linux-kernel - Re: [RFC][PATCH] Make io

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Message-ID: <20120724223702.GB6723@lenny.home.zabbo.net>
Date:	Tue, 24 Jul 2012 15:37:02 -0700
From:	Zach Brown <zab@...bo.net>
To:	Ankit Jain <jankit@...e.de>
Cc:	Al Viro <viro@...iv.linux.org.uk>, bcrl@...ck.org,
	linux-fsdevel@...r.kernel.org, linux-aio@...ck.org,
	linux-kernel@...r.kernel.org, Jan Kara <jack@...e.cz>
Subject: Re: [RFC][PATCH] Make io_submit non-blocking

On Tue, Jul 24, 2012 at 05:11:05PM +0530, Ankit Jain wrote:
> 
> Currently, io_submit tries to execute the io requests on the
> same thread, which could block because of various reaons (eg.
> allocation of disk blocks). So, essentially, io_submit ends
> up being a blocking call.

Yup, sadly that's how its built.  A blocking submission phase that
returns once completion doesn't need the submitters's context.  It
happens to mostly work for O_DIRECT block IO most of the time.

> With this patch, io_submit prepares all the kiocbs and then
> adds (kicks) them to ctx->run_list (kicked) in one go and then
> schedules the workqueue. The actual operations are not executed
> on io_submit's process context, so it can return very quickly.

Strong nack; this isn't safe without having done the work to ensure that
all the task_struct references under the f_op->aio_*() paths won't be
horribly confused to find a kernel thread instead of the process that
called io_submit().

The one-off handling of the submitters's cred is an indication that
there might be other cases to worry about :).

> 3. Also, I tried not using aio_queue_work from io_submit call, and instead
> depending on an already scheduled one or the iocbs being run when
> io_getevents gets called. This seemed to give improved perfomance. But
> does this constitute as change of api semantics?

You can't rely on io_getevents() being called for forward progress.  Its
perfectly reasonable for a task to wait for io completion by polling an
eventfd that aio_complete() notifies, for instance.

- z
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