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

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Date:	Thu, 21 May 2009 10:41:41 +0800
From:	Li Zefan <lizf@...fujitsu.com>
To:	Jiaying Zhang <jiayingz@...gle.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

Jiaying Zhang wrote:
> But if we convert blktrace to use event tracer interface, we can have:
> 

Actually we are converting block tracepoints to TRACE_EVENT, and we are
not going to remove relay+ioctl-based blktrace or ftrace-plugin blktrace.

> 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;
> ...
>         ),
> );
> 

I'm not sure if this piece of code can pass compile when !CONFIG_MD
if we put it in include/trace/events/block.h with other block tracepoints.

And what if we have another trace_block_bio_complete(rq, bio) in blk-core.c?

Even it works, still this is not the key to this issue.

We have some other ptr-deref in block tracepoints and other places:

  static inline void blk_partition_remap(struct bio *bio)
  {
	...
	trace_block_remap(bdev_get_queue(bio->bi_bdev), bio,
			    bdev->bd_dev,
			    bio->bi_sector - p->start_sect);
	...
  }

  static int __end_that_request_first(struct request *req, int error,
				    int nr_bytes)
  {
	...
	trace_block_rq_complete(req->q, req);
	...
  }

> 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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