blk-throttle: make blk_throtl_drain() ready for hierarchy

The current blk_throtl_drain() assumes that all active throtl_grps are
queued on throtl_data->service_queue, which won't be true once
hierarchy support is implemented.

This patch makes blk_throtl_drain() perform post-order walk of the
blkg hierarchy draining each associated throtl_grp, which guarantees
that all bios will eventually be pushed to the top-level service_queue
in throtl_data.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Vivek Goyal <vgoyal@redhat.com>

1 files changed, 40 insertions, 11 deletions

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 918d22240856..8c6e13359781 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -1299,6 +1299,28 @@ out:
         return throttled;
 }
 
+/*
+ * Dispatch all bios from all children tg's queued on @parent_sq.  On
+ * return, @parent_sq is guaranteed to not have any active children tg's
+ * and all bios from previously active tg's are on @parent_sq->bio_lists[].
+ */
+static void tg_drain_bios(struct throtl_service_queue *parent_sq)
+{
+        struct throtl_grp *tg;
+
+        while ((tg = throtl_rb_first(parent_sq))) {
+                struct throtl_service_queue *sq = &tg->service_queue;
+                struct bio *bio;
+
+                throtl_dequeue_tg(tg);
+
+                while ((bio = bio_list_peek(&sq->bio_lists[READ])))
+                        tg_dispatch_one_bio(tg, bio_data_dir(bio));
+                while ((bio = bio_list_peek(&sq->bio_lists[WRITE])))
+                        tg_dispatch_one_bio(tg, bio_data_dir(bio));
+        }
+}
+
 /**
  * blk_throtl_drain - drain throttled bios
  * @q: request_queue to drain throttled bios for
@@ -1309,27 +1331,34 @@ void blk_throtl_drain(struct request_queue *q)
         __releases(q->queue_lock) __acquires(q->queue_lock)
 {
         struct throtl_data *td = q->td;
-        struct throtl_service_queue *parent_sq = &td->service_queue;
-        struct throtl_grp *tg;
+        struct blkcg_gq *blkg;
+        struct cgroup *pos_cgrp;
         struct bio *bio;
         int rw;
 
         queue_lockdep_assert_held(q);
+        rcu_read_lock();
 
-        while ((tg = throtl_rb_first(parent_sq))) {
-                struct throtl_service_queue *sq = &tg->service_queue;
+        /*
+         * Drain each tg while doing post-order walk on the blkg tree, so
+         * that all bios are propagated to td->service_queue.  It'd be
+         * better to walk service_queue tree directly but blkg walk is
+         * easier.
+         */
+        blkg_for_each_descendant_post(blkg, pos_cgrp, td->queue->root_blkg)
+                tg_drain_bios(&blkg_to_tg(blkg)->service_queue);
 
-                throtl_dequeue_tg(tg);
+        tg_drain_bios(&td_root_tg(td)->service_queue);
 
-                while ((bio = bio_list_peek(&sq->bio_lists[READ])))
-                        tg_dispatch_one_bio(tg, bio_data_dir(bio));
-                while ((bio = bio_list_peek(&sq->bio_lists[WRITE])))
-                        tg_dispatch_one_bio(tg, bio_data_dir(bio));
-        }
+        /* finally, transfer bios from top-level tg's into the td */
+        tg_drain_bios(&td->service_queue);
+
+        rcu_read_unlock();
         spin_unlock_irq(q->queue_lock);
 
+        /* all bios now should be in td->service_queue, issue them */
         for (rw = READ; rw <= WRITE; rw++)
-                while ((bio = bio_list_pop(&parent_sq->bio_lists[rw])))
+                while ((bio = bio_list_pop(&td->service_queue.bio_lists[rw])))
                         generic_make_request(bio);
 
         spin_lock_irq(q->queue_lock);