From: Pavel Begunkov <asml.silence@gmail.com>
To: Jens Axboe <axboe@kernel.dk>,
linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] io_uring: remove wait loop spurious wakeups
Date: Wed, 9 Oct 2019 01:05:45 +0300 [thread overview]
Message-ID: <4004b5bc-edf0-cc8f-8efc-7f848c95f0ba@gmail.com> (raw)
In-Reply-To: <a1f8de23-fcad-7252-cbd4-8f5e617056cd@kernel.dk>
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On 09/10/2019 00:22, Jens Axboe wrote:
> On 10/8/19 2:58 PM, Pavel Begunkov wrote:
>> On 08/10/2019 20:00, Jens Axboe wrote:
>>> On 10/8/19 10:43 AM, Pavel Begunkov wrote:
>>>> On 08/10/2019 06:16, Jens Axboe wrote:
>>>>> On 10/7/19 5:18 PM, Pavel Begunkov (Silence) wrote:
>>>>>> From: Pavel Begunkov <asml.silence@gmail.com>
>>>>>>
>>>>>> Any changes interesting to tasks waiting in io_cqring_wait() are
>>>>>> commited with io_cqring_ev_posted(). However, io_ring_drop_ctx_refs()
>>>>>> also tries to do that but with no reason, that means spurious wakeups
>>>>>> every io_free_req() and io_uring_enter().
>>>>>>
>>>>>> Just use percpu_ref_put() instead.
>>>>>
>>>>> Looks good, this is a leftover from when the ctx teardown used
>>>>> the waitqueue as well.
>>>>>
>>>> BTW, is there a reason for ref-counting in struct io_kiocb? I understand
>>>> the idea behind submission reference, but don't see any actual part
>>>> needing it.
>>>
>>> In short, it's to prevent the completion running before we're done with
>>> the iocb on the submission side.
>>
>> Yep, that's what I expected. Perhaps I missed something, but what I've
>> seen following code paths all the way down, it either
>> 1. gets error / completes synchronously and then frees req locally
>> 2. or passes it further (e.g. async list) and never accesses it after
>
> As soon as the IO is passed on, it can complete. In fact, it can complete
> even _before_ that call returns. That's the issue. Obviously this isn't
> true for purely polled IO, but it is true for IRQ based IO.
And the idea was to not use io_kiocb after submission. Except when we know,
that it won't complete asynchronously (e.g. error), that could be checked
with return code, I guess.
Anyway, thanks for the explanation!
--
Yours sincerely,
Pavel Begunkov
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next prev parent reply other threads:[~2019-10-08 22:06 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-07 23:18 [PATCH] io_uring: remove wait loop spurious wakeups Pavel Begunkov (Silence)
2019-10-08 3:16 ` Jens Axboe
2019-10-08 16:43 ` Pavel Begunkov
2019-10-08 17:00 ` Jens Axboe
2019-10-08 20:58 ` Pavel Begunkov
2019-10-08 21:22 ` Jens Axboe
2019-10-08 22:05 ` Pavel Begunkov [this message]
2019-10-09 2:54 ` Jens Axboe
2019-10-09 9:19 ` Pavel Begunkov
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