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

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Date:	Wed, 20 May 2009 19:15:23 -0700
From:	Jiaying Zhang <jiayingz@...gle.com>
To:	Li Zefan <lizf@...fujitsu.com>
Cc:	Jason Baron <jbaron@...hat.com>, 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

But if we convert blktrace to use event tracer interface, we can have:

trace_block_bio_complete(md, bio);

TRACE_EVENT(block_bio_complete,
        TP_PROTO(struct mapped_device *md, struct bio *bio),
...
        TP_fast_assign(
                __entry->queue            = md->queue;
...
        ),
);

Jiaying

On Wed, May 20, 2009 at 6:59 PM, Li Zefan <lizf@...fujitsu.com> wrote:
> Jiaying Zhang wrote:
>> Is it possible to convert blktrace to use event tracer? Then in this case we
>
> Yes, I'm doing this, see:
>        http://marc.info/?l=linux-kernel&m=124228198011297&w=2
>
>> can pass 'md' as the parameter to trace_block_bio_complete and dereference
>> md->queue during assignment.
>>
>
> But the problem discussed here exists whether you use plain tracepoints
> or TRACE_EVENT.
>
> Though we can add a new tracepoint named trace_md_bio_complete, this is
> not the way to solve it.
>
>> 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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