linux-kernel - Re: [PATCH 0/3] tracepoints: delay argument evaluation

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Date:	Tue, 19 May 2009 18:36:56 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	Jason Baron <jbaron@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Peter Zijlstra <peterz@...radead.org>, jiayingz@...gle.com,
	mbligh@...gle.com, roland@...hat.com,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH 0/3] tracepoints: delay argument evaluation


[ added Christoph ]

On Tue, 19 May 2009, Mathieu Desnoyers wrote:

> * Jason Baron (jbaron@...hat.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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