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Date:   Sun, 23 Jul 2023 09:31:26 -0600
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Subject: Re: [PATCH 6.4 800/800] io_uring: Use io_schedule* in cqring wait
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To:     Oleksandr Natalenko <oleksandr@natalenko.name>,
        stable@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Pavel Begunkov <asml.silence@gmail.com>,
        io-uring@vger.kernel.org, linux-kernel@vger.kernel.org,
        Andres Freund <andres@anarazel.de>
References: <20230716194949.099592437@linuxfoundation.org>
 <20230716195007.731909670@linuxfoundation.org>
 <12251678.O9o76ZdvQC@natalenko.name>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <12251678.O9o76ZdvQC@natalenko.name>
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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@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 |   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