linux-kernel - Re: [RFC][PATCH 2/3] perf: Take a hot regs snapshot for trace events

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Date:	Thu, 04 Mar 2010 10:16:41 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>,
	Paul Mackerras <paulus@...ba.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>
Subject: Re: [RFC][PATCH 2/3] perf: Take a hot regs snapshot for trace
 events

On Thu, 2010-03-04 at 12:25 +0100, Ingo Molnar wrote:
> * Peter Zijlstra <peterz@...radead.org> wrote:
> 
> > On Wed, 2010-03-03 at 12:07 -0500, Steven Rostedt wrote:
> > > oops, my bad :-), I thought this was in the x86 arch directory. For the
> > > University, I was helping them with adding trace points for page faults
> > > when I came across this in arch/x86/mm/fault.c:
> > > 
> > >         perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, 0, regs, address);
> > > 
> > > 
> > > This is what I actually was wondering about. Why is it a "perf only" trace 
> > > point instead of a TRACE_EVENT()?
> > 
> > Because I wanted to make perf usable without having to rely on funny 
> > tracepoints. That is, I am less worried about committing software counters 
> > to ABI than I am about TRACE_EVENT(), which still gives me a terribly 
> > uncomfortable feeling.
> 
> I'd still like a much less error-prone and work-intense way of doing it.
> 
> I'd suggest we simply add a TRACE_EVENT_ABI() for such cases, where we really 
> want to expose a tracepoint to tooling, programmatically. Maybe even change 
> the usage sites to trace_foo_ABI(), to make it really clear and to make people 
> aware of the consequences.

Would this still be available as a normal trace event?

> 
> > Also, building with all CONFIG_TRACE_*=n will still yield a usable perf, 
> > which is something the embedded people might fancy, all that TRACE stuff 
> > adds lots of code.
> 
> Not a real issue i suspect when you do lock profiling ...
> 
> Or if it is, some debloating might be in order - and the detaching of event 
> enumeration and ftrace TRACE_EVENT infrastructure from other ftrace bits. (i 
> suggested an '/eventfs' special filesystem before, for nicely layed out 
> hierarchy of ftrace/perf events.)

Actually, we already have a way to decouple it.

include/trace/define_trace.h is the file that just adds the tracepoint
that is needed.

include/trace/ftrace.h is the file that does the magic and adds the code
for callbacks and tracing.

The perf hooks probably should not have gone in that file and been put
into a include/trace/perf.h file, and then in define_trace.h we would
add:

 #ifdef CONFIG_EVENT_TRACING
 #include <trace/ftrace.h>
 #endif

+#ifdef CONFIG_PERF_EVENTS
+#include <trace/perf.h>
+#endif

This should be done anyway. But it would also let you decouple ftrace
trace events from perf trace events but still let the two use the same
trace points.

-- Steve



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