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Subject[PATCH 6/6 v5.1] blkio-cgroup: Document for blkio.use_hierarchy interface
Document for blkio.use_hierarchy interface

Signed-off-by: Gui Jianfeng <guijianfen@cn.fujitsu.com>
---
Documentation/cgroups/blkio-controller.txt | 81 +++++++++++++++++++++-------
1 files changed, 62 insertions(+), 19 deletions(-)
diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
index 4ed7b5c..24399f4 100644
--- a/Documentation/cgroups/blkio-controller.txt
+++ b/Documentation/cgroups/blkio-controller.txt
@@ -91,30 +91,62 @@ Throttling/Upper Limit policy

Hierarchical Cgroups
====================
-- Currently none of the IO control policy supports hierarhical groups. But
- cgroup interface does allow creation of hierarhical cgroups and internally
- IO policies treat them as flat hierarchy.
+- Cgroup interface allows creation of hierarchical cgroups. Currently,
+ internally IO policies are able to treat them as flat hierarchy or
+ hierarchical hierarchy. Both hierarchical bandwidth division and flat
+ bandwidth division are supported. "blkio.use_hierarchy" can be used to
+ switch between flat mode and hierarchical mode.

- So this patch will allow creation of cgroup hierarhcy but at the backend
- everything will be treated as flat. So if somebody created a hierarchy like
- as follows.
+ Note: Currently, "blkio.use_hierarchy" only effects proportional bandwidth
+ division. For Throttling logic, it still continues to treat everything as flat.

- root
- / \
- test1 test2
- |
- test3
+ Consider the following CGroup hierarchy:

- CFQ and throttling will practically treat all groups at same level.
+ Root
+ / | \
+ Grp1 Grp2 tsk1
+ / \
+ Grp3 tsk2

- pivot
- / | \ \
- root test1 test2 test3
+ If blkio.use_hierarchy is disabled in all CGroups, CFQ will practically treat all groups
+ at the same level.

- Down the line we can implement hierarchical accounting/control support
- and also introduce a new cgroup file "use_hierarchy" which will control
- whether cgroup hierarchy is viewed as flat or hierarchical by the policy..
- This is how memory controller also has implemented the things.
+ Pivot tree
+ / | | \
+ Root Grp1 Grp2 Grp3
+ / |
+ tsk1 tsk2
+
+ If blkio.use_hierarchy is enabled in Root group, then all children will inherit it, thus
+ all children group have use_hierarchy=1 set automatically and looks as follows.
+
+ Pivot tree
+ |
+ Root
+ / | \
+ Grp1 Grp2 tsk1
+ / \
+ Grp3 tsk2
+
+ If blkio.use_hierarchy is enabled in Grp1 and Grp3, CFQ will treat groups and tasks as the
+ same view in CGroup hierarchy, it looks as follows.
+
+
+ Pivot tree
+ / | \
+ Root Grp1 Grp2
+ / / \
+ tsk1 Grp3 tsk2
+
+ Root, Grp1 and Grp2 are treated at the same level under Pivot tree. tsk1 stays under Root.
+ Grp3 and tsk2 are treated at the same level under Grp1. Below is the mapping between
+ task io priority and io weight:
+
+ prio 0 1 2 3 4 5 6 7
+ weight 1000 868 740 612 484 356 228 100
+
+ Note: Regardless of the use_hierarchy setting in Root group, Root group is always put onto
+ Pivot tree.

Various user visible config options
===================================
@@ -169,6 +201,17 @@ Proportional weight policy files
dev weight
8:16 300

+- blkio.use_hierarchy
+ - Switch between hierarchical mode and flat mode as stated above.
+ blkio.use_hierarchy == 1 means hierarchical mode is enabled.
+ blkio.use_hierarchy == 0 means flat mode is enabled.
+ You can set this interface only if there isn't any child CGroup under
+ this CGroup. If one CGroup's blkio.use_hierarchy is set, the created
+ children will inherit it. it's not allowed to unset it in children.
+ The default mode in Root CGroup is flat.
+ blkio.use_hierarchy only works for proportional bandwidth division
+ as of today and doesn't have any effect on throttling logic.
+
- blkio.time
- disk time allocated to cgroup per device in milliseconds. First
two fields specify the major and minor number of the device and
--
1.7.1


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