using get_cycles() to get the real elapsed time

[Li Guanglei]

The following is the internal discussion that didn't copy the 
SystemTap mailinglist. FYI.


Paul Mackerras wrote:
> 
> 
> 
> Correct - IBM's POWER4 and POWER5 systems don't provide any hardware or
> firmware mechanism to change the CPU frequency.  There are 64-bit PowerPC
> systems which do, for instance Apple's G5 machines.
> 
> --
> Paul Mackerras, Ph.D.
> PowerPC Linux Kernel Architect
> STSM
> IBM Linux Technology Center OzLabs
> Canberra, Australia
> 
> 
>                                                                            
>              guanglei@cn.ltcfw                                             
>              d.linux.ibm.com                                               
>                                                                         To 
>              08/08/06 12:09 PM         Paul Mackerras/Australia/IBM@IBMAU  
>                                                                         cc 
>                                        Vara Prasad <prasadav@us.ibm.com>,  
>                                        ananth@in.ltcfwd.linux.ibm.com,     
>                                        Anton                               
>                                        Blanchard/Australia/IBM@IBMAU,      
>                                        guij@cn.ltcfwd.linux.ibm.com, Jim   
>                                        Keniston <jkenisto@us.ibm.com>,     
>                                        paulus@ozlabs.au.ibm.com            
>                                                                    Subject 
>                                        Re: using get_cycles() to get the   
>                                        real elapsed time                   
>                                                                            
>                                                                            
>
> 
> 
> 
> Paul Mackerras wrote:
>>
>>
>> Vara,
>>
>> No, this is not a bug, in fact it is one place where PowerPC machines do
>> better than x86. :)  PowerPC has a constant-frequency, high-resolution
>> counter called the timebase register, which is cheap to read and which
>> counts synchronously across all processors in an SMP system, and that's
>> what get_cycles and sched_clock read.  The timebase frequency doesn't
>> change when the CPU frequency changes, which is why the tb_to_ns_*
>> variables don't get changed when the CPU frequency changes.
>>
>> --
>> Paul Mackerras, Ph.D.
>> PowerPC Linux Kernel Architect
>> STSM
>> IBM Linux Technology Center OzLabs
>> Canberra, Australia
> 
> I ever tried to enable CONFIG_CPU_FREQ in the Kernel and used the
> CONSERVATIVE governor on Power5 platform. But the cpu frequency won't
> change when CPU is idle and I didn't find a entry existed in
> /sys/devices/system/cpu/cpu0/cpufreq
> 
> I searched inside IBM intranet and found the Time Base on Power5 will
> be driven at 1/8 of the CPU frequency. I think the Power5 CPU doesn't
> has the CPU frequency scaling feature. So the CPU frequency will be
> constant, and then the Time Base will also be increased at a
> constant-frequency. And I think it's the same with Power4. Am I right?
> 
> Another question: Does anybody know how to call sched_clock() in a
> kernel module? insmod complains of unknown symbol of sched_clock
> 
> Thanks.
> 
> - Guanglei
> 
>