linux-kernel - blktrace: event traces displayed wrong while ftrace blktrace is active

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Date:	Mon, 13 Apr 2009 17:59:19 -0400
From:	"Theodore Ts'o" <tytso@....edu>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Jens Axboe <jens.axboe@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: blktrace: event traces displayed wrong while ftrace blktrace is active

Hi Li,

Thanks for working on the ftrace blktrace code; I was wondering if you
could take a look at something.   While the nop or syscall tracer is
selected ("echo nop > /sys/kernel/debug/tracing/current_tracer"), the
event traces display like this:

           <...>-4491  [001]  9556.305696: jbd2_start_commit: dev dm-0:8 transaction 335442
           <...>-4491  [001]  9556.305700: jbd2_commit_locking: dev dm-0:8 transaction 335442
           <...>-4491  [001]  9556.305713: jbd2_commit_flushing: dev dm-0:8 transaction 335442
           <...>-4491  [001]  9556.305720: jbd2_submit_inode_data: dev dm-0 transaction ino 3807454
           <...>-4491  [001]  9556.305833: jbd2_commit_logging: dev dm-0:8 transaction 335442
           <...>-4491  [000]  9556.307241: jbd2_end_commit: dev dm-0:8 transaction 335442 head 334303

That is, the timestamp information what CPU, what pid, etc., is present.

However, if the blk tracer is active, this information is missing:

jbd2_start_commit: dev dm-0:8 transaction 333227
jbd2_commit_locking: dev dm-0:8 transaction 333227
jbd2_commit_flushing: dev dm-0:8 transaction 333227
jbd2_submit_inode_data: dev dm-0 transaction ino 1466869
jbd2_submit_inode_data: dev dm-0 transaction ino 1310793
jbd2_commit_logging: dev dm-0:8 transaction 333227
jbd2_end_commit: dev dm-0:8 transaction 333227 head 332814

The context information for the blktrace is present:

              rm-25433 [001]  9578.307485:   8,18   m   N cfq25433 slice expired t=0
              rm-25433 [001]  9578.307486:   8,18   m   N cfq25433 put_queue
      kjournald2-14059 [001]  9579.032457:   8,18   A   W 5631242 + 8 <- (8,18) 3671312
      kjournald2-14059 [001]  9579.032462:   8,18   Q   W 5631242 + 8 [kjournald2]
      kjournald2-14059 [001]  9579.032470:   8,18   G   W 5631242 + 8 [kjournald2]

It's just the context information for the event traces which are
missing.

Anyway, if you could take a look at this, it would be much appreciated.
Usually I can interpolate the the timestamps from the surrounding
blktrace lines, but it would be nice if I didn't have to do that.

Regards,

	 	       	     	     	- Ted
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