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Subject: Re: [PATCH v2 0/2] Optimise io_uring completion waiting
To:     Pavel Begunkov <asml.silence@gmail.com>,
        Ingo Molnar <mingo@kernel.org>
Cc:     Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
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From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <405d24d1-6f73-0437-e3e9-f862d1437c21@kernel.dk>
Date:   Tue, 24 Sep 2019 12:09:05 +0200
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On 9/24/19 3:20 AM, Pavel Begunkov wrote:
> 
> 
> On 24/09/2019 11:27, Jens Axboe wrote:
>> On 9/24/19 2:02 AM, Jens Axboe wrote:
>>> On 9/24/19 1:06 AM, Pavel Begunkov wrote:
>>>> On 24/09/2019 02:00, Jens Axboe wrote:
>>>>>> I think we can do the same thing, just wrapping the waitqueue in a
>>>>>> structure with a count in it, on the stack. Got some flight time
>>>>>> coming up later today, let me try and cook up a patch.
>>>>>
>>>>> Totally untested, and sent out 5 min before departure... But something
>>>>> like this.
>>>> Hmm, reminds me my first version. Basically that's the same thing but
>>>> with macroses inlined. I wanted to make it reusable and self-contained,
>>>> though.
>>>>
>>>> If you don't think it could be useful in other places, sure, we could do
>>>> something like that. Is that so?
>>>
>>> I totally agree it could be useful in other places. Maybe formalized and
>>> used with wake_up_nr() instead of adding a new primitive? Haven't looked
>>> into that, I may be talking nonsense.
>>>
>>> In any case, I did get a chance to test it and it works for me. Here's
>>> the "finished" version, slightly cleaned up and with a comment added
>>> for good measure.
>>
>> Notes:
>>
>> This version gets the ordering right, you need exclusive waits to get
>> fifo ordering on the waitqueue.
>>
>> Both versions (yours and mine) suffer from the problem of potentially
>> waking too many. I don't think this is a real issue, as generally we
>> don't do threaded access to the io_urings. But if you had the following
>> tasks wait on the cqring:
>>
>> [min_events = 32], [min_events = 8], [min_events = 8]
>>
>> and we reach the io_cqring_events() == threshold, we'll wake all three.
>> I don't see a good solution to this, so I suspect we just live with
>> until proven an issue. Both versions are much better than what we have
>> now.
>>
> If io_cqring_events() == 8, only the last two would be woken up in both
> implementations, as to_wait/threshold is specified per waiter. Isn't it?

If io_cqring_events() == 8, then none would be woken in my
implementation since the first one will break the wakeup loop.

> Agree with waiting, I don't see a good real-life case for that, that
> couldn't be done efficiently in userspace.

Exactly

-- 
Jens Axboe