io_uring/io-wq: add check free worker before create new worker

After commit 0b2b066 ("io_uring/io-wq: only create a new worker
if it can make progress"), in our produce environment, we still
observe that part of io_worker threads keeps creating and destroying.
After analysis, it was confirmed that this was due to a more complex
scenario involving a large number of fsync operations, which can be
abstracted as frequent write + fsync operations on multiple files in
a single uring instance. Since write is a hash operation while fsync
is not, and fsync is likely to be suspended during execution, the
action of checking the hash value in
io_wqe_dec_running cannot handle such scenarios.
Similarly, if hash-based work and non-hash-based work are sent at the
same time, similar issues are likely to occur.
Returning to the starting point of the issue, when a new work
arrives, io_wq_enqueue may wake up free worker A, while
io_wq_dec_running may create worker B. Ultimately, only one of A and
B can obtain and process the task, leaving the other in an idle
state. In the end, the issue is caused by inconsistent logic in the
checks performed by io_wq_enqueue and io_wq_dec_running.
Therefore, the problem can be resolved by checking for available
workers in io_wq_dec_running.

Signed-off-by: Fengnan Chang <[email protected]>
Reviewed-by: Diangang Li <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: Fengnan Chang

io_uring/io-wq.c

@@ -357,6 +357,13 @@ static void create_worker_cb(struct callback_head *cb)
 	worker = container_of(cb, struct io_worker, create_work);
 	wq = worker->wq;
 	acct = worker->acct;
+
+	rcu_read_lock();
+	do_create = !io_acct_activate_free_worker(acct);
+	rcu_read_unlock();
+	if (!do_create)
+		goto no_need_create;
+
 	raw_spin_lock(&acct->workers_lock);
 
 	if (acct->nr_workers < acct->max_workers) {
@@ -367,6 +374,7 @@ static void create_worker_cb(struct callback_head *cb)
 	if (do_create) {
 		create_io_worker(wq, acct);
 	} else {
+no_need_create:
 		atomic_dec(&acct->nr_running);
 		io_worker_ref_put(wq);
 	}