[PATCH 2/2] io_uring: replace use of system_unbound_wq with system_dfl_wq

[Marco Crivellari <marco.crivellari@suse.com>]

Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistentcy cannot be addressed without refactoring the API.

system_unbound_wq should be the default workqueue so as not to enforce
locality constraints for random work whenever it's not required.

Adding system_dfl_wq to encourage its use when unbound work should be used.

queue_work() / queue_delayed_work() / mod_delayed_work() will now use the
new unbound wq: whether the user still use the old wq a warn will be
printed along with a wq redirect to the new one.

The old system_unbound_wq will be kept for a few release cycles.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
---
 io_uring/io_uring.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index 2a6ead3c7d36..74972ecf2045 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -2983,7 +2983,7 @@ static __cold void io_ring_ctx_wait_and_kill(struct io_ring_ctx *ctx)
 
 	INIT_WORK(&ctx->exit_work, io_ring_exit_work);
 	/*
-	 * Use system_unbound_wq to avoid spawning tons of event kworkers
+	 * Use system_dfl_wq to avoid spawning tons of event kworkers
 	 * if we're exiting a ton of rings at the same time. It just adds
 	 * noise and overhead, there's no discernable change in runtime
 	 * over using system_percpu_wq.
-- 
2.51.0