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

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Date:	Wed, 04 Sep 2013 23:38:14 +0530
From:	Hemant <hkshaw@...ux.vnet.ibm.com>
To:	Masami Hiramatsu <masami.hiramatsu.pt@...achi.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: [RFC PATCH 0/2] Perf support to SDT markers

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?

Thanks
Hemant

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