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reset! causes incorrect scaling for Stats #54

@topolarity

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@topolarity
julia> bench = LinuxPerf.make_bench();
julia> enable!(bench); sleep(1.0); disable!(bench);
julia> stats = LinuxPerf.ThreadStats(bench);
julia> stats.groups[1][1]
LinuxPerf.Counter(hw:cycles, 0x0000000001bdc0f6, 0x0000000000a08a80, 0x00000000007a6ed7)

julia> reset!(bench)
julia> stats = LinuxPerf.ThreadStats(bench);
julia> stats.groups[1][1]
LinuxPerf.Counter(hw:cycles, 0x0000000000000000, 0x0000000003dad501, 0x00000000026bd3ce)

Notice that IOC_RESET resets the value of the counter, but doesn't affect time_enabled or time_running

It's hard to say what the correct behavior is here, but this means that the Counter object is no longer valid because its running / enabled ratio is not what it thinks it is:

julia> stats
╶ cpu-cycles               0.00e+00   76.3%  #  0.0 cycles per ns
┌ cache-references         0.00e+00   62.0%
└ cache-misses             0.00e+00   62.0%  #  NaN% of cache refs
┌ branch-instructions      0.00e+00   61.8%  #  NaN% of insns
│ branch-misses            0.00e+00   61.8%  #  NaN% of branch insns
└ instructions             0.00e+00   61.8%  #  NaN insns per cycle
┌ context-switches         0.00e+00  100.0%
│ page-faults              0.00e+00  100.0%
│ minor-faults             0.00e+00  100.0%
│ major-faults             0.00e+00  100.0%
└ cpu-migrations           0.00e+00  100.0%

If you try to reset in a loop, this means you end up scaling the measurement by its average running time instead of its true running time for the latest sample.

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