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SubjectRe: [PATCH 4/4] tracing/filters: use ring_buffer_discard_commit for discarded events

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



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