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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 <> 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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