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Subject: Re: [PATCH] io_uring: wakeup threads waiting for EPOLLOUT events
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     Alexander Viro <viro@zeniv.linux.org.uk>, io-uring@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org
References: <20200116134946.184711-1-sgarzare@redhat.com>
 <2d2dda92-3c50-ee62-5ffe-0589d4c8fc0d@kernel.dk>
 <20200116155557.mwjc7vu33xespiag@steredhat>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <5723453a-9326-e954-978e-910b8b495b38@kernel.dk>
Date:   Thu, 16 Jan 2020 09:00:24 -0700
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On 1/16/20 8:55 AM, Stefano Garzarella wrote:
> On Thu, Jan 16, 2020 at 08:29:07AM -0700, Jens Axboe wrote:
>> On 1/16/20 6:49 AM, Stefano Garzarella wrote:
>>> io_uring_poll() sets EPOLLOUT flag if there is space in the
>>> SQ ring, then we should wakeup threads waiting for EPOLLOUT
>>> events when we expose the new SQ head to the userspace.
>>>
>>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>>> ---
>>>
>>> Do you think is better to change the name of 'cq_wait' and 'cq_fasync'?
>>
>> I honestly think it'd be better to have separate waits for in/out poll,
>> the below patch will introduce some unfortunate cacheline traffic
>> between the submitter and completer side.
> 
> Agree, make sense. I'll send a v2 with a new 'sq_wait'.
> 
> About fasync, do you think could be useful the POLL_OUT support?
> In this case, maybe is not simple to have two separate fasync_struct,
> do you have any advice?

The fasync should not matter, it's all in the checking of whether the sq
side has any sleepers. This is rarely going to be the case, so as long
as we can keep the check cheap, then I think we're fine.

Since the use case is mostly single submitter, unless you're doing
something funky or unusual, you're not going to be needing POLLOUT ever.
Hence I don't want to add any cost for it, I'd even advocate just doing
waitqueue_active() perhaps, if we can safely pull it off.

-- 
Jens Axboe