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Subject: Re: [PATCH] eventfd: convert to using ->write_iter()
To:     Matthew Wilcox <willy@infradead.org>
Cc:     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>
 <7caa3703-af14-2ff6-e409-77284da11e1f@kernel.dk>
 <20210503180207.GD1847222@casper.infradead.org>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <7263c088-22f5-d125-cf80-5ebbd9d110e5@kernel.dk>
Date:   Mon, 3 May 2021 12:05:13 -0600
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On 5/3/21 12:02 PM, Matthew Wilcox wrote:
> On Mon, May 03, 2021 at 11:57:08AM -0600, Jens Axboe wrote:
>> 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.
> 
> We do!
> 
> https://lore.kernel.org/linux-fsdevel/20210107151125.GB5270@casper.infradead.org/
> is one thread on it.  There have been others.

But then we really must get that fixed, imho ->read() and ->write()
should go away, and if the iter variants are 10% slower, then that should
get fixed up.

I'll go over that thread.

-- 
Jens Axboe