Ingo Molnar a écrit :

* Eric Dumazet [off-list ref] wrote:

quoted

Eric Dumazet a écrit :

quoted

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

ah! That makes quite a bit of sense. The BIOS interfering with an OS 
feature ... Was that the default setting in the BIOS?

This was default setting in BIOS, yes.

quoted

# 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

That looks perfect now.

It would also be really nice to have a sysrq-p dump of your PMU 
state before you've done any profiling. Is there any trace of the 
BIOS meddling with them, that we could detect (and warn about) 
during bootup?

Difference is that on BIOS set to 'OS Control Mode' I see one more entry in ACPI list :

[    0.000000] ACPI: SSDT cfe5b000 004C9 (v01 HP        SSDTP 00000001 INTL 20030228)

...
And these 8 additional lines after (one per cpu)
[    0.706697] ACPI: SSDT cfe5c000 002DA (v01 HP        SSDT0 00000001 INTL 20030228)
[    0.707250] ACPI: SSDT cfe5c300 002DA (v01 HP        SSDT1 00000001 INTL 20030228)
[    0.707768] ACPI: SSDT cfe5c600 002DA (v01 HP        SSDT2 00000001 INTL 20030228)
[    0.708376] ACPI: SSDT cfe5c900 002DF (v01 HP        SSDT3 00000001 INTL 20030228)
[    0.708964] ACPI: SSDT cfe5cc00 002DA (v01 HP        SSDT4 00000001 INTL 20030228)
[    0.709567] ACPI: SSDT cfe5cf00 002DA (v01 HP        SSDT5 00000001 INTL 20030228)
[    0.710122] ACPI: SSDT cfe5d200 002DA (v01 HP        SSDT6 00000001 INTL 20030228)
[    0.710713] ACPI: SSDT cfe5d500 002DA (v01 HP        SSDT7 00000001 INTL 20030228)

Also, if this option is set to default (HP Dynamic Power Savings Mode) I get :

# modprobe acpi-cpufreq
FATAL: Error inserting acpi_cpufreq (/lib/modules/2.6.30-rc4-tip-01560-gdd5fa92/kernel/arch/x86/kernel/cpu/cpufreq/acpi-cpufreq.ko): No such device

but no kernel message logged.

Might be possible to add some kind of warning yes, I can test a patch if you want.