Re: [PATCH 2/4] ftrace - add function_duration tracer

[Frederic Weisbecker <fweisbec@gmail.com>]

On Thu, Dec 10, 2009 at 09:29:27AM -0500, Steven Rostedt wrote:
> On Thu, 2009-12-10 at 13:03 +0100, Frederic Weisbecker wrote:
> 
> > This makes me feel I'm going to try converting the function graph tracer
> > into an event during the next cycle. It does not mean I could make it
> > usable as a perf event right away in the same shot that said, as you can
> > guess this is not a trivial plug. The current perf fast path is not yet
> > adapted for that.
> 
> I curious how you plan on doing this. The current event system shows one
> event per trace point. A straight forward approach would make every
> entry and exit of a function a trace point and that would lead to a very
> large kernel to handle that.


Oh no, I'm not planning to use tracepoints for that.


> Perhaps we could abstract out all entries and exits. We need to be able
> to link to a single point (entry or exit) not all.  This also has the
> added issue of using the ftrace infrastructure of nop the mcount call.
> We also need a way to enable a set of functions.
> 
> We may be able to abstract this out, but I'm hesitant on making this the
> only interface.


Hmm, yeah. The idea was just to move the use the struct trace to struct
trace_event. This would be about straightforward. A bit like kprobes: by
not using the TRACE_EVENT macros (would be impossible anyway) but
specific callbacks.

It would be one event.

set_ftrace_filter and set_graph_function can still be used to further
control dynamic patching. That's what I intended for a first conversion.

Another idea would be to abstract it through one trace event subsystem
that has one event per function. But that sounds a bit too much in term
of memory footprint. Also it's perhaps sufficient to abstract the
dynamic patching, but not enough to abstract set_graph_function.

But later on, a full trace event integration would probably imply
dicossiating dynamic tracing from the two function tracers.
For example if the function graph tracer asks to nop a function,
this shouldn't be propagated to a parallel function tracer user.
That's even worse once we get a perf integration, we can have
multiple parallel users of the function tracer. And patching
should probably adapt to parallel uses, maintaining a kind of
refcounting, extending the current function hashlist we have
for function profiling could probably help for that.