Re: [RFC PATCH 0/2] Perf support to SDT markers
From: Hemant
Date: Wed Sep 04 2013 - 14:08:36 EST
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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