[PATCH V2] powerpc/perf: Enable PMU counters post partition migration if PMU is active

[Nicholas Piggin]

Excerpts from Michael Ellerman's message of October 22, 2021 10:19 am:
> Nathan Lynch <nathanl at linux.ibm.com> writes:
>> Athira Rajeev <atrajeev at linux.vnet.ibm.com> writes:
>>> During Live Partition Migration (LPM), it is observed that perf
>>> counter values reports zero post migration completion. However
>>> 'perf stat' with workload continues to show counts post migration
>>> since PMU gets disabled/enabled during sched switches. But incase
>>> of system/cpu wide monitoring, zero counts were reported with 'perf
>>> stat' after migration completion.
>>>
>>> Example:
>>>  ./perf stat -e r1001e -I 1000
>>>            time             counts unit events
>>>      1.001010437         22,137,414      r1001e
>>>      2.002495447         15,455,821      r1001e
>>> <<>> As seen in next below logs, the counter values shows zero
>>>         after migration is completed.
>>> <<>>
>>>     86.142535370    129,392,333,440      r1001e
>>>     87.144714617                  0      r1001e
>>>     88.146526636                  0      r1001e
>>>     89.148085029                  0      r1001e
>>
>> Confirmed in my environment:
>>
>>     51.099987985            300,338      cache-misses
>>     52.101839374            296,586      cache-misses
>>     53.116089796            263,150      cache-misses
>>     54.117949249            232,290      cache-misses
>>     55.602029375     68,700,421,711      cache-misses
>>     56.610073969                  0      cache-misses
>>     57.614732000                  0      cache-misses
>>
>> I wonder what it means that there is a very unlikely huge value before
>> the counter stops working -- I believe your example has this phenomenon
>> too.
> 
> AFAICS the patch is not reading the PMC values before the migration, so

My suggested change I think should take care of that.

Thanks,
Nick