linux-kernel - Re: [PATCH] [RFC] um: Convert ubd driver to blk-mq

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Date:   Sun, 03 Dec 2017 22:54:43 +0100
From:   Richard Weinberger <richard@....at>
To:     Christoph Hellwig <hch@....de>
Cc:     user-mode-linux-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, axboe@...com,
        anton.ivanov@...bridgegreys.com, linux-block@...r.kernel.org,
        david@...ma-star.at
Subject: Re: [PATCH] [RFC] um: Convert ubd driver to blk-mq

Christoph,

Am Mittwoch, 29. November 2017, 22:46:51 CET schrieb Christoph Hellwig:
> On Sun, Nov 26, 2017 at 02:10:53PM +0100, Richard Weinberger wrote:
> > MAX_SG is 64, used for blk_queue_max_segments(). This comes from
> > a0044bdf60c2 ("uml: batch I/O requests"). Is this still a good/sane
> > value for blk-mq?
> 
> blk-mq itself doesn't change the tradeoff.
> 
> > The driver does IO batching, for each request it issues many UML struct
> > io_thread_req request to the IO thread on the host side.
> > One io_thread_req per SG page.
> > Before the conversion the driver used blk_end_request() to indicate that
> > a part of the request is done.
> > blk_mq_end_request() does not take a length parameter, therefore we can
> > only mark the whole request as done. See the new is_last property on the
> > driver.
> > Maybe there is a way to partially end requests too in blk-mq?
> 
> You can, take a look at scsi_end_request which handles this for blk-mq
> and the legacy layer.  That being said I wonder if batching really
> makes that much sene if you execute each segment separately?

Anton did a lot of performance improvements in this area.
He has all the details.
AFAIK batching brings us more throughput because in UML all IO is done by
a different thread and the IPC has a certain overhead. 

> > Another obstacle with IO batching is that UML IO thread requests can
> > fail. Not only due to OOM, also because the pipe between the UML kernel
> > process and the host IO thread can return EAGAIN.
> > In this case the driver puts the request into a list and retried later
> > again when the pipe turns writable.
> > I’m not sure whether this restart logic makes sense with blk-mq, maybe
> > there is a way in blk-mq to put back a (partial) request?
> 
> blk_mq_requeue_request requeues requests that have been partially
> exectuted (or not at all for that matter).

Thanks this is what I needed.
BTW: How can I know which blk functions are not usable in blk-mq?
I didn't realize that I can use blk_update_request().

Thanks,
//richard