Re: user-defined static trace points

[Andi Kleen <andi@firstfloor.org>]

Milian Wolff <milian.wolff@kdab.com> writes:

> Hey all,
>
> I'm aware that we can add custum user-defined dynamic trace points using `perf 
> probe`. But this is unwieldy for some larger applications, esp. if we 
> recompile them often. Is there a public API (that I could not find so far), to 
> define static trace points in userspace code?
>
> I found
> http://www.brendangregg.com/blog/2015-07-03/hacking-linux-usdt-ftrace.html
> which references https://www.sourceware.org/systemtap/wiki/
> AddingUserSpaceProbingToApps for systemtap, but on my machine I don't have a 
> <sys/sdt.h> header.

You need to install systemtap-devel or similar.

> Would such trace points be useable by perf? https://
> lwn.net/Articles/570818/ doesn't seem to be included, i.e. I don't have `perf 
> probe --markers` with a recent build of perf.

There is a patch to teach perf about SDT probes, but so far it has not
been integrated. You can use them from systemtap or gdb though.

https://lwn.net/Articles/569622/

Perhaps Hemant needs to resubmit?


>
> If someone could point me into the right direction, that would be highly 
> appreciated. I would also like to know about the performance impact of adding 
> static trace points to code. If it is low, I'd then try to add such trace 
> points to Qt upstream in some GUI related areas, e.g. to investigate FPS or 
> similar.

When they are disabled they are just nops (so basically free)
When enabled it is an exception so not super cheap.

-Andi


-- 
ak@linux.intel.com -- Speaking for myself only