Eric Dumazet a écrit :

Eric Dumazet a écrit :
 

quoted

But if I use plain "perf stat -a sleep 10"
it seems I get wrong values again (16 G cycles/sec) for all next perf sessions

Well, I confirm all my cpus switched from 3GHz to 2GHz, after

"perf stat -a sleep 10"

(but "perf stat -e instructions -e cycles -a sleep 10" doesnt trigger this problem)

Nothing logged, and /proc/cpuinfo stills reports 3 GHz frequencies

# cat unit.c
main() {
  int i;
  for (i = 0 ; i < 10000000; i++)
        getppid();
}
# time ./unit

real    0m0.818s
user    0m0.289s
sys     0m0.529s
# perf stat -a sleep 10 2>/dev/null
# time ./unit

real    0m1.122s
user    0m0.482s
sys     0m0.640s

# tail -n 27 /proc/cpuinfo
processor       : 7
vendor_id       : GenuineIntel
cpu family      : 6
model           : 23
model name      : Intel(R) Xeon(R) CPU           E5450  @ 3.00GHz
stepping        : 6
cpu MHz         : 3000.102
cache size      : 6144 KB
physical id     : 1
siblings        : 1
core id         : 3
cpu cores       : 4
apicid          : 7
initial apicid  : 7
fdiv_bug        : no
hlt_bug         : no
f00f_bug        : no
coma_bug        : no
fpu             : yes
fpu_exception   : yes
cpuid level     : 10
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe lm constant_tsc arch_perfmon pebs bts pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm dca sse4_1 lahf_lm tpr_shadow vnmi flexpriority
bogomips        : 6000.01
clflush size    : 64
power management:

# grep CPU_FREQ .config
# CONFIG_CPU_FREQ is not set


perf_counter seems promising, but still... needs some bug hunting :)

Update :

Mike Galbraith suggested me to try various things, and finally, I discovered
this frequency change was probably a BIOS problem on my HP BL460c G1

System Options -> Power regulator for Proliant

[*] HP Dynamic Power Savings Mode
[ ] HP Static Low Power Mode
[ ] HP Static High Performance Mode
[ ] OS Control Mode


I switched it to 'OS Control Mode'

Then acpi-cpufreq could load, and no more frequencies changes on a "perf -a sleep 10" 
session, using or not cpufreq.
(Supported cpufreq speeds on these cpus : 1999 & 2999 MHz)

So it was a BIOS issue

# perf stat -a sleep 10

 Performance counter stats for 'sleep':

   80005.418223  task clock ticks     (msecs)
          80266  context switches     (events)
              3  CPU migrations       (events)
            486  pagefaults           (events)
   240013851624  CPU cycles           (events) << good >>
   239076501419  instructions         (events)
         679464  cache references     (events)
  <not counted>  cache misses

 Wall-clock time elapsed: 10000.468808 msecs

Thank you