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SubjectRe: [RFC] [PATCH 7/7] Ftrace plugin for Uprobes
On Tue, 2010-01-12 at 05:54 +0100, Frederic Weisbecker wrote:

> Now what if I want to launch ls and want to profile a function
> inside. What can I do with a trace event. I can't create the
> probe event based on a pid as I don't know it in advance.
> I could give it the ls cmdline and it manages to activate
> on the next ls launched. This is racy as another ls can
> be launched concurrently.

You make a wrapper script:

<add probe to ls with pid> $$
exec $*

I do this all the time to limit the function tracer to a specific app.

echo $$ > /debug/tracing/set_ftrace_pid
echo function > /debug/tracing/current_tracer
exec $*

The exec will cause the ls to have the pid of $$.

-- Steve

> So I can only say there that an ftrace plugin or an ftrace trace
> event would be only a half-useful interface to exploit utrace
> possibilities because it only lets us trace already running
> apps. Moreover I bet the most chosen workflow to profile/trace
> uprobes is by launching an app and profile it from the beginning,
> not by profiling an already running one, which makes an ftrace
> interface even less than half useful there.
> ftrace is cool to trace the kernel, but this kind of tricky
> userspace tracing workflow is not adapted to it.
> What do you think?

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