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Subject[PATCH v2 2/5] fuse: Optimize request_end() by not taking fiq->waitq.lock
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We take global fiq->waitq.lock every time, when we are
in this function, but interrupted requests are just small
subset of all requests. This patch optimizes request_end()
and makes it to take the lock when it's really needed.

queue_interrupt() needs small change for that. After req
is linked to interrupt list, we do smp_mb() and check
for FR_FINISHED again. In case of FR_FINISHED bit has
appeared, we remove req and leave the function:

CPU 0 CPU 1
queue_interrupt() request_end()

spin_lock(&fiq->waitq.lock)


list_add_tail(&req->intr_entry, &fiq->interrupts) test_and_set_bit(FR_FINISHED, &req->flags)
smp_mb() <memory barrier implied test_and_set_bit()>
if (test_bit(FR_FINISHED, &req->flags)) if (!list_empty(&req->intr_entry))

list_del_init(&req->intr_entry) spin_lock(&fiq->waitq.lock)
list_del_init(&req->intr_entry)

Check the change is visible in perf report:

1)Firstly mount fusexmp_fh:
$fuse/example/.libs/fusexmp_fh mnt

2)Run test doing
futimes(fd, tv1);
futimes(fd, tv2);
in many threads endlessly.

3)perf record -g (all the system load)

Without the patch in request_end() we spend 62.58% of do_write() time:
(= 12.58 / 20.10 * 100%)

55,22% entry_SYSCALL_64
20,10% do_writev
...
18,08% fuse_dev_do_write
12,58% request_end
10,08% __wake_up_common_lock
1,97% queued_spin_lock_slowpath
1,31% fuse_copy_args
1,04% fuse_copy_one
0,85% queued_spin_lock_slowpath

With the patch, the perf report becomes better, and only 58.16%
of do_write() time we spend in request_end():

54,15% entry_SYSCALL_64
18,24% do_writev
...
16,25% fuse_dev_do_write
10,61% request_end
10,25% __wake_up_common_lock
1,34% fuse_copy_args
1,06% fuse_copy_one
0,88% queued_spin_lock_slowpath

v2: Remove unlocked test of FR_FINISHED from queue_interrupt()

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
---
fs/fuse/dev.c | 28 ++++++++++++++++++++--------
1 file changed, 20 insertions(+), 8 deletions(-)

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 1ecec7fcb841..7374a23b1bc8 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -427,10 +427,16 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)

if (test_and_set_bit(FR_FINISHED, &req->flags))
goto put_request;
-
- spin_lock(&fiq->waitq.lock);
- list_del_init(&req->intr_entry);
- spin_unlock(&fiq->waitq.lock);
+ /*
+ * test_and_set_bit() implies smp_mb() between bit
+ * changing and below intr_entry check. Pairs with
+ * smp_mb() from queue_interrupt().
+ */
+ if (!list_empty(&req->intr_entry)) {
+ spin_lock(&fiq->waitq.lock);
+ list_del_init(&req->intr_entry);
+ spin_unlock(&fiq->waitq.lock);
+ }
WARN_ON(test_bit(FR_PENDING, &req->flags));
WARN_ON(test_bit(FR_SENT, &req->flags));
if (test_bit(FR_BACKGROUND, &req->flags)) {
@@ -469,12 +475,18 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
static void queue_interrupt(struct fuse_iqueue *fiq, struct fuse_req *req)
{
spin_lock(&fiq->waitq.lock);
- if (test_bit(FR_FINISHED, &req->flags)) {
- spin_unlock(&fiq->waitq.lock);
- return;
- }
if (list_empty(&req->intr_entry)) {
list_add_tail(&req->intr_entry, &fiq->interrupts);
+ /*
+ * Pairs with smp_mb() implied by test_and_set_bit()
+ * from request_end().
+ */
+ smp_mb();
+ if (test_bit(FR_FINISHED, &req->flags)) {
+ list_del_init(&req->intr_entry);
+ spin_unlock(&fiq->waitq.lock);
+ return;
+ }
wake_up_locked(&fiq->waitq);
kill_fasync(&fiq->fasync, SIGIO, POLL_IN);
}
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