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

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After commit 0b2b066f8a ("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 <changfengnan@bytedance.com>
Reviewed-by: Diangang Li <lidiangang@bytedance.com>
Link: https://lore.kernel.org/r/20250813120214.18729-1-changfengnan@bytedance.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Fengnan Chang 2025-08-13 20:02:14 +08:00 committed by Jens Axboe
parent 41b70df5b3
commit 9d83e1f05c
1 changed files with 8 additions and 0 deletions
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8
io_uring/io-wq.c
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@ -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);
}