linux-kernel - Re: Re: [RFC PATCH 0/2] Perf support to SDT markers

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Date:	Thu, 05 Sep 2013 13:59:51 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Hemant <hkshaw@...ux.vnet.ibm.com>
Cc:	Mark Wielaard <mjw@...hat.com>, Namhyung Kim <namhyung@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
	srikar@...ux.vnet.ibm.com, peterz@...radead.org, oleg@...hat.com,
	mingo@...hat.com, anton@...hat.com, systemtap@...rceware.org
Subject: Re: Re: [RFC PATCH 0/2] Perf support to SDT markers

(2013/09/05 3:08), Hemant wrote:
> On 09/04/2013 02:09 PM, Masami Hiramatsu wrote:
>> (2013/09/04 17:25), Mark Wielaard wrote:
>>> On Wed, 2013-09-04 at 15:49 +0900, Namhyung Kim wrote:
>>>> On Tue, 03 Sep 2013 18:53:17 +0530, Hemant wrote:
>>>>> On 09/03/2013 02:47 PM, Masami Hiramatsu wrote:
>>>>>> Indeed, and also I'd like to know what versions of SDT this support,
>>>>>> and where we can see the technical document of that. As far as I know,
>>>>>> the previous(?) SDT implementation also involves ugly semaphores.
>>>>>> Have that already gone?
>>>> It seems it's not. I see the SDT v3 document still mentions semaphores.
>>> It mentions them, but should normally not be used. They are there for
>>> dtrace (source) compatibility. And you don't have to use them.
>>>
>>> Since normally a SDT probe marker is just a NOP it doesn't have any
>>> overhead. But if you want to add complicated arguments that you would
>>> normally not generate in your code, then you might want to add a
>>> semaphore. That way you can have probes with a bit more overhead that
>>> still have zero overhead when not being probed.
>>>
>>> Note that if you use the normal DTRACE_PROBE macros no semaphore will be
>>> inserted. And you can opt to not support probes that have a semaphore in
>>> perf if you think that is easier (just check the semaphore link-time
>>> address for the probe, it should normally be zero). Just warn: "No way I
>>> am going to probe something that might have a little extra overhead! I
>>> am no debugger..." :)
>> OK, I see. And in that case, we'd better filter out the markers which
>> use a semaphore when list it up, since we can not enable it.
>>
>> Thank you,
>>
>>
> 
> But isn't it a better idea to handle semaphores, because there can be 
> many important markers using semaphores and people may still want to 
> probe on them?

Yeah, I didn't mean that all markers with semaphore keep disabled forever.
Instead, I meant that should not be for the first step. And in the first step,
we'd better support only markers without semaphore, and to avoid confusing
users, it should list only probable markers at this point.

I agree that we need to continue discussing how we handle those semaphores.

Thank you!

-- 
Masami HIRAMATSU
IT Management Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@...achi.com


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