Eric Dumazet a écrit :
quoted text
> Eric Dumazet a écrit :
>  
>> 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

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