Re: Does oprofile suffer from aliasing?

On 04/30/2015 12:43 PM, Grant Edwards wrote:
> On 2015-04-30, William Cohen <[email protected]> wrote:
>>
>>> AFAICT the ARM926 core doesn't have a PMU at all -- I can find no
>>> mention of one in either ARM docs for the ARM926 or Atmel docs for the
>>> at91 parts.
> [...]
>>> Is there such a thing as software-only PMU support?
> 
>> The perf command included with newer kernels (since 2.6.31) allow
>> recording both performance monitoring hardware events and software
>> events.
> 
> So that would be a sort of software-only PMU support (from the user's
> POV)?

Hi Grant,

Yes, in theory perf should provide software-only events if no pmu is available. I have seen that happen on trimslice when the device tree didn't describe performance monitoring hardware.  However, using perf depends on having a newer kernel that works on the particular device.  In the past I have encountered arm devices that only worked with particular kernels with various patches (for example genesi efika smartbook).  I hope that you have a reasonable new kernel that works on the arm926.

> 
>> Assuming that perf is working you should be able to do following to
>> see what events it supports:
>>
>> # perf list
> [...]
>> # perf record --freq 1000 -e cpu-clock -a sleep 10
> [...]
>> # perf report
> 
> OK, thanks.  It's probably time to try to get a newer kernel running. 
> IIRC, Atmel provides kernel config examples for a few different
> versions up to 3.6 for my platform.
> 
> So, let's say I have a hardware interrupt I can hook stuff to (I do),
> and I can generate "random enough" edges on the pin in question with a
> predictable frequency distribution (that might take a bit of
> soldering, but shouldn't be too difficult).
> 
> Can I stick something in the ISR for that interrupt that will cause a
> "sample"?
> 

You could do something like that with the "perf probe" described on:

https://lwn.net/Articles/355456/
http://www.brendangregg.com/perf.html

It looks like you would need to have a script to do process to group things by the argument.

-Will


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