blk-throttle: add throtl_qnode for dispatch fairness
With flat hierarchy, there's only single level of dispatching happening and fairness beyond that point is the responsibility of the rest of the block layer and driver, which usually works out okay; however, with the planned hierarchy support, service_queue->bio_lists[] can be filled up by bios from a single source. While the limits would still be honored, it'd be very easy to starve IOs from siblings or children. To avoid such starvation, this patch implements throtl_qnode and converts service_queue->bio_lists[] to lists of per-source qnodes which in turn contains the bio's. For example, when a bio is dispatched from a child group, the bio doesn't get queued on ->bio_lists[] directly but it first gets queued on the group's qnode which in turn gets queued on service_queue->queued[]. When dispatching for the upper level, the ->queued[] list is consumed in round-robing order so that the dispatch windows is consumed fairly by all IO sources. There are two ways a bio can come to a throtl_grp - directly queued to the group or dispatched from a child. For the former throtl_grp->qnode_on_self[rw] is used. For the latter, the child's ->qnode_on_parent[rw]. Note that this means that the child which is contributing a bio to its parent should stay pinned until all its bios are dispatched to its grand-parent. This patch moves blkg refcnting from bio add/remove spots to qnode activation/deactivation so that the blkg containing an active qnode is always pinned. As child pins the parent, this is sufficient for keeping the relevant sub-tree pinned while bios are in flight. The starvation issue was spotted by Vivek Goyal. v2: The original patch used the same throtl_grp->qnode_on_self/parent for reads and writes causing RWs to be queued incorrectly if there already are outstanding IOs in the other direction. They should be throtl_grp->qnode_on_self/parent[2] so that READs and WRITEs can use different qnodes. Spotted by Vivek Goyal. Signed-off-by: Tejun Heo <tj@kernel.org> Acked-by: Vivek Goyal <vgoyal@redhat.com>
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