Date: Thu, 18 Mar 2004 23:52:00 -0800
From: Andrew Morton <akpm@osdl.org>
To: Jens Axboe <axboe@suse.de>
Cc: markw@osdl.org, linux-kernel@vger.kernel.org
Subject: Re: 2.6.4-mm2
Message-Id: <20040318235200.25c376a9.akpm@osdl.org>
In-Reply-To: <20040319073919.GY22234@suse.de>
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	<20040318192707.GV22234@suse.de>
	<20040318191530.34e04cb2.akpm@osdl.org>
	<20040319073919.GY22234@suse.de>
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Jens Axboe <axboe@suse.de> wrote:
>
> > --- 25/drivers/md/dm-table.c~a	2004-03-18 19:03:15.130004696 -0800
>  > +++ 25-akpm/drivers/md/dm-table.c	2004-03-18 19:03:41.656971984 -0800
>  > @@ -893,7 +893,7 @@ void dm_table_unplug_all(struct dm_table
>  >  		struct dm_dev *dd = list_entry(d, struct dm_dev, list);
>  >  		request_queue_t *q = bdev_get_queue(dd->bdev);
>  >  
>  > -		if (q->unplug_fn)
>  > +		if (q->unplug_fn && queue_needs_unplug(q)))
>  >  			q->unplug_fn(q);
>  >  	}
>  >  }
>  > 
>  > 
>  > to reduce the computational expense of dm_table_unplug_all() a bit.
>  > 
>  > But we're barking up the wrong tree here.  Mark, if it's OK I'll run up
>  > some kernels for you to test.
> 
>  I thought about this last night, and I have a better idea that gets the
>  same accomplished. The problem right now is indeed that we aren't
>  tracking who needs to be unplugged, like we used to. The solution is to
>  do the exact same style plugging (with block helpers) that we used to,
>  except the plug_list is maintained in the driver. So when you do
>  dm_unplug(), it doesn't _have_ to iterate the full device list, only
>  those that do need kicking.
> 
>  I'll produce a patch to fix this this morning. First coffee.

Yes, it would be nice but I fear that it gets complicated.

Is it not the case that two dm maps can refer to the same queue?  Say, one
map uses /dev/hda1 and another map uses /dev/hda2?

If so, then when the /dev/hda queue is plugged we need to tell both the
higher-level maps that this queue needs an unplug.  So blk_plug_device()
and the various unplug functions need to perform upcalls to an arbitrary
number of higher-level drivers, and those drivers need to keep track of the
currently-plugged queues without adding data structures to the
request_queue structure.

It can be done of course, but could get messy.

Or am I missing something?
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