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

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Message-ID: <20090519235245.GD6066@nowhere>
Date:	Wed, 20 May 2009 01:52:47 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	Jason Baron <jbaron@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, 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

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@...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.



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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