linux-kernel - Re: [PATCH] io_uring: remove wait loop spurious wakeups

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Message-ID: <62a8a6c7-9c5b-c9a4-9c73-c77db87c6637@kernel.dk>
Date:   Tue, 8 Oct 2019 11:00:19 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Pavel Begunkov <asml.silence@...il.com>,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] io_uring: remove wait loop spurious wakeups

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@...il.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.

> Tested with another ref-counting patch and got +5-8% to
> nops performance.
> 
> 


-- 
Jens Axboe