linux-kernel - Re: [PATCH-v2 2/2] Initialize mempool and elevator only for request-based dm devices

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Message-Id: <200908121417.28760.knikanth@suse.de>
Date:	Wed, 12 Aug 2009 14:17:28 +0530
From:	Nikanth Karthikesan <knikanth@...e.de>
To:	Kiyoshi Ueda <k-ueda@...jp.nec.com>
Cc:	Alasdair G Kergon <agk@...hat.com>,
	Mike Snitzer <snitzer@...hat.com>,
	Jens Axboe <jens.axboe@...cle.com>, dm-devel@...hat.com,
	linux-kernel@...r.kernel.org, Hannes Reinecke <hare@...e.de>
Subject: Re: [PATCH-v2 2/2] Initialize mempool and elevator only for request-based dm devices

Hi Kiyoshi Ueda,

On Wednesday 12 August 2009 07:45:56 Kiyoshi Ueda wrote:
> Hi Nikanth,
>
> On 08/11/2009 06:05 PM +0900, Nikanth Karthikesan wrote:
> > On Tuesday 11 August 2009 13:36:24 Kiyoshi Ueda wrote:
> >> On 08/10/2009 07:48 PM +0900, Nikanth Karthikesan wrote:
> >>> +
> >>> +		/*
> >>> +		 * reinitialize make_request_fn as it was reset to the
> >>> +		 * default __make_request by blk_init_allocate_queue
> >>> +		 */
> >>> +		md->saved_make_request_fn = md->queue->make_request_fn;
> >>> +		blk_queue_make_request(md->queue, dm_request);
> >>> +
> >>> +		blk_queue_softirq_done(md->queue, dm_softirq_done);
> >>> +		blk_queue_prep_rq(md->queue, dm_prep_fn);
> >>> +		blk_queue_lld_busy(md->queue, dm_lld_busy);
> >>> +	}
> >>> +
> >>>  	__unbind(md);
> >>>  	r = __bind(md, table, &limits);
> >>
> >> The queue has been registered at the device creation time by
> >> add_disk() in alloc_dev().
> >> Since the queue is reconfigured (elevator is attached), you have to
> >> update the queue registration (e.g. unregister, then re-register).
> >> But it may not be easy.  At least, there is no exported interface to
> >> unregister/re-register queue.
> >
> > Ah, yes. The scheduler attributes will not be exported in
> > /sys/block/dm*/queue/iosched. Exporting elv_register_queue() and calling
> > it here solves it. Something like..
> >
> > @@ -2203,6 +2199,29 @@ int dm_swap_table(struct mapped_device *md, struct
> > dm_table *table)
> >  		goto out;
> >  	}
> >
> > +	/* new device is being marked as request-based */
> > +	if (!md->map && dm_table_request_based(table)) {
> > +		/* initialize queue for request-based dm */
> > +		r = blk_init_allocated_queue(md->queue, dm_request_fn, NULL);
> > +		if (r)
> > +			goto out;
> > +
> > +		r = elv_register_queue(md->queue);
> > +		/* if (r)
> > +		 *	 goto out; Better to ignore, just like add_disk does ;-)
> > +		 */
> > +		/*
> > +		 * reinitialize make_request_fn as it was reset to the
> > +		 * default __make_request by blk_init_allocate_queue
> > +		 */
> > +		md->saved_make_request_fn = md->queue->make_request_fn;
> > +		blk_queue_make_request(md->queue, dm_request);
> > +
> > +		blk_queue_softirq_done(md->queue, dm_softirq_done);
> > +		blk_queue_prep_rq(md->queue, dm_prep_fn);
> > +		blk_queue_lld_busy(md->queue, dm_lld_busy);
> > +	}
> > +
> >  	__unbind(md);
> >  	r = __bind(md, table, &limits);
> >
> > I would post the v3 of the patches with this change. Do you see any
> > problems in this?
>
> Humm, it might work for now, but I disagree with that.
>
> Since elevator is block internal and dm doesn't really care
> (its initialization is actually hidden in blk_init_allocated_queue()),
> directly calling elv_register_queue() from dm seems not right.
> It will likely introduce a bug by future changes in block layer.
>
> I think the right approach is to define a proper block layer interface
> to reflect the queue configuration change.
> That's why I said "Updating the queue registration may not be easy".
>

I do not see too much of overhead in the future with this approach, atleast no 
more than "proper block layer interface". IMHO, unregistering the queue and 
registering the queue again with the elevator, is basically wasting CPU cycles 
and possibly would confuse the user-space, which may be watching the sysfs... 
Or asking block layer to recheck and find what we have changed in the 
request_queue. It does not sound like the best solution.

It is better to tell the block-layer that we have added a q->request_fn 
function, so initialize the elevator.

If block layer, exports elv_register_queue() and document it, it would become 
a proper block layer interface, right? Device-mapper would always depend on 
internals of block-layer to some extent. ;-)

Thanks
Nikanth
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