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    tracing/filters: add run-time field descriptions to TRACE_EVENT_FORMAT events · e1112b4d
    Tom Zanussi authored
    This patch adds run-time field descriptions to all the event formats
    exported using TRACE_EVENT_FORMAT.  It also hooks up all the tracers
    that use them (i.e. the tracers in the 'ftrace subsystem') so they can
    also have their output filtered by the event-filtering mechanism.
    When I was testing this, there were a couple of things that fooled me
    into thinking the filters weren't working, when actually they were -
    I'll mention them here so others don't make the same mistakes (and file
    bug reports. ;-)
    One is that some of the tracers trace multiple events e.g. the
    sched_switch tracer uses the context_switch and wakeup events, and if
    you don't set filters on all of the traced events, the unfiltered output
    from the events without filters on them can make it look like the
    filtering as a whole isn't working properly, when actually it is doing
    what it was asked to do - it just wasn't asked to do the right thing.
    The other is that for the really high-volume tracers e.g. the function
    tracer, the volume of filtered events can be so high that it pushes the
    unfiltered events out of the ring buffer before they can be read so e.g.
    cat'ing the trace file repeatedly shows either no output, or once in
    awhile some output but that isn't there the next time you read the
    trace, which isn't what you normally expect when reading the trace file.
    If you read from the trace_pipe file though, you can catch them before
    they disappear.
    Changes from v1:
    As suggested by Frederic Weisbecker:
    - get rid of externs in functions
    - added unlikely() to filter_check_discard()
    Signed-off-by: default avatarTom Zanussi <>
    Signed-off-by: default avatarSteven Rostedt <>
    Signed-off-by: default avatarIngo Molnar <>