Re: [PATCH 6.4 800/800] io_uring: Use io_schedule* in cqring wait

From: Jens Axboe
Date: Sun Jul 23 2023 - 11:32:21 EST


On 7/23/23 3:39?AM, Oleksandr Natalenko wrote:
> Hello.
>
> On ned?le 16. ?ervence 2023 21:50:53 CEST Greg Kroah-Hartman wrote:
>> From: Andres Freund <andres@xxxxxxxxxxx>
>>
>> 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@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>
>> Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
>> ---
>> io_uring/io_uring.c | 15 +++++++++++++--
>> 1 file changed, 13 insertions(+), 2 deletions(-)
>>
>> --- a/io_uring/io_uring.c
>> +++ b/io_uring/io_uring.c
>> @@ -2575,6 +2575,8 @@ int io_run_task_work_sig(struct io_ring_
>> 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)))
>> @@ -2585,11 +2587,20 @@ static inline int io_cqring_wait_schedul
>> 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;
>> }
>>
>> /*
>
> Reportedly, this caused a regression as reported in [1] [2] [3]. Not only v6.4.4 is affected, v6.1.39 is affected too.
>
> Reverting this commit fixes the issue.
>
> Please check.
>
> Thanks.
>
> [1] https://bbs.archlinux.org/viewtopic.php?id=287343
> [2] https://bugzilla.kernel.org/show_bug.cgi?id=217700
> [3] https://bugzilla.kernel.org/show_bug.cgi?id=217699

Just read the first one, but this is very much expected. It's now just
correctly reflecting that one thread is waiting on IO. IO wait being
100% doesn't mean that one core is running 100% of the time, it just
means it's WAITING on IO 100% of the time.

--
Jens Axboe