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

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Date:	Wed, 15 Apr 2009 08:57:39 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Theodore Tso <tytso@....edu>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Li Zefan <lizf@...fujitsu.com>,
	Jens Axboe <jens.axboe@...cle.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: 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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