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To:     Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <6256169d519f72fe592e70be47a04aa0e9c3b9a1.1576333754.git.asml.silence@gmail.com>
 <c6f625bdb27ea3b929d0717ebf2aaa33ad5410da.1576335142.git.asml.silence@gmail.com>
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From:   Pavel Begunkov <asml.silence@gmail.com>
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Subject: Re: [PATCH v3] io_uring: don't wait when under-submitting
Message-ID: <900dbb63-ae9e-40e6-94f9-8faa1c14389e@gmail.com>
Date:   Sun, 15 Dec 2019 18:48:20 +0300
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From: Pavel Begunkov <asml.silence@gmail.com>
To: Jens Axboe <axboe@kernel.dk>, io-uring@vger.kernel.org,
 linux-kernel@vger.kernel.org
Message-ID: <900dbb63-ae9e-40e6-94f9-8faa1c14389e@gmail.com>
Subject: Re: [PATCH v3] io_uring: don't wait when under-submitting
References: <6256169d519f72fe592e70be47a04aa0e9c3b9a1.1576333754.git.asml.silence@gmail.com>
 <c6f625bdb27ea3b929d0717ebf2aaa33ad5410da.1576335142.git.asml.silence@gmail.com>
 <a1f0a9ed-085f-dd6f-9038-62d701f4c354@kernel.dk>
 <3a102881-3cc3-ba05-2f86-475145a87566@kernel.dk>
In-Reply-To: <3a102881-3cc3-ba05-2f86-475145a87566@kernel.dk>

--DCNTUnmK3E0hZSIvJaRjxh8hQ2GTe7IcP
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On 15/12/2019 08:42, Jens Axboe wrote:
> On 12/14/19 11:43 AM, Jens Axboe wrote:
>> On 12/14/19 7:53 AM, Pavel Begunkov wrote:
>>> There is no reliable way to submit and wait in a single syscall, as
>>> io_submit_sqes() may under-consume sqes (in case of an early error).
>>> Then it will wait for not-yet-submitted requests, deadlocking the use=
r
>>> in most cases.
>>>
>>> In such cases adjust min_complete, so it won't wait for more than
>>> what have been submitted in the current call to io_uring_enter(). It
>>> may be less than totally in-flight including previous submissions,
>>> but this shouldn't do harm and up to a user.
>>
>> Thanks, applied.
>=20
> This causes a behavioral change where if you ask to submit 1 but
> there's nothing in the SQ ring, then you would get 0 before. Now
> you get -EAGAIN. This doesn't make a lot of sense, since there's no
> point in retrying as that won't change anything.
>=20
> Can we please just do something like the one I sent, instead of trying
> to over-complicate it?
>=20

Ok, when I get to a compiler.

--=20
Pavel Begunkov


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