[RFC 1/3] io_uring: reduce frequent add_wait_queue() overhead for multi-shot poll request

Xiaoguang Wang
2021-09-22T12:34:27Z

Run echo_server to evaluate io_uring's multi-shot poll performance, perf
shows that add_wait_queue() has obvious overhead. Intruduce a new state
'active' in io_poll_iocb to indicate whether io_poll_wake() should queue
a task_work. This new state will be set to true initially, be set to false
when starting to queue a task work, and be set to true again when a poll
cqe has been committed. One concern is that this method may lost waken-up
event, but seems it's ok.

io_poll_wake io_poll_task_func
t1 |
t2 | WRITE_ONCE(req->poll.active, true);
t3 |
t4 | io_commit_cqring(ctx);
t5 |
t6 |

If waken-up events happens before or at t4, it's ok, user app will always
see a cqe. If waken-up events happens after t4 and IIUC, io_poll_wake()
will see the new req->poll.active value by using READ_ONCE().

With this patch, a pure echo_server(1000 connections, packet is 16 bytes)
shows about 1.6% reqs improvement.

Signed-off-by: Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>
---
fs/io_uring.c | 20 ++++++++++++++++----
1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 1294b1ef4acb..ca4464a75c7b 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -487,6 +487,7 @@ struct io_poll_iocb {
__poll_t events;
bool done;
bool canceled;
+ bool active;
struct wait_queue_entry wait;
};

@@ -5025,8 +5026,6 @@ static int __io_async_wake(struct io_kiocb *req, struct io_poll_iocb *poll,

trace_io_uring_task_add(req->ctx, req->opcode, req->user_data, mask);

- list_del_init(&poll->wait.entry);
-
req->result = mask;
req->io_task_work.func = func;

@@ -5057,7 +5056,10 @@ static bool io_poll_rewait(struct io_kiocb *req, struct io_poll_iocb *poll)

spin_lock(&ctx->completion_lock);
if (!req->result && !READ_ONCE(poll->canceled)) {
- add_wait_queue(poll->head, &poll->wait);
+ if (req->opcode == IORING_OP_POLL_ADD)
+ WRITE_ONCE(req->poll.active, true);
+ else
+ add_wait_queue(poll->head, &poll->wait);
return true;
}

@@ -5133,6 +5135,9 @@ static inline bool io_poll_complete(struct io_kiocb *req, __poll_t mask)
return done;
}

+static bool __io_poll_remove_one(struct io_kiocb *req,
+ struct io_poll_iocb *poll, bool do_cancel);
+
static void io_poll_task_func(struct io_kiocb *req, bool *locked)
{
struct io_ring_ctx *ctx = req->ctx;
@@ -5146,10 +5151,11 @@ static void io_poll_task_func(struct io_kiocb *req, bool *locked)
done = __io_poll_complete(req, req->result);
if (done) {
io_poll_remove_double(req);
+ __io_poll_remove_one(req, io_poll_get_single(req), true);
hash_del(&req->hash_node);
} else {
req->result = 0;
- add_wait_queue(req->poll.head, &req->poll.wait);
+ WRITE_ONCE(req->poll.active, true);
}
io_commit_cqring(ctx);
spin_unlock(&ctx->completion_lock);
@@ -5204,6 +5210,7 @@ static void io_init_poll_iocb(struct io_poll_iocb *poll, __poll_t events,
poll->head = NULL;
poll->done = false;
poll->canceled = false;
+ poll->active = true;
#define IO_POLL_UNMASK (EPOLLERR|EPOLLHUP|EPOLLNVAL|EPOLLRDHUP)
/* mask in events that we always want/need */
poll->events = events | IO_POLL_UNMASK;
@@ -5301,6 +5308,7 @@ static int io_async_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
trace_io_uring_poll_wake(req->ctx, req->opcode, req->user_data,
key_to_poll(key));

+ list_del_init(&poll->wait.entry);
return __io_async_wake(req, poll, key_to_poll(key), io_async_task_func);
}

@@ -5569,6 +5577,10 @@ static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
struct io_kiocb *req = wait->private;
struct io_poll_iocb *poll = &req->poll;

+ if (!READ_ONCE(poll->active))
+ return 0;
+
+ WRITE_ONCE(poll->active, false);
return __io_async_wake(req, poll, key_to_poll(key), io_poll_task_func);
}

--
2.14.4.44.g2045bb6