blk-throttle: implement proper hierarchy support

With the recent updates, blk-throttle is finally ready for proper
hierarchy support.  Dispatching now honors service_queue->parent_sq
and propagates correctly.  The only thing missing is setting
->parent_sq correctly so that throtl_grp hierarchy matches the cgroup
hierarchy.

This patch updates throtl_pd_init() such that service_queues form the
same hierarchy as the cgroup hierarchy if sane_behavior is enabled.
As this concludes proper hierarchy support for blkcg, the shameful
.broken_hierarchy tag is removed from blkio_subsys.

v2: Updated blkio-controller.txt as suggested by Vivek.

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

4 files changed, 38 insertions, 23 deletions

diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
index da272c8f44e7..cd556b914786 100644
--- a/Documentation/cgroups/blkio-controller.txt
+++ b/Documentation/cgroups/blkio-controller.txt
@@ -94,11 +94,13 @@ Throttling/Upper Limit policy
 
 Hierarchical Cgroups
 ====================
-- Currently only CFQ supports hierarchical groups. For throttling,
-  cgroup interface does allow creation of hierarchical cgroups and
-  internally it treats them as flat hierarchy.
 
-  If somebody created a hierarchy like as follows.
+Both CFQ and throttling implement hierarchy support; however,
+throttling's hierarchy support is enabled iff "sane_behavior" is
+enabled from cgroup side, which currently is a development option and
+not publicly available.
+
+If somebody created a hierarchy like as follows.
 
                         root
                         /  \
@@ -106,21 +108,20 @@ Hierarchical Cgroups
                         |
                      test3
 
-  CFQ will handle the hierarchy correctly but and throttling will
-  practically treat all groups at same level. For details on CFQ
-  hierarchy support, refer to Documentation/block/cfq-iosched.txt.
-  Throttling will treat the hierarchy as if it looks like the
-  following.
+CFQ by default and throttling with "sane_behavior" will handle the
+hierarchy correctly.  For details on CFQ hierarchy support, refer to
+Documentation/block/cfq-iosched.txt.  For throttling, all limits apply
+to the whole subtree while all statistics are local to the IOs
+directly generated by tasks in that cgroup.
+
+Throttling without "sane_behavior" enabled from cgroup side will
+practically treat all groups at same level as if it looks like the
+following.
 
                                 pivot
                              /  /   \  \
                         root  test1 test2  test3
 
-  Nesting cgroups, while allowed, isn't officially supported and blkio
-  genereates warning when cgroups nest. Once throttling implements
-  hierarchy support, hierarchy will be supported and the warning will
-  be removed.
-
 Various user visible config options
 ===================================
 CONFIG_BLK_CGROUP
diff --git a/block/blk-cgroup.c b/block/blk-cgroup.c
index d0747605f56c..290792a13e3c 100644
--- a/block/blk-cgroup.c
+++ b/block/blk-cgroup.c
@@ -911,14 +911,6 @@ struct cgroup_subsys blkio_subsys = {
         .subsys_id = blkio_subsys_id,
         .base_cftypes = blkcg_files,
         .module = THIS_MODULE,
-
-        /*
-         * blkio subsystem is utterly broken in terms of hierarchy support.
-         * It treats all cgroups equally regardless of where they're
-         * located in the hierarchy - all cgroups are treated as if they're
-         * right below the root.  Fix it and remove the following.
-         */
-        .broken_hierarchy = true,
 };
 EXPORT_SYMBOL_GPL(blkio_subsys);
 
diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 27f006bb363b..08a32dfd3844 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -397,10 +397,30 @@ static void throtl_pd_init(struct blkcg_gq *blkg)
 {
         struct throtl_grp *tg = blkg_to_tg(blkg);
         struct throtl_data *td = blkg->q->td;
+        struct throtl_service_queue *parent_sq;
         unsigned long flags;
         int rw;
 
-        throtl_service_queue_init(&tg->service_queue, &td->service_queue);
+        /*
+         * If sane_hierarchy is enabled, we switch to properly hierarchical
+         * behavior where limits on a given throtl_grp are applied to the
+         * whole subtree rather than just the group itself.  e.g. If 16M
+         * read_bps limit is set on the root group, the whole system can't
+         * exceed 16M for the device.
+         *
+         * If sane_hierarchy is not enabled, the broken flat hierarchy
+         * behavior is retained where all throtl_grps are treated as if
+         * they're all separate root groups right below throtl_data.
+         * Limits of a group don't interact with limits of other groups
+         * regardless of the position of the group in the hierarchy.
+         */
+        parent_sq = &td->service_queue;
+
+        if (cgroup_sane_behavior(blkg->blkcg->css.cgroup) && blkg->parent)
+                parent_sq = &blkg_to_tg(blkg->parent)->service_queue;
+
+        throtl_service_queue_init(&tg->service_queue, parent_sq);
+
         for (rw = READ; rw <= WRITE; rw++) {
                 throtl_qnode_init(&tg->qnode_on_self[rw], tg);
                 throtl_qnode_init(&tg->qnode_on_parent[rw], tg);
diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 5047355b9a0f..09f1a1408ae0 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -272,6 +272,8 @@ enum {
          * - memcg: use_hierarchy is on by default and the cgroup file for
          *   the flag is not created.
          *
+         * - blkcg: blk-throttle becomes properly hierarchical.
+         *
          * The followings are planned changes.
          *
          * - release_agent will be disallowed once replacement notification