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, 9 insertions, 0 deletions

diff --git a/block/elevator.c b/block/elevator.c
index 76e3702d5381..5e734592bb40 100644
--- a/block/elevator.c
+++ b/block/elevator.c
@@ -539,6 +539,15 @@ void elv_merge_requests(struct request_queue *q, struct request *rq,
         q->last_merge = rq;
 }
 
+void elv_bio_merged(struct request_queue *q, struct request *rq,
+                        struct bio *bio)
+{
+        struct elevator_queue *e = q->elevator;
+
+        if (e->ops->elevator_bio_merged_fn)
+                e->ops->elevator_bio_merged_fn(q, rq, bio);
+}
+
 void elv_requeue_request(struct request_queue *q, struct request *rq)
 {
         /*