Re: [PATCH 3/3] io_uring: briefly loose locks while reaping events

[Jens Axboe <axboe@kernel.dk>]

On 7/6/20 8:45 AM, Pavel Begunkov wrote:
> 
> 
> On 06/07/2020 17:42, Pavel Begunkov wrote:
>> On 06/07/2020 17:31, Jens Axboe wrote:
>>> On 7/6/20 8:14 AM, Pavel Begunkov wrote:
>>>> It's not nice to hold @uring_lock for too long io_iopoll_reap_events().
>>>> For instance, the lock is needed to publish requests to @poll_list, and
>>>> that locks out tasks doing that for no good reason. Loose it
>>>> occasionally.
>>>>
>>>> Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
>>>> ---
>>>>  fs/io_uring.c | 3 +++
>>>>  1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/fs/io_uring.c b/fs/io_uring.c
>>>> index 020944a193d0..568e25bcadd6 100644
>>>> --- a/fs/io_uring.c
>>>> +++ b/fs/io_uring.c
>>>> @@ -2059,11 +2059,14 @@ static void io_iopoll_reap_events(struct io_ring_ctx *ctx)
>>>>  
>>>>  		io_iopoll_getevents(ctx, &nr_events, 1);
>>>>  
>>>> +		/* task_work takes the lock to publish a req to @poll_list */
>>>> +		mutex_unlock(&ctx->uring_lock);
>>>>  		/*
>>>>  		 * Ensure we allow local-to-the-cpu processing to take place,
>>>>  		 * in this case we need to ensure that we reap all events.
>>>>  		 */
>>>>  		cond_resched();
>>>> +		mutex_lock(&ctx->uring_lock);
>>>>  	}
>>>>  	mutex_unlock(&ctx->uring_lock);
>>>>  }
>>>
>>> This would be much better written as:
>>>
>>> if (need_resched()) {
>>> 	mutex_unlock(&ctx->uring_lock);
>>> 	cond_resched();
>>> 	mutex_lock(&ctx->uring_lock);
>>> }
>>>
>>> to avoid shuffling the lock when not needed to. Every cycle counts for
>>> polled IO.
>>
>> It happens only when io_uring is being killed, can't imagine any sane app
>> trying to catch CQEs after doing that. I'll resend
> 
> Also, io_iopoll_getevents() already does need_resched(). Hmm, do you wan't
> me to resend?

I missed this was the "reap on dead" path. But yes, please resend.

-- 
Jens Axboe