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    SubjectRe: [PATCH 0/3] tracepoints: delay argument evaluation
    On Tue, May 19, 2009 at 06:36:56PM -0400, Steven Rostedt wrote:
    >
    > [ added Christoph ]
    >
    > On Tue, 19 May 2009, Mathieu Desnoyers wrote:
    >
    > > * Jason Baron (jbaron@redhat.com) wrote:
    > > > hi,
    > > >
    > > > After disassembling some of the tracepoints, I've noticed that arguments that
    > > > are passed as macros or that perform dereferences, evaluate prior to the
    > > > tracepoint on/off check. This means that we are needlessly impacting the
    > > > off case.
    > > >
    > > > I am proposing to fix this by adding a macro that first checks for on/off and
    > > > then calls 'trace_##name', preserving type checking. Thus, callsites have to
    > > > move from:
    > > >
    > > > trace_block_bio_complete(md->queue, bio);
    > > >
    > > > to:
    > > >
    > > > tracepoint_call(block_bio_complete, md->queue, bio);
    > > >
    > >
    > > I knew this limitation in the first place, but decided it was not worth
    > > uglifying the tracepoint call site for it.
    > >
    > > The expected use is to pass a pointer or a value as tracepoint argument
    > > and dereference it in the callback attached to it.
    > >
    > > Is there any _real_ added value for going through this API change pain ?
    > >
    >
    > I agree with Mathieu that I don't think we want to "uglify" the callers.
    > But I also agree with Jason that we must not add any overhead to the "off"
    > state when we can avoid it.
    >
    > If it comes down to the two, I would lean towards the "uglify" if it shows
    > performance benefits in the "off" case.



    Yeah, I agree with you, if we have no choice, the most important goal
    is to drop any overhead in tracing off-case.



    > Perhaps I'll try to see if I can fool CPP to getting both worlds. But this
    > will be tricky :-/
    >
    > When are we going to get our own C pre-processor?


    It starts to be really required....


    > -- Steve
    >



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