blkio: Add io_merged stat

This includes both the number of bios merged into requests belonging to this
cgroup as well as the number of requests merged together.
In the past, we've observed different merging behavior across upstream kernels,
some by design some actual bugs. This stat helps a lot in debugging such
problems when applications report decreased throughput with a new kernel
version.

This needed adding an extra elevator function to capture bios being merged as I
did not want to pollute elevator code with blkiocg knowledge and hence needed
the accounting invocation to come from CFQ.

Signed-off-by: Divyesh Shah<dpshah@google.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

1 files changed, 5 insertions, 0 deletions

diff --git a/Documentation/cgroups/blkio-controller.txt b/Documentation/cgroups/blkio-controller.txt
index ed04fe9cce1a..810e30171a54 100644
--- a/Documentation/cgroups/blkio-controller.txt
+++ b/Documentation/cgroups/blkio-controller.txt
@@ -134,6 +134,11 @@ Details of cgroup files
           minor number of the device, third field specifies the operation type
           and the fourth field specifies the io_wait_time in ns.
 
+- blkio.io_merged
+        - Total number of bios/requests merged into requests belonging to this
+          cgroup. This is further divided by the type of operation - read or
+          write, sync or async.
+
 - blkio.dequeue
         - Debugging aid only enabled if CONFIG_DEBUG_CFQ_IOSCHED=y. This
           gives the statistics about how many a times a group was dequeued