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

[Michael Ellerman <mpe@ellerman.id.au>]

Nathan Lynch <nathanl@linux.ibm.com> writes:
> Athira Rajeev <atrajeev@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
I suspect we're losing some counts just before the migration and then
the delta is going negative somewhere, leading to an implausibly large
count.

cheers