlock_irq(&pipe->rd_wait.lock);
continue;
}

pipe->head = head + 1;
- spin_unlock_irq(&pipe->rd_wait.lock);

/* Insert it into the buffer array */
buf = &pipe->bufs[head & mask];
@@ -1260,14 +1244,12 @@ int pipe_resize_ring(struct pipe_inode_info *pipe, unsigned int nr_slots)
if (unlikely(!bufs))
return -ENOMEM;

- spin_lock_irq(&pipe->rd_wait.lock);
mask = pipe->ring_size - 1;
head = pipe->head;
tail = pipe->tail;

n = pipe_occupancy(head, tail);
if (nr_slots < n) {
- spin_unlock_irq(&pipe->rd_wait.lock);
kfree(bufs);
return -EBUSY;
}
@@ -1303,8 +1285,6 @@ int pipe_resize_ring(struct pipe_inode_info *pipe, unsigned int nr_slots)
pipe->tail = tail;
pipe->head = head;

- spin_unlock_irq(&pipe->rd_wait.lock);
-
/* This might have made more room for writers */
wake_up_interruptible(&pipe->wr_wait);
return 0;
diff --git a/include/linux/pipe_fs_i.h b/include/linux/pipe_fs_i.h
index 6cb65df3e3ba..f5084daf6eaf 100644
--- a/include/linux/pipe_fs_i.h
+++ b/include/linux/pipe_fs_i.h
@@ -2,6 +2,8 @@
#ifndef _LINUX_PIPE_FS_I_H
#define _LINUX_PIPE_FS_I_H

+#include <linux/fs.h>
+
#define PIPE_DEF_BUFFERS 16

#define PIPE_BUF_FLAG_LRU 0x01 /* page is on the LRU */
@@ -223,6 +225,16 @@ static inline void pipe_discard_from(struct pipe_inode_info *pipe,
#define PIPE_SIZE PAGE_SIZE

/* Pipe lock and unlock operations */
+static inline void __pipe_lock(struct pipe_inode_info *pipe)
+{
+ mutex_lock_nested(&pipe->mutex, I_MUTEX_PARENT);
+}
+
+static inline void __pipe_unlock(struct pipe_inode_info *pipe)
+{
+ mutex_unlock(&pipe->mutex);
+}
+
void pipe_lock(struct pipe_inode_info *);
void pipe_unlock(struct pipe_inode_info *);
void pipe_double_lock(struct pipe_inode_info *, struct pipe_inode_info *);
diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index a6f9bdd956c3..92e46cfe9419 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -108,7 +108,7 @@ static bool post_one_notification(struct watch_queue *wqueue,
if (!pipe)
return false;

- spin_lock_irq(&pipe->rd_wait.lock);
+ __pipe_lock(pipe);

mask = pipe->ring_size - 1;
head = pipe->head;
@@ -135,17 +135,17 @@ static bool post_one_notification(struct watch_queue *wqueue,
buf->offset = offset;
buf->len = len;
buf->flags = PIPE_BUF_FLAG_WHOLE;
- smp_store_release(&pipe->head, head + 1); /* vs pipe_read() */
+ pipe->head = head + 1;

if (!test_and_clear_bit(note, wqueue->notes_bitmap)) {
- spin_unlock_irq(&pipe->rd_wait.lock);
+ __pipe_unlock(pipe);
BUG();
}
wake_up_interruptible_sync_poll_locked(&pipe->rd_wait, EPOLLIN | EPOLLRDNORM);
done = true;

out:
- spin_unlock_irq(&pipe->rd_wait.lock);
+ __pipe_unlock(pipe);
if (done)
kill_fasync(&pipe->fasync_readers, SIGIO, POLL_IN);
return done;
--
2.34.1