Corey,
On Sun, Aug 23, 2009 at 4:00 AM, Corey
Ashford<cjash...@linux.vnet.ibm.com> wrote:
> Hello,
>
> On Friday, we decided to double check that the recently checked-in changes
> for Power7 are working for perfmon2 (and perf_counters).
>
good idea.
> So I set up a system with the latest 2.6.30 perfmon2 kernel, latest
> libpfm, and latest PAPI code. I ran the PAPI tests and immediately
> had a number of problems.
>
> The first problem is that there are missing groups in the power7_events.h
> file due to an error in the source file that I use
> to generate the C code. This can be corrected easily, and I will
> submit a patch later for it early next week.
>
> The second problem is that is that the mapping from PAPI events
> to native events required too many native events for PAPI_FP_OPS.
> This works OK for perf_counters, since it doesn't use the group
> tables to determine the values for the mmcr registers. However,
> the dispatch implementation in libpfm does use the groups, and there
> didn't exist a group containing all of the needed events. I can
> work around this by just removing one of the events so that a
> matching group can be found. We can probably generate a new
> group containing the needed event, and I will work on creating
> such a patch. (another solution would be to port the
> perf_counters code to libpfm, but that would be quite a lot
> of work)
>
> I left the worst for last: even running a simple test like
> libpfm/examples_v2.x/self, I get very inconsistent counts.
> Sometimes I get zeros! The numbers I get are almost always
> consistently less than they should be, which leads me to
> suspect that the pmd registers are getting zeroed out
> somewhere where they shouldn't be.
>
> I wasn't sure if this was due to some difference between Power7
> and earlier chips, so I went and set up the same experiment
> on a Power5 machine, and the problem replicates there too.
> So this isn't a Power7-specific problem. Something changed
> between 2.6.29 and 2.6.30. When I run the same experiment on
> a perfmon2 2.6.29 kernel, the results are very consistent and
> look correct.
>
> I diffed the Power-specific source files that might have had some
> changes:
> arch/powerpc/perfmon/perfmon.c and
> arch/powerpc/perfmon/perfmon_power5.c
> but there are no differences between 2.6.29 and 2.6.30.
>
I don't see any major changes.
Here are a couple of tests you could try and run to narrow it down:
- taskset -c 0 self
- syst
Do those produce more stable and valid results?
> Stephane, can you think of anything that you had to do in porting
> perfmon2 to 2.6.30 that might be causing this?
>
Not enough data to figure it out.
> Can you run examples_v2.x/self on x86 to see if you are seeing
> the same issue?
>
I did and it is stable and correct. So must be something in the
Power code or something in the generic code which impacts only
Power.
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