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


On 09/05/2013 12:22 AM, Mark Wielaard wrote:
On Wed, 2013-09-04 at 23:42 +0530, Hemant wrote:
On 09/04/2013 01:55 PM, Mark Wielaard wrote:
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..." :)
I agree. There will be an extra overhead but there may be some important
markers (on which we need to probe) may be worth this overhead?
Yes, there maybe. And gdb and stap do support them. But it means not
just setting the probe, but also incrementing (and decrementing) the
semaphore. See "Semaphore Handling" under
https://sourceware.org/systemtap/wiki/UserSpaceProbeImplementation

Which is extra work, so for a minimal implementation that just supports
normal (no-overhead) probes you might want to skip the extra work
required to support them. I believe they are normally not used. I
wouldn't recommend them and when I have added SDT probes myself I never
used/needed them, but I haven't actually looked what others do.

Hmm, I agree as normally they aren't used. Also in normal usage, they aren't needed. Avoiding this seems the right choice for now. Will just filter out them as suggested by Masami.

Thanks
Hemant


Cheers,

Mark



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