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perf: massive perf_event slowdown between 4.9 and 4.11-rc




Hello

My student actually noticed this before I did, I was hoping it was some 
sort of error in her data.

Anyway all perf_event functionality (especially reads) has become about 
20x slower, at least on Intel machines (haswell and skylake are the only 
ones I've tested) sometime between 4.9 and 4.11-rc

For example, in the PAPI tests:

4.11-rc2

Total cost for PAPI_read (2 counters) over 1000000 iterations
min cycles   : 15192
max cycles   : 3887735
mean cycles  : 15662.057418
std deviation: 19079.398693


4.9

Total cost for PAPI_read (2 counters) over 1000000 iterations
min cycles   : 864
max cycles   : 78459
mean cycles  : 908.010315
std deviation: 144.875697


The perf_event_test validation tests are also showing this, even when 
using rdpmc() rather than read.

Is there a likely change that might have caused this?  Hoping to avoid 
bisecting it as that will kill the rest of the week probably.

Vince