Re: Some newbie questions

[Avi Kivity <avi at scylladb dot com>]

On 08/10/2016 07:47 PM, Frank Ch. Eigler wrote:
> 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.

Like you say, those are unavoidable.  But at least those costs are 
handled by scalable resources.


>    Your perf
> report may well be misattributing the cost.

I think it's unlikely.  When perf says __raw_spin_lock is guilty, it 
usually is.

>    (Have you tried a stap
> script that merely traps all the same the function calls, and has
> empty probe handlers?)

I can try it.

> > > 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.

Will timer.profile work with dyninst?