linux-kernel - Re: [PATCH] eventfd: convert to using ->write

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Message-ID: <20210503180207.GD1847222@casper.infradead.org>
Date:   Mon, 3 May 2021 19:02:07 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     David Laight <David.Laight@...lab.com>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] eventfd: convert to using ->write_iter()

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.

> That should not be a consideration, since the non-iov ones are
> legacy and should not be adopted in new code.
> 
> -- 
> Jens Axboe
>