linux-kernel - Re: [PATCH v2 0/2] Optimise io

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Message-ID: <b999490f-6138-b685-5472-5cd1843b747d@kernel.dk>
Date:   Tue, 24 Sep 2019 10:27:47 +0200
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>,
        Ingo Molnar <mingo@...nel.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] Optimise io_uring completion waiting

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.

-- 
Jens Axboe