Re: [PATCH 0/3] A kernel tracing interface

[David Wilder <dwilder at us dot ibm dot com>]

Andrew Morton wrote:
> On Tue, 26 Feb 2008 14:38:37 -0800 "David J. Wilder" <dwilder@us.ibm.com> wrote:
>
> > These patches provide a kernel tracing interface called "trace".
> >
> > ChangeLog:
> > -Updated to 2.6.25-rc2-mm1
> > -Removed sem_watch example, to hard to maintain.
> > -Added a new example that demonstrates per-cpu continuous tracing
> >  of data generated using marker probes.
> > -Removed inline from relay patch.
> > -Moved examples into /sample directory.
> >
> > The motivation for "trace" is to:
> > - Provide a simple set of tracing primitives that will utilize the high-
> >   performance and low-overhead of relayfs for passing traces data from
> >   kernel to user space.
> > - Provide a common user interface for managing kernel traces.
> > - Allow for binary as well as ascii trace data.
> > - Incorporate features from the systemtap runtime that are
> >   useful to others.
>
> So... what's the story on this versus lttng?

Trace is simpler to use and a smaller bit of code than lttng.
It may not have all the features of lttng but it is good starting point 
that can be expanded upon.  I have several users of trace that just 
wanted a simple way to get data out of the kernel without the overhead 
of lttng trace is working well for them.  Trace also works well with 
markers and kprobes.  For example systemtap is is using the basic trace 
code to gather data from kprobes and markers.
> Is there some userspace code available for people to test this?  Maybe it's
> mentioned and I missed it.

I purposely designed trace so no special user code is required to gather 
trace data. Standard user utilities like cat, grep, sort and more is all 
that is needed.  I show an example in /samples/trace/fork_trace.c.

However since trace uses relay as its transport any user interfaces 
provided by relay can be used to read trace data. (documented in 
/Documentation/relay.txt)

> It'd be interesting to see a writeup of the proposed kernel<->userspace
> interfaces.  I see a description of kernel-internal interfaces, but how do
> users use it?

I will add some text in the documentation.
> I see it uses things from blktrace.  Can blktrace be switched over to being
> a client of this code?

yep.

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