Re: [PATCH 5/6] perf_counter: add more context information

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2009-04-06 at 14:33 -0700, Corey Ashford wrote:
> 
> Peter Zijlstra wrote:
> > On Mon, 2009-04-06 at 14:15 -0700, Corey Ashford wrote:
> >> Peter Zijlstra wrote:
> >>> On Mon, 2009-04-06 at 13:16 -0700, Corey Ashford wrote:
> >>>
> >>>>>> One downside of this approach is that you if you specify "no header" 
> >>>>>> (currently not possible, but maybe later?), you will not be able to get 
> >>>>>> the level bits.
> >>>>> Would this be desirable? 
> >>>> I think it would.  For one use case I'm working on right now, simple 
> >>>> profiling, all I need are ip's.  If I could omit the header, that would 
> >>>> reduce the frequency of sigio's by a factor of three, and make it faster 
> >>>> to read up the ip's when the SIGIO's occur.
> >>> Self-profiling?
> >>>
> >>> So you're interested in getting the smallest possible record size, that
> >>> would still be 2 u64, right? Otherwise you don't get the IP context that
> >>> started this.
> >>>
> >>>
> >> Self-profiling mainly, yes.  PAPI specs an ability for remote monitoring 
> >> of processes and threads, but I think it's only partially implemented.
> >>
> >> So when you are talking about IP context, you mean pid/tid?
> > 
> > Ah, we called it level before, the hv/kernel/user thing. For remote
> > profiling you'd want to have the mmap thing too.
> 
> Oh I see.  In PAPI, the user specifies the range(s) of addresses he's 
> interested in profiling (any sampled IP's outside the requested ranges 
> are discarded), and so as long as the kernel space IP's don't overlap 
> with user space IP's, we should be fine.

Ah, while this would be true for most 'sane' architectures, Paul was
right in pointing out that this is not true for all architectures -- and
we should therefore not rely on address range alone.

You could of course use: hw_event.exclude_{hv,kernel} = 1 to ensure you
only get userspace thingies I suppose (but then you have no way of
telling how many you missed I guess).