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

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Message-Id: <1223592050.7382.48.camel@lappy.programming.kicks-ass.net>
Date:	Fri, 10 Oct 2008 00:40:49 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Chris Friesen <cfriesen@...tel.com>
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

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:
> > 
> >>Hi Sam,
> >>
> >>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.


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