Return-Path: <linux-kernel-owner+w=401wt.eu-S936599AbZDDBLd@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S936599AbZDDBLd (ORCPT <rfc822;w@1wt.eu>);
	Fri, 3 Apr 2009 21:11:33 -0400
Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1763858AbZDDBLY
	(ORCPT <rfc822;linux-kernel-outgoing>);
	Fri, 3 Apr 2009 21:11:24 -0400
Received: from hrndva-omtalb.mail.rr.com ([71.74.56.124]:50134 "EHLO
	hrndva-omtalb.mail.rr.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1763471AbZDDBLX (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Fri, 3 Apr 2009 21:11:23 -0400
Date: Fri, 3 Apr 2009 21:11:20 -0400 (EDT)
From: Steven Rostedt <rostedt@goodmis.org>
X-X-Sender: rostedt@gandalf.stny.rr.com
To: Ingo Molnar <mingo@elte.hu>
cc: Tom Zanussi <tzanussi@gmail.com>, linux-kernel@vger.kernel.org,
       Andrew Morton <akpm@linux-foundation.org>,
       Thomas Gleixner <tglx@linutronix.de>,
       Peter Zijlstra <peterz@infradead.org>,
       Frederic Weisbecker <fweisbec@gmail.com>,
       Steven Rostedt <srostedt@redhat.com>
Subject: Re: [PATCH 4/4] tracing/filters: use ring_buffer_discard_commit for
 discarded events
In-Reply-To: <20090403115100.GH31399@elte.hu>
Message-ID: <alpine.DEB.2.00.0904032109400.32083@gandalf.stny.rr.com>
References: <20090402052721.013878388@goodmis.org> <20090402053522.834546042@goodmis.org> <1238663187.6621.13.camel@bookworm> <alpine.DEB.2.00.0904021033080.965@gandalf.stny.rr.com> <20090403115100.GH31399@elte.hu>
User-Agent: Alpine 2.00 (DEB 1167 2008-08-23)
MIME-Version: 1.0
Content-Type: TEXT/PLAIN; charset=US-ASCII
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Content-Length: 2316
Lines: 57


On Fri, 3 Apr 2009, Ingo Molnar wrote:

> 
> * Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > > [  240.462396]  [<ffffffff802c843b>] trace_function+0xab/0xc0
> > 
> > Oo, you have trace filters in trace_function. I'm not sure we want 
> > that. If we do, please make it a separate function register. That 
> > is, the function tracer is such a extreme hot path, that I would 
> > like to avoid as many branch conditionals as possible. With the 
> > dynamic function tracing, we can pick functions that do conditions 
> > when we want them, and we can pick those that do not. If you look 
> > at some of the register_ftrace_function callers, you will see 
> > where I've picked different functions to be called by the tracer 
> > depending on what the options are.
> 
> Actually, i like Tom's idea very much: as it allows pretty flexible 
> context based filtering - not just function based filtering.
> 
> So we could filter for:
> 
>  -    a specific PID
> 
>  - or a pattern of ->comm strings ['bash' or 'sshd' or 'hackbench']
> 
>  - or we could filter for non-zero preempt-counts
>    (i.e. critical sections only).
> 
> So this extends the function symbol based regexp mechanism in a 
> quite natural way, and it would be sad to not do this just because 
> it's ... arguably hard to implement robustly ;-)
> 
> The filter expression predicaments are pre-constructed and static at 
> the point of execution, so they rely on no external facility other 
> than some internal C code. So it should be possible to do this.
> 
> And if the tracepoint causes runtime overhead .. we need to optimize 
> that. We could register a different, filter-aware mcount callback 
> depending on whether there's any filter defined there.

This is exactly what I was talking about in my ramblings up above ;-)

That is, if filtering is not on, we can just enable the non filtered 
version of ftrace. As soon as the function tracer filtering is modified, 
we switch to a "filter-aware" version of the function tracer.

I'll look at modifying it do to something like this.

-- Steve

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/