linux-kernel - Re: [PATCH 4/4] tracing/filters: use ring_buffer_discard

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Date:	Fri, 3 Apr 2009 21:11:20 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Tom Zanussi <tzanussi@...il.com>, linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <srostedt@...hat.com>
Subject: Re: [PATCH 4/4] tracing/filters: use ring_buffer_discard_commit for
 discarded events


On Fri, 3 Apr 2009, Ingo Molnar wrote:

> 
> * Steven Rostedt <rostedt@...dmis.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

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