linux-kernel - Re: Building a tracing userspace tool in the kernel tree

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Message-ID: <48EE8FA4.8040003@nortel.com>
Date:	Thu, 09 Oct 2008 17:11:32 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Sam Ravnborg <sam@...nborg.org>, linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: Building a tracing userspace tool in the kernel tree

Peter Zijlstra wrote:
> On Thu, 2008-10-09 at 16:35 -0600, Chris Friesen wrote:
>>Peter Zijlstra wrote:
>>>On Thu, 2008-10-09 at 15:16 -0400, Mathieu Desnoyers wrote:

>>>>At the kernel summit, people seemed to be interested to have the basic
>>>>userspace tools required to extract and pretty-print a trace available
>>>>within the kernel tree. Therefore, what I am trying to do is something
>>>>along the lines of
>>>>
>>>>ltt/usr/
>>>>ltt/usr/tracectl/    (control tracing)
>>>>ltt/usr/tracesplice/ (splice buffers to disk)
>>>>ltt/usr/tracecat/    (merge sort and format the binary buffers into
>>>>                     human-readable text)

>>>I'd rather have you provide that interface from the kernel much like
>>>ftrace does. So we can do:
>>>
>>># cat /debug/tracing/lttng/trace

>>Do we really want to reserve memory in the kernel to store all the data? 
>>  Assuming not, do we really want to have to deal with filesystem 
>>namespaces in the kernel when interpreting which file we want to log to?
> 
> 
> Not quite sure I get what you mean here. The kernel already needs the
> memory anyway, as we keep the trace buffers in memory in either case.
> 
> All this does is provide a debugfs interface that does the exact same
> thing the tracecat proglet would otherwise do.
> 
> I don't know how filesystem namespaces and debugfs interact, but seeing
> as non of the debugfs users seem to be bothered with that, I don't see
> why we should be.

Maybe I misunderstood something.  I was under the impression that the 
standard LTT usage is to stream raw trace data to disk and then 
post-process it.  If we're writing to disk, we should probably think 
about filesystem namespaces.

Chris
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