From: Andres Freund andres@anarazel.de
Commit 8a796565cec3601071cbbd27d6304e202019d014 upstream.
I observed poor performance of io_uring compared to synchronous IO. That turns out to be caused by deeper CPU idle states entered with io_uring, due to io_uring using plain schedule(), whereas synchronous IO uses io_schedule().
The losses due to this are substantial. On my cascade lake workstation, t/io_uring from the fio repository e.g. yields regressions between 20% and 40% with the following command: ./t/io_uring -r 5 -X0 -d 1 -s 1 -c 1 -p 0 -S$use_sync -R 0 /mnt/t2/fio/write.0.0
This is repeatable with different filesystems, using raw block devices and using different block devices.
Use io_schedule_prepare() / io_schedule_finish() in io_cqring_wait_schedule() to address the difference.
After that using io_uring is on par or surpassing synchronous IO (using registered files etc makes it reliably win, but arguably is a less fair comparison).
There are other calls to schedule() in io_uring/, but none immediately jump out to be similarly situated, so I did not touch them. Similarly, it's possible that mutex_lock_io() should be used, but it's not clear if there are cases where that matters.
Cc: stable@vger.kernel.org # 5.10+ Cc: Pavel Begunkov asml.silence@gmail.com Cc: io-uring@vger.kernel.org Cc: linux-kernel@vger.kernel.org Signed-off-by: Andres Freund andres@anarazel.de Link: https://lore.kernel.org/r/20230707162007.194068-1-andres@anarazel.de [axboe: minor style fixup] Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- io_uring/io_uring.c | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)
--- a/io_uring/io_uring.c +++ b/io_uring/io_uring.c @@ -7796,7 +7796,7 @@ static inline int io_cqring_wait_schedul struct io_wait_queue *iowq, ktime_t *timeout) { - int ret; + int token, ret;
/* make sure we run task_work before checking for signals */ ret = io_run_task_work_sig(); @@ -7806,9 +7806,17 @@ static inline int io_cqring_wait_schedul if (test_bit(0, &ctx->check_cq_overflow)) return 1;
+ /* + * 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. + */ + token = io_schedule_prepare(); + ret = 1; if (!schedule_hrtimeout(timeout, HRTIMER_MODE_ABS)) - return -ETIME; - return 1; + ret = -ETIME; + io_schedule_finish(token); + return ret; }
/*