Re: Building a tracing userspace tool in the kernel tree

["Chris Friesen" <cfriesen@nortel.com>]

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