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Subjectblktrace: event traces displayed wrong while ftrace blktrace is active
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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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