io_uring: cache req->apoll->events in req->cflags

When we arm poll on behalf of a different type of request, like a network
receive, then we allocate req->apoll as our poll entry. Running network
workloads shows io_poll_check_events() as the most expensive part of
io_uring, and it's all due to having to pull in req->apoll instead of
just the request which we have hot already.

Cache poll->events in req->cflags, which isn't used until the request
completes anyway. This isn't strictly needed for regular poll, where
req->poll.events is used and thus already hot, but for the sake of
unification we do it all around.

This saves 3-4% of overhead in certain request workloads.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -5876,13 +5876,13 @@ static int io_poll_check_events(struct io_kiocb *req)
 			return -ECANCELED;
 
 		if (!req->result) {
-			struct poll_table_struct pt = { ._key = poll->events };
+			struct poll_table_struct pt = { ._key = req->cflags };
 
-			req->result = vfs_poll(req->file, &pt) & poll->events;
+			req->result = vfs_poll(req->file, &pt) & req->cflags;
 		}
 
 		/* multishot, just fill an CQE and proceed */
-		if (req->result && !(poll->events & EPOLLONESHOT)) {
+		if (req->result && !(req->cflags & EPOLLONESHOT)) {
 			__poll_t mask = mangle_poll(req->result & poll->events);
 			bool filled;
 
@@ -5953,9 +5953,16 @@ static void io_apoll_task_func(struct io_kiocb *req, bool *locked)
 		io_req_complete_failed(req, ret);
 }
 
-static void __io_poll_execute(struct io_kiocb *req, int mask)
+static void __io_poll_execute(struct io_kiocb *req, int mask, int events)
 {
 	req->result = mask;
+	/*
+	 * This is useful for poll that is armed on behalf of another
+	 * request, and where the wakeup path could be on a different
+	 * CPU. We want to avoid pulling in req->apoll->events for that
+	 * case.
+	 */
+	req->cflags = events;
 	if (req->opcode == IORING_OP_POLL_ADD)
 		req->io_task_work.func = io_poll_task_func;
 	else
@@ -5965,17 +5972,17 @@ static void __io_poll_execute(struct io_kiocb *req, int mask)
 	io_req_task_work_add(req, false);
 }
 
-static inline void io_poll_execute(struct io_kiocb *req, int res)
+static inline void io_poll_execute(struct io_kiocb *req, int res, int events)
 {
 	if (io_poll_get_ownership(req))
-		__io_poll_execute(req, res);
+		__io_poll_execute(req, res, events);
 }
 
 static void io_poll_cancel_req(struct io_kiocb *req)
 {
 	io_poll_mark_cancelled(req);
 	/* kick tw, which should complete the request */
-	io_poll_execute(req, 0);
+	io_poll_execute(req, 0, 0);
 }
 
 static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
@@ -5989,7 +5996,7 @@ static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 	if (unlikely(mask & POLLFREE)) {
 		io_poll_mark_cancelled(req);
 		/* we have to kick tw in case it's not already */
-		io_poll_execute(req, 0);
+		io_poll_execute(req, 0, poll->events);
 
 		/*
 		 * If the waitqueue is being freed early but someone is already
@@ -6020,7 +6027,7 @@ static int io_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 			list_del_init(&poll->wait.entry);
 			poll->head = NULL;
 		}
-		__io_poll_execute(req, mask);
+		__io_poll_execute(req, mask, poll->events);
 	}
 	return 1;
 }
@@ -6124,7 +6131,7 @@ static int __io_arm_poll_handler(struct io_kiocb *req,
 		/* can't multishot if failed, just queue the event we've got */
 		if (unlikely(ipt->error || !ipt->nr_entries))
 			poll->events |= EPOLLONESHOT;
-		__io_poll_execute(req, mask);
+		__io_poll_execute(req, mask, poll->events);
 		return 0;
 	}
 	io_add_napi(req->file, req->ctx);
@@ -6135,7 +6142,7 @@ static int __io_arm_poll_handler(struct io_kiocb *req,
 	 */
 	v = atomic_dec_return(&req->poll_refs);
 	if (unlikely(v & IO_POLL_REF_MASK))
-		__io_poll_execute(req, 0);
+		__io_poll_execute(req, 0, poll->events);
 	return 0;
 }
 
@@ -6333,7 +6340,7 @@ static int io_poll_add_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe
 		return -EINVAL;
 
 	io_req_set_refcount(req);
-	poll->events = io_poll_parse_events(sqe, flags);
+	req->cflags = poll->events = io_poll_parse_events(sqe, flags);
 	return 0;
 }