Messages in this thread |  | | | Date | Fri, 3 Apr 2009 13:51:00 +0200 | | From | Ingo Molnar <> | | Subject | Re: [PATCH 4/4] tracing/filters: use ring_buffer_discard_commit for discarded events |
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* 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.
Ingo
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