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SubjectRe: [PATCH 0/3] tracepoints: delay argument evaluation

[ 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.

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?

-- Steve



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