Some newbie questions

[Frank Ch. Eigler]

Hi -

On Wed, Aug 10, 2016 at 06:40:02PM +0300, Avi Kivity wrote:
> [...]
> Yes.  The problem is that if the function is called often (with a usual 
> short running time), then systemtap will eat all of the cpu time 
> spinning on an internal lock.

Well, not just that ...  trapping each function entry/exit has
unavoidable kernel uprobes context-switchy-type overheads.  Your perf
report may well be misattributing the cost.  (Have you tried a stap
script that merely traps all the same the function calls, and has
empty probe handlers?)


> >Note though that such analysis probably cannot be performed based only
> >upon PC samples - or even backtrace samples.  We seem to require
> >trapping individual function entry/exit events.
> 
> That's why I tried systemtap.  It worked well on my desktop, but very 
> badly in production.

It may be worth experimenting with "stap --runtime=dyninst" if your
function analysis were restricted to basic Cish userspace that dyninst
can handle.


- FChE