From: Andres Freund <andres@xxxxxxxxxxx> 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@xxxxxxxxxxxxxxx # 5.10+ Cc: Pavel Begunkov <asml.silence@xxxxxxxxx> Cc: io-uring@xxxxxxxxxxxxxxx Cc: linux-kernel@xxxxxxxxxxxxxxx Signed-off-by: Andres Freund <andres@xxxxxxxxxxx> Link: https://lore.kernel.org/r/20230707162007.194068-1-andres@xxxxxxxxxxx [axboe: minor style fixup] Signed-off-by: Jens Axboe <axboe@xxxxxxxxx> --- io_uring/io_uring.c | 15 +++++++++++++-- 1 file changed, 13 insertions(+), 2 deletions(-) diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c index e8096d502a7c..7505de2428e0 100644 --- a/io_uring/io_uring.c +++ b/io_uring/io_uring.c @@ -2489,6 +2489,8 @@ int io_run_task_work_sig(struct io_ring_ctx *ctx) static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx, struct io_wait_queue *iowq) { + int token, ret; + if (unlikely(READ_ONCE(ctx->check_cq))) return 1; if (unlikely(!llist_empty(&ctx->work_llist))) @@ -2499,11 +2501,20 @@ static inline int io_cqring_wait_schedule(struct io_ring_ctx *ctx, return -EINTR; if (unlikely(io_should_wake(iowq))) 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. + */ + token = io_schedule_prepare(); + ret = 0; if (iowq->timeout == KTIME_MAX) schedule(); else if (!schedule_hrtimeout(&iowq->timeout, HRTIMER_MODE_ABS)) - return -ETIME; - return 0; + ret = -ETIME; + io_schedule_finish(token); + return ret; } /* -- 2.40.1
