blktrace: Provide event for request merging

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@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-merge.c b/block/blk-merge.c
index f0b0bae075a0c8ea98c7158985b8d16fec7e9c4d..9c9fb21584b64e22c8acdc7d232cf41b3f6a4029 100644 (file)
--- 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 93b114226af8bf56d9725c38a6c993011ad54cf0..34d64ca306b1c7abc5639ff2cf715a4b69ca6fe2 100644 (file)
--- 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 c086c38f495427bc1ff48c8f3d9600193a9260b9..7ba62d68885aec065731d22ebb267d5a26fac191 100644 (file)
--- a/kernel/trace/blktrace.c
+++ b/kernel/trace/blktrace.c
@@ -860,6 +860,13 @@ 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)
@@ -1144,6 +1151,8 @@ 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);
@@ -1190,6 +1199,7 @@ 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);