Messages in this thread | | | Date | Fri, 19 Mar 2004 08:57:05 +0100 | From | Jens Axboe <> | Subject | Re: 2.6.4-mm2 |
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On Thu, Mar 18 2004, Andrew Morton wrote: > 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.
That would get nasty, it's much more natural to track it from the other end. I view it as a dm (or whatever problem) that they need to track who has pending io on their behalf, which is pretty easy to to from eg __map_bio().
The only addition is putting the old q->plug_list back, but using it on stacking drivers like dm. dm then maintains a per-dm plugged list that it can use when dm_unplug() runs.
-- Jens Axboe
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