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

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Date:	Thu, 4 Mar 2010 12:25:31 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	rostedt@...dmis.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


* 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.

> 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.)

Thanks,

	Ingo
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