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

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Date:	Fri, 3 Apr 2009 13:51:00 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
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

* 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.

	Ingo
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