netdev - [BUG] perf_counter: change cpu frequencies

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Date:	Thu, 30 Apr 2009 18:07:44 +0200
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Christoph Lameter <cl@...ux.com>,
	linux kernel <linux-kernel@...r.kernel.org>,
	Andi Kleen <andi@...stfloor.org>,
	David Miller <davem@...emloft.net>, jesse.brandeburg@...el.com,
	netdev@...r.kernel.org, haoki@...hat.com, mchan@...adcom.com,
	davidel@...ilserver.org
Subject: [BUG] perf_counter: change cpu frequencies 

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 :)

Thank you

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