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

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Date:	Wed, 20 May 2009 09:12:04 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	linux-kernel@...r.kernel.org, fweisbec@...il.com, mingo@...e.hu,
	laijs@...fujitsu.com, rostedt@...dmis.org,
	mathieu.desnoyers@...ymtl.ca, jiayingz@...gle.com,
	mbligh@...gle.com, roland@...hat.com, fche@...hat.com
Subject: Re: [PATCH 0/3] tracepoints: delay argument evaluation

On Tue, 2009-05-19 at 17:03 -0400, Jason Baron 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've tried '__always_inline', but that did not fix this issue. Obviously this
> change will require changes to all the callsites. But, that shouldn't be
> very hard, I've already included the scheduler and block changes with this
> patch. I think its important to minimize code execution in the off case, and
> thus going through all the callsites is well worth it. If we agree on this
> change, I can change the rest in very short order.
> 
> Below I'm also showing the assembly in the 'dec_pending()' function before and
> after this change to show the difference it makes. The arguments to the
> tracepoint are as above, 'md->queue' and 'bio'. Notice the 2 extra instructions,
> before the initial 'je', that could be moved after the 'je'.

I really really hate this.

Why can't 

trace_block_bio_complete(arg..)
   if (__trace_block_bio_compete.state)
     __trace_block_bio_complete(arg)

work?

Surely its possible to wrap the whole stuff in yet another macro layer
that will do the conditional before evaluating the arguments?
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