Re: [patch] trace: Add user-space event tracing/injection

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2010-11-17 at 14:33 +0200, Pekka Enberg wrote:
> On 11/17/10 2:30 PM, Ingo Molnar wrote:
> > What does the duration in milliseconds mean there? For things like
> >> GC and JIT, I want something like:
> >>
> >> void gc(void)
> >> {
> >>          prctl(PR_TASK_PERF_USER_TRACE_START, ...)
> >>
> >>          collect();
> >>
> >>          prctl(PR_TASK_PERF_USER_TRACE_END, ...)
> >> }
> >>
> >> So that it's clear from the tracing output that the VM was busy
> >> doing GC for n milliseconds. Barring background JIT'ing and
> >> pauseless GC, I'd also be interested in showing how much time the VM
> >> was actually _blocking_ the running application (which can happen
> >> with signals too, btw, for things like accessing data that's lazily
> >> initialized).
> > We can add two events: user_event_entry/user_event_exit - or we could use the string
> > to differentiate, and start it with:
> >
> >    "entry: ..."
> >    "exit: ..."
> >
> > And then the event timestamps (which are absolute and are available) could be used
> > to calculate the duration of this period.
> >
> > 'trace' could even be taught to treat such entry:/exit: strings in a special way, so
> > that you dont have to write Jato specific trace decoding bits?
> 
> Yes, makes sense. I like the API so lets convince others that it's 
> important enough to be merged. :-)

I don't much like it, Jato already does its own tracing for the anon_vma
symbols, it might as well write its own event log too (would need a
proper VDSO clock thingy though).

Both these, the anon_vma symbols and the events, should be implemented
in a libperf.so or the like so as not to rely on the crummy /tmp file
interface we've been using so far.

Apps can link against libperf_dummy.so to get weak stubs and LD_PRELOAD
libperf.so to get the real ones (perf record can do that LD_PRELOAD
automagically).

perf report will need to be taught to not only pick up the anon_vma
symbols but also look for the events to be merge sorted.

We already have this communication channel for Jato (due to the anon_vma
muck), might as well extend it.