On Fri, 19 Sep 2008 19:11:22 -0400 (EDT) Kiyoshi Ueda <k-ueda@xxxxxxxxxxxxx> wrote: > > Back in the days when we first did the backing_dev_info.congested_fn() > > logic it was decided that there basically was no single place in which > > the congested state could be stored. > > > > So we ended up deciding that whenever a caller wants to know a > > backing_dev's congested status, it has to call in to the > > ->congested_fn() and that congested_fn would then call down into all > > the constituent low-level drivers/queues/etc asking each one if it is > > congested. > > bdi_lld_congested() also does that using bdi_congested(), which calls > ->congested_fn(). > And only real device drivers (e.g. scsi, ide) set/clear the flag. > Stacking drivers like request-based dm don't. umm, OK, that should work. > So stacking drivers always check the BDI_lld_congested flag of > the bottom device of the device stack. How does a stacking driver know that the backing_device which it is looking at is a "lowest level" device? I don't think it does - only the code which implements that device knows this, so the stacking driver has to call into that device's congested_fn(), yes? In which case one wonders why the state was stored in the backing_dev_info at all. Why not store it in the device-private data to avoid confusion and abuse? > BDI_[write|read]_congested flags have been using for queue's > congestion, so that I/O queueing/merging can be proceeded even if > the lld is congested. So I added a new flag. iirc, BDI_read/write_congested predated the introduction of the congested_fn() and perhaps should have been removed once we went to the congested_fn approach. But it's been a while since I spent a lot of time looking in there. > > > I mean, as a simple example which is probably wrong - what happens if a > > single backing_dev is implemented via two different disks and > > controllers, and they both become congested and then one of them comes > > uncongested. Is there no way in which the above implemention can > > incorrectly flag the backing_dev as being uncongested? > > Do you mean that "a single backing_dev via two disks/controllers" is > a dm device (e.g. a dm-multipath device having 2 paths, a dm-mirror > device having 2 disks)? Something along those lines, sure. > If so, dm doesn't set/clear the flag, and the decision, whether > the dm device itself is congested or not, depends on dm's target driver. > > For example of dm-multipath, > o call bdi_lld_congested() for each path. > o if one of the paths are uncongested, dm-multipath will decide > the dm device is uncongested and dispatch incoming I/Os to > the uncongested path. hm, OK. > For example of dm-mirror, > o call bdi_lld_congested() for each disk. > o if the incoming I/O is READ, same logic as dm-multipath above. > if the incoming I/O is WRITE, dm-mirror will decide the dm device > is uncongested only when all disks are uncongested. > > Thanks, > Kiyoshi Ueda -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
