io-controller: Document for blkio.weight_device

Here is the document for blkio.weight_device

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 30 insertions, 1 deletions

diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
index db054ea3e7fb..d422b410a995 100644
--- a/Documentation/cgroups/blkio-controller.txt
+++ b/Documentation/cgroups/blkio-controller.txt
@@ -76,9 +76,38 @@ CONFIG_DEBUG_BLK_CGROUP
 Details of cgroup files
 =======================
 - blkio.weight
-        - Specifies per cgroup weight.
+        - Specifies per cgroup weight. This is default weight of the group
+          on all the devices until and unless overridden by per device rule.
+          (See blkio.weight_device).
           Currently allowed range of weights is from 100 to 1000.
 
+- blkio.weight_device
+        - One can specify per cgroup per device rules using this interface.
+          These rules override the default value of group weight as specified
+          by blkio.weight.
+
+          Following is the format.
+
+          #echo dev_maj:dev_minor weight > /path/to/cgroup/blkio.weight_device
+          Configure weight=300 on /dev/sdb (8:16) in this cgroup
+          # echo 8:16 300 > blkio.weight_device
+          # cat blkio.weight_device
+          dev     weight
+          8:16    300
+
+          Configure weight=500 on /dev/sda (8:0) in this cgroup
+          # echo 8:0 500 > blkio.weight_device
+          # cat blkio.weight_device
+          dev     weight
+          8:0     500
+          8:16    300
+
+          Remove specific weight for /dev/sda in this cgroup
+          # echo 8:0 0 > blkio.weight_device
+          # cat blkio.weight_device
+          dev     weight
+          8:16    300
+
 - blkio.time
         - disk time allocated to cgroup per device in milliseconds. First
           two fields specify the major and minor number of the device and