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

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Date:	Wed, 20 May 2009 18:49:50 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Jason Baron <jbaron@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	fweisbec@...il.com, laijs@...fujitsu.com, rostedt@...dmis.org,
	peterz@...radead.org, mathieu.desnoyers@...ymtl.ca,
	mbligh@...gle.com, roland@...hat.com, fche@...hat.com
Subject: Re: [PATCH 0/3] tracepoints: delay argument evaluation

Is it possible to convert blktrace to use event tracer? Then in this case we
can pass 'md' as the parameter to trace_block_bio_complete and dereference
md->queue during assignment.

Jiaying

On Wed, May 20, 2009 at 8:42 AM, Jason Baron <jbaron@...hat.com> wrote:
> On Wed, May 20, 2009 at 09:33:48AM +0200, Ingo Molnar wrote:
>> hm, this is really a compiler bug in essence - the compiler should
>> delay the construction of arguments into unlikely branches - if the
>> arguments are only used there.
>>
>> We'd basically open-code a clear-cut:
>>
>>    trace_block_bio_complete(md->queue, bio);
>>
>> into this form:
>>
>>    trace(block_bio_complete, md->queue, bio);
>>
>> .. and this latter form could become moot (and a nuisance) if the
>> compiler is fixed.
>>
>> Have you tried very latest GCC, does it still have this optimization
>> problem?
>>
>> Note that the compiler getting this right would help a _lot_ of
>> other inline functions in the kernel as well. Arguments only used
>> within unlikely() branches are quite common.
>>
>>       Ingo
>
> hi,
>
> I e-mailed the gcc list, where they suggested using a macro, as I've
> done. They also suggested filing an enhancement request for this, which
> I've done: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40207 It seems
> like they agree with the suggestion.
>
> It still might make sense to make this requirement explicit (by adding
> the extra macro), as the tracepoint off case should really be as optimized as
> possible.
>
> thanks,
>
> -Jason
>
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