Commit 7b72d661 authored by Jens Axboe's avatar Jens Axboe
Browse files

io_uring: gate iowait schedule on having pending requests 

A previous commit made all cqring waits marked as iowait, as a way to
improve performance for short schedules with pending IO. However, for
use cases that have a special reaper thread that does nothing but
wait on events on the ring, this causes a cosmetic issue where we
know have one core marked as being "busy" with 100% iowait.

While this isn't a grave issue, it is confusing to users. Rather than
always mark us as being in iowait, gate setting of current->in_iowait
to 1 by whether or not the waiting task has pending requests.

Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/io-uring/CAMEGJJ2RxopfNQ7GNLhr7X9=bHXKo+G5OOe0LUq=+UgLXsv1Xg@mail.gmail.com/
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217699
Link: https://bugzilla.kernel.org/show_bug.cgi?id=217700


Reported-by: default avatarOleksandr Natalenko <oleksandr@natalenko.name>
Reported-by: default avatarPhil Elwell <phil@raspberrypi.com>
Tested-by: default avatarAndres Freund <andres@anarazel.de>
Fixes: 8a796565 ("io_uring: Use io_schedule* in cqring wait")
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 07e98113

io_uring/io_uring.c

+17 −6
Original line number Diff line number Diff line
@@ -2493,11 +2493,20 @@ int io_run_task_work_sig(struct io_ring_ctx *ctx) 
	return 0;

}



static bool current_pending_io(void)

{

	struct io_uring_task *tctx = current->io_uring;



	if (!tctx)

		return false;

	return percpu_counter_read_positive(&tctx->inflight);

}



/* when returns >0, the caller should retry */

static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx,

					  struct io_wait_queue *iowq)

{

	int token, ret;

	int io_wait, ret;



	if (unlikely(READ_ONCE(ctx->check_cq)))

		return 1;
@@ -2511,17 +2520,19 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx, 
		return 0;



	/*

	 * Use io_schedule_prepare/finish, so cpufreq can take into account

	 * that the task is waiting for IO - turns out to be important for low

	 * QD IO.

	 * Mark us as being in io_wait if we have pending requests, so cpufreq

	 * can take into account that the task is waiting for IO - turns out

	 * to be important for low QD IO.

	 */

	token = io_schedule_prepare();

	io_wait = current->in_iowait;

	if (current_pending_io())

		current->in_iowait = 1;

	ret = 0;

	if (iowq->timeout == KTIME_MAX)

		schedule();

	else if (!schedule_hrtimeout(&iowq->timeout, HRTIMER_MODE_ABS))

		ret = -ETIME;

	io_schedule_finish(token);

	current->in_iowait = io_wait;

	return ret;

}