Re: perf_counters: page fault trace record

From: Ingo Molnar
Date: Sat Jun 13 2009 - 06:09:32 EST

* Paul Mackerras <paulus@xxxxxxxxx> 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.


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