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Subject: Re: [PATCH] eventfd: convert to using ->write_iter()
To:     David Laight <David.Laight@ACULAB.COM>,
        "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
        "linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
References: <7b98e3c2-2d9f-002b-1da1-815d8522b594@kernel.dk>
 <de316af8f88947fabd1422b04df8a66e@AcuMS.aculab.com>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <7caa3703-af14-2ff6-e409-77284da11e1f@kernel.dk>
Date:   Mon, 3 May 2021 11:57:08 -0600
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On 5/3/21 10:12 AM, David Laight wrote:
> From: Jens Axboe
>> Sent: 03 May 2021 15:58
>>
>> Had a report on writing to eventfd with io_uring is slower than it
>> should be, and it's the usual case of if a file type doesn't support
>> ->write_iter(), then io_uring cannot rely on IOCB_NOWAIT being honored
>> alongside O_NONBLOCK for whether or not this is a non-blocking write
>> attempt. That means io_uring will punt the operation to an io thread,
>> which will slow us down unnecessarily.
>>
>> Convert eventfd to using fops->write_iter() instead of fops->write().
> 
> Won't this have a measurable performance degradation on normal
> code that does write(event_fd, &one, 4);

If ->write_iter() or ->read_iter() is much slower than the non-iov
versions, then I think we have generic issues that should be solved.
That should not be a consideration, since the non-iov ones are
legacy and should not be adopted in new code.

-- 
Jens Axboe