[BUG] cpufreq: constant cpu_khz

Peter Zijlstra
2008-08-25T06:35:25Z

Hi,

I noticed that my sched_clock() was slow on a number of machine, so I
started looking at cpufreq. I inserted the following printk in the
set_cyc2ns_scale() function, which is called from the cpufreq change
notifier:

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 46af716..8dffd82 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -261,6 +263,9 @@ static void set_cyc2ns_scale(unsigned long cpu_khz, int cpu)
if (cpu_khz)
*scale = (NSEC_PER_MSEC << CYC2NS_SCALE_FACTOR)/cpu_khz;

+ printk(KERN_DEBUG "set_cyc2ns_scale: %lu %lu %d\n",
+ *scale, cpu_khz, cpu);
+
sched_clock_idle_wakeup_event(0);
local_irq_restore(flags);
}

And on my laptop:

root@lappy:~# dmesg | grep cyc2ns | tail -10
[ 91.378693] set_cyc2ns_scale: 854 1198980 0
[ 91.378713] set_cyc2ns_scale: 854 1198980 0
[ 110.548064] set_cyc2ns_scale: 854 1198980 0
[ 110.548106] set_cyc2ns_scale: 854 1198980 0
[ 110.756121] set_cyc2ns_scale: 854 1198980 0
[ 110.756135] set_cyc2ns_scale: 854 1198980 0
[ 440.812063] set_cyc2ns_scale: 854 1198980 0
[ 440.812106] set_cyc2ns_scale: 854 1198980 0
[ 441.593140] set_cyc2ns_scale: 854 1198980 0
[ 441.593159] set_cyc2ns_scale: 854 1198980 0

even though I'm sure there were some 800 <-> 1200 MHz changes there
esp. as:

root@lappy:~# cat /proc/cpuinfo
processor : 0
vendor_id : GenuineIntel
cpu family : 6
model : 11
model name : Intel(R) Pentium(R) III Mobile CPU 1200MHz
stepping : 1
cpu MHz : 798.000
cache size : 512 KB
fdiv_bug : no
hlt_bug : no
f00f_bug : no
coma_bug : no
fpu : yes
fpu_exception : yes
cpuid level : 2
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 sep mtrr pge mca cmov pat pse36 mmx fxsr sse up nopl
bogomips : 1598.64
clflush size : 32
power management:

On my opteron:

[root@opteron ~]# dmesg | grep cyc2ns | tail -10
[ 0.000000] set_cyc2ns_scale: 427 2394004 0
[ 0.000000] set_cyc2ns_scale: 427 2394004 1
[ 14.784154] set_cyc2ns_scale: 427 2394004 0
[ 14.788342] set_cyc2ns_scale: 427 2394004 1
[ 14.792789] set_cyc2ns_scale: 427 2394004 0
[ 14.797028] set_cyc2ns_scale: 427 2394004 1
[ 15.841049] set_cyc2ns_scale: 427 2394004 0
[ 15.845250] set_cyc2ns_scale: 427 2394004 1
[ 15.849503] set_cyc2ns_scale: 427 2394004 0
[ 15.853785] set_cyc2ns_scale: 427 2394004 1
[root@opteron ~]# cat /proc/cpuinfo
processor : 0
vendor_id : AuthenticAMD
cpu family : 15
model : 67
model name : Dual-Core AMD Opteron(tm) Processor 1216
stepping : 2
cpu MHz : 1000.000
cache size : 1024 KB
physical id : 0
siblings : 2
core id : 0
cpu cores : 2
apicid : 0
initial apicid : 0
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow rep_good nopl pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy
bogomips : 1995.00
TLB size : 1024 4K pages
clflush size : 64
cache_alignment : 64
address sizes : 40 bits physical, 48 bits virtual
power management: ts fid vid ttp tm stc

processor : 1
vendor_id : AuthenticAMD
cpu family : 15
model : 67
model name : Dual-Core AMD Opteron(tm) Processor 1216
stepping : 2
cpu MHz : 1000.000
cache size : 1024 KB
physical id : 0
siblings : 2
core id : 1
cpu cores : 2
apicid : 1
initial apicid : 1
fpu : yes
fpu_exception : yes
cpuid level : 1
wp : yes
flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt rdtscp lm 3dnowext 3dnow rep_good nopl pni cx16 lahf_lm cmp_legacy svm extapic cr8_legacy
bogomips : 1995.00
TLB size : 1024 4K pages
clflush size : 64
cache_alignment : 64
address sizes : 40 bits physical, 48 bits virtual
power management: ts fid vid ttp tm stc