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

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Message-ID: <5226F1B6.3000801@hitachi.com>
Date:	Wed, 04 Sep 2013 17:39:18 +0900
From:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>
To:	Mark Wielaard <mjw@...hat.com>
Cc:	Namhyung Kim <namhyung@...nel.org>,
	Hemant <hkshaw@...ux.vnet.ibm.com>,
	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

(2013/09/04 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,


-- 
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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