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SubjectRe: blktrace: event traces displayed wrong while ftrace blktrace is active
Theodore Tso wrote:
> On Tue, Apr 14, 2009 at 11:25:33PM +0200, Frederic Weisbecker wrote:
>> Indeed. Note that you can overwrite this default by typing:
>>
>> echo context-info > trace_option
>>
>> after setting bkltrace as the current tracer.
>
> The problem with doing that is that blktrace will then display this:
>
> fsync-tester-27934 [001] 208.031278: fsync-tester-27934 [001] 208.031278: 254,4 Q WS 268392 + 8 [fsync-tester]
>
> I think the real problem is that blk trace is displaying the standard
> context-info, so it's suppressing the normal context info. So I have
> the choice on having no context information on my event trace lines,
> or two copies of the context information for the block trace lines.
>
> :-(
>

I'll provide a patch to solve this confusing behavior.

> I'm guessing things were done this way in order to support the old
> legancy blktrace format?
>

Right, in 2 ways:

1. we can make the output the same as userspace 'btrace /dev/sda':
# echo blk > current_tracer
# echo 1 > options/blk_classic

2. we can make the binary output parsable by userspace blkparse:
# echo blk > current_tracer
# echo bin > trace_options
# cat trace_pipe | blkparse -i -


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