Re: perf_counters: page fault trace record

[Ingo Molnar <mingo@elte.hu>]

* Paul Mackerras <paulus@samba.org> wrote:

> Corey Ashford writes:
> 
> > Paul Mackerras wrote:
> > > Can't you do what you need just using a page fault software 
> > > counter with sample_type = PERF_SAMPLE_IP | PERF_SAMPLE_ADDR 
> > > and sample_period = 1?
> > 
> > I thought about that, but I was under the (incorrect?) 
> > impression that on Power, the PERF_SAMPLE_ADDR would be set by 
> > the value of the SDAR register, which wouldn't be correct for 
> > the case of a page fault.
> 
> No, the PERF_SAMPLE_ADDR value only comes from SDAR for a hardware 
> counter overflow event.  For the page-fault software counter the 
> PERF_SAMPLE_ADDR value will always be the faulting address.

Corey, could you please add support for it in 'perf'? We dont want 
such sw-counter features to be in the kernel code without matching 
support in tools/perf/.

While user data symbols wont be resolved, if we have a 
--target-address switch in perf record we could see the faulting 
frequency (and the fault coverage - and ordering as well) of shared 
libraries, in perf report and perf annotate.

This would be a very useful facility.

Thanks,

	Ingo