Currently blk-mq does not report any event when two requests get merged
in the elevator. This then results in difficult to understand sequence
of events like:
...
8,0 34 1579 0.608765271 2718 I WS 215023504 + 40 [dbench]
8,0 34 1584 0.609184613 2719 A WS 215023544 + 56 <- (8,4) 2160568
8,0 34 1585 0.609184850 2719 Q WS 215023544 + 56 [dbench]
8,0 34 1586 0.609188524 2719 G WS 215023544 + 56 [dbench]
8,0 3 602 0.609684162 773 D WS 215023504 + 96 [kworker/3:1H]
8,0 34 1591 0.609843593 0 C WS 215023504 + 96 [0]
and you can only guess (after quite some headscratching since the above
excerpt is intermixed with a lot of other IO) that request 215023544+56
got merged to request 215023504+40. Provide proper event for request
merging like we used to do in the legacy block layer.
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
block/blk-merge.c | 2 ++
include/trace/events/block.h | 15 +++++++++++++++
kernel/trace/blktrace.c | 10 ++++++++++
3 files changed, 27 insertions(+)
diff --git a/block/blk-merge.c b/block/blk-merge.c
index f0b0bae075a0..9c9fb21584b6 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -793,6 +793,8 @@ static struct request *attempt_merge(struct request_queue *q,
*/
blk_account_io_merge_request(next);
+ trace_block_rq_merge(q, next);
+
/*
* ownership of bio passed from next to req, return 'next' for
* the caller to free
diff --git a/include/trace/events/block.h b/include/trace/events/block.h
index 81b43f5bdf23..b81205560782 100644
--- a/include/trace/events/block.h
+++ b/include/trace/events/block.h
@@ -211,6 +211,21 @@ DEFINE_EVENT(block_rq, block_rq_issue,
TP_ARGS(q, rq)
);
+/**
+ * block_rq_merge - merge request with another one in the elevator
+ * @q: queue holding operation
+ * @rq: block IO operation operation request
+ *
+ * Called when block operation request @rq from queue @q is merged to another
+ * request queued in the elevator.
+ */
+DEFINE_EVENT(block_rq, block_rq_merge,
+
+ TP_PROTO(struct request_queue *q, struct request *rq),
+
+ TP_ARGS(q, rq)
+);
+
/**
* block_bio_bounce - used bounce buffer when processing block operation
* @q: queue holding the block operation
diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
index ea47f2084087..41521216d3eb 100644
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -834,6 +834,13 @@ static void blk_add_trace_rq_issue(void *ignore,
blk_trace_request_get_cgid(q, rq));
}
+static void blk_add_trace_rq_merge(void *ignore,
+ struct request_queue *q, struct request *rq)
+{
+ blk_add_trace_rq(rq, 0, blk_rq_bytes(rq), BLK_TA_BACKMERGE,
+ blk_trace_request_get_cgid(q, rq));
+}
+
static void blk_add_trace_rq_requeue(void *ignore,
struct request_queue *q,
struct request *rq)
@@ -1115,6 +1122,8 @@ static void blk_register_tracepoints(void)
WARN_ON(ret);
ret = register_trace_block_rq_issue(blk_add_trace_rq_issue, NULL);
WARN_ON(ret);
+ ret = register_trace_block_rq_merge(blk_add_trace_rq_merge, NULL);
+ WARN_ON(ret);
ret = register_trace_block_rq_requeue(blk_add_trace_rq_requeue, NULL);
WARN_ON(ret);
ret = register_trace_block_rq_complete(blk_add_trace_rq_complete, NULL);
@@ -1161,6 +1170,7 @@ static void blk_unregister_tracepoints(void)
unregister_trace_block_bio_bounce(blk_add_trace_bio_bounce, NULL);
unregister_trace_block_rq_complete(blk_add_trace_rq_complete, NULL);
unregister_trace_block_rq_requeue(blk_add_trace_rq_requeue, NULL);
+ unregister_trace_block_rq_merge(blk_add_trace_rq_merge, NULL);
unregister_trace_block_rq_issue(blk_add_trace_rq_issue, NULL);
unregister_trace_block_rq_insert(blk_add_trace_rq_insert, NULL);
--
2.16.4
