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Subjectperf stat stddev reporting broken since "perf stat: Introduce read_counters function"
I noticed that perf stat in 4.4 is not printing stddev figures as
expected and bisected it to commit 106a94a0f8c2 ("perf stat: Introduce
read_counters function").

# Kernel 4.4
# make clean && make prefix=/usr && ./perf stat -r 5 sleep 1
Performance counter stats for 'sleep 1' (5 runs):

0.466683 task-clock (msec) # 0.000 CPUs utilized
1 context-switches # 0.002 M/sec
1 cpu-migrations # 0.002 M/sec
66 page-faults # 0.144 M/sec
1,630,220 cycles # 3.562 GHz
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
902,686 instructions # 0.66 insns per cycle
185,692 branches # 405.786 M/sec
8,640 branch-misses # 4.63% of all branches

1.000855883 seconds time elapsed ( +- 0.01% )

# git checkout 106a94a0f8c207ef4113ce7e32f34a00b3b174e7^
Performance counter stats for 'sleep 1' (5 runs):

0.397939 task-clock (msec) # 0.000 CPUs utilized ( +- 3.37% )
1 context-switches # 0.003 M/sec
0 cpu-migrations # 0.000 K/sec
67 page-faults # 0.169 M/sec ( +- 0.56% )
1,360,763 cycles # 3.420 GHz ( +- 2.14% )
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
903,901 instructions # 0.66 insns per cycle ( +- 0.24% )
186,472 branches # 468.595 M/sec ( +- 0.29% )
8,599 branch-misses # 4.61% of all branches ( +- 0.77% )

1.000761882 seconds time elapsed ( +- 0.00% )

# git checkout 106a94a0f8c207ef4113ce7e32f34a00b3b174e7

Performance counter stats for 'sleep 1' (5 runs):

0.583178 task-clock (msec) # 0.001 CPUs utilized
1 context-switches # 0.002 M/sec
0 cpu-migrations # 0.000 K/sec
67 page-faults # 0.141 M/sec
1,224,416 cycles # 2.585 GHz
<not supported> stalled-cycles-frontend
<not supported> stalled-cycles-backend
892,568 instructions # 0.68 insns per cycle
184,975 branches # 390.554 M/sec
8,512 branch-misses # 4.57% of all branches

1.000889249 seconds time elapsed ( +- 0.01% )

I checked and current master in Linus' tree is also broken. Is this
change in documented perf stat behaviour intentional?

--
Mel Gorman
SUSE Labs

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