writeback, blkcg: propagate non-root blkcg congestion state

Now that bdi layer can handle per-blkcg bdi_writeback_congested state,
blk_{set|clear}_congested() can propagate non-root blkcg congestion
state to them.

This can be easily achieved by disabling the root_rl tests in
blk_{set|clear}_congested().  Note that we still need those tests when
!CONFIG_CGROUP_WRITEBACK as otherwise we'll end up flipping root blkcg
wb's congestion state for events happening on other blkcgs.

v2: Updated for bdi_writeback_congested.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Jan Kara <jack@suse.cz>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

block/blk-core.c

@@ -65,23 +65,26 @@ static struct workqueue_struct *kblockd_workqueue;
 
 static void blk_clear_congested(struct request_list *rl, int sync)
 {
-	if (rl != &rl->q->root_rl)
-		return;
 #ifdef CONFIG_CGROUP_WRITEBACK
 	clear_wb_congested(rl->blkg->wb_congested, sync);
 #else
-	clear_wb_congested(rl->q->backing_dev_info.wb.congested, sync);
+	/*
+	 * If !CGROUP_WRITEBACK, all blkg's map to bdi->wb and we shouldn't
+	 * flip its congestion state for events on other blkcgs.
+	 */
+	if (rl == &rl->q->root_rl)
+		clear_wb_congested(rl->q->backing_dev_info.wb.congested, sync);
 #endif
 }
 
 static void blk_set_congested(struct request_list *rl, int sync)
 {
-	if (rl != &rl->q->root_rl)
-		return;
 #ifdef CONFIG_CGROUP_WRITEBACK
 	set_wb_congested(rl->blkg->wb_congested, sync);
 #else
-	set_wb_congested(rl->q->backing_dev_info.wb.congested, sync);
+	/* see blk_clear_congested() */
+	if (rl == &rl->q->root_rl)
+		set_wb_congested(rl->q->backing_dev_info.wb.congested, sync);
 #endif
 }