Re: [PATCH] io_uring: fix io_sq_thread no schedule when busy

[Jens Axboe <axboe@kernel.dk>]

On 6/21/20 11:50 PM, Xuan Zhuo wrote:
> When the user consumes and generates sqe at a fast rate,
> io_sqring_entries can always get sqe, and ret will not be equal to -EBUSY,
> so that io_sq_thread will never call cond_resched or schedule, and then
> we will get the following system error prompt:
> 
> rcu: INFO: rcu_sched self-detected stall on CPU
> or
> watchdog: BUG: soft lockup-CPU#23 stuck for 112s! [io_uring-sq:1863]
> 
> This patch adds a check after io_submit_sqes. If io_sq_thread does not call
> cond_resched or schedule for more than HZ/2, it will call them.

This looks reasonable. It'd be easier if we could just cond_resched() after
the call unconditionally, but that would not drop the mm.

But maybe we can just drop the timeout and just rely on need_resched()
for this, ala the below?

diff --git a/fs/io_uring.c b/fs/io_uring.c
index c686061c3762..955d0765f302 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -6248,7 +6248,7 @@ static int io_sq_thread(void *data)
 		 * If submit got -EBUSY, flag us as needing the application
 		 * to enter the kernel to reap and flush events.
 		 */
-		if (!to_submit || ret == -EBUSY) {
+		if (!to_submit || ret == -EBUSY || need_resched()) {
 			/*
 			 * Drop cur_mm before scheduling, we can't hold it for
 			 * long periods (or over schedule()). Do this before
@@ -6264,7 +6264,7 @@ static int io_sq_thread(void *data)
 			 * more IO, we should wait for the application to
 			 * reap events and wake us up.
 			 */
-			if (!list_empty(&ctx->poll_list) ||
+			if (!list_empty(&ctx->poll_list) || need_resched() ||
 			    (!time_after(jiffies, timeout) && ret != -EBUSY &&
 			    !percpu_ref_is_dying(&ctx->refs))) {
 				if (current->task_works)

-- 
Jens Axboe