Re: [REGRESSION] x86, perf: counter freezing breaks rr

[Andi Kleen]

> It does seem that FREEZE_PERFMON_ON_PMI (misnamed as it is) is of
> rather limited use (or even negative, in our case) to a counter that's
> already restricted to ring 3.

It's much faster. The PMI cost goes down dramatically.

I still the the right fix is to add an perf event opt-out and let it be 
used by rr.

   V3 is without counter freezing.
    V4 is with counter freezing.
    The value is the average cost of the PMI handler.
    (lower is better)

    perf options    `           V3(ns) V4(ns)  delta
    -c 100000                   1088   894     -18%
    -g -c 100000                1862   1646    -12%
    --call-graph lbr -c 100000  3649   3367    -8%
    --c.g. dwarf -c 100000      2248   1982    -12%


-Andi