linux-kernel - Re: [PATCH 1/2] ftrace: Add separate handler for ftrace:function event

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Date:	Wed, 25 Nov 2015 18:12:43 +0100
From:	Jiri Olsa <jolsa@...hat.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jiri Olsa <jolsa@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
	David Ahern <dsahern@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH 1/2] ftrace: Add separate handler for ftrace:function
 event

On Wed, Nov 25, 2015 at 12:02:57PM -0500, Steven Rostedt wrote:
> On Wed, 25 Nov 2015 17:50:57 +0100
> Jiri Olsa <jolsa@...hat.com> wrote:
> 
> > > This seems redundant. I never understood the control_ops that perf uses
> > > in the function tracing infrastructure. Why can't you just register the
> > > event->ops and have that ops set the filtering? Then the ftrace
> > > infrastructure will only call that event handler for the functions its
> > > filtered on. Then you don't need to do it again. Right now ftrace
> > > already does that with the generic "control_ops" that perf uses, but
> > > now you are doing it again. Seems rather pointless.  
> > 
> > well thats exactly what we are doing.. but as all ops
> > share single callback we need to find the proper event
> > this callback was triggered for
> 
> The ftrace_ops has a "private" field for the user to set. Could you
> make that point back to the event that allocated the ftrace_ops?
> Then the callback function could easily get the event that matches the
> ftrace_ops.

aaah did not see this one ;-) that will do.. nice

> 
> > 
> > currently we use tracepoint callback (perf_tp_event)
> > where the proper event is found based on the event->filter
> > 
> > however this is not the case for ftrace:function because
> > filter will not change the event->filter, but the ops filter
> 
> Yeah, I'm trying to figure out the paths here. I would love to remove
> the control_ops as that complicates the function tracing code a bit
> more than I would like it to be.

perf is using the control interface to enable and disable function
tracing any time the process (and its events) is scheduled in/out

sched_in(event)  -> ftrace_function_local_enable(&event->ftrace_ops)
sched_out(event) -> ftrace_function_local_disable(&event->ftrace_ops)

> 
> I just crashed function tracing by function tracing perf doing function
> tracing :-) I'm currently debugging that (and adding more code to help
> debug things like this).

;-)

jirka
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