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

Excerpts from Michael Ellerman's message of October 22, 2021 10:19 am:

Nathan Lynch [off-list ref] writes:

quoted

Athira Rajeev [off-list ref] writes:

quoted

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