Re: perf_counters: page fault trace record

[Corey Ashford <cjashfor@linux.vnet.ibm.com>]

Ingo Molnar wrote:
> * Paul Mackerras <paulus@samba.org> wrote:
> 
>> Corey Ashford writes:
>>
>>> Paul Mackerras wrote:
>>>> Can't you do what you need just using a page fault software 
>>>> counter with sample_type = PERF_SAMPLE_IP | PERF_SAMPLE_ADDR 
>>>> and sample_period = 1?
>>> I thought about that, but I was under the (incorrect?) 
>>> impression that on Power, the PERF_SAMPLE_ADDR would be set by 
>>> the value of the SDAR register, which wouldn't be correct for 
>>> the case of a page fault.
>> No, the PERF_SAMPLE_ADDR value only comes from SDAR for a hardware 
>> counter overflow event.  For the page-fault software counter the 
>> PERF_SAMPLE_ADDR value will always be the faulting address.
> 
> Corey, could you please add support for it in 'perf'? We dont want 
> such sw-counter features to be in the kernel code without matching 
> support in tools/perf/.
> 
> While user data symbols wont be resolved, if we have a 
> --target-address switch in perf record we could see the faulting 
> frequency (and the fault coverage - and ordering as well) of shared 
> libraries, in perf report and perf annotate.
> 
> This would be a very useful facility.
> 
> Thanks,
> 
> 	Ingo

I'll see what I can do.  This will be my first time modifying perf, so expect 
that I won't do things the way you prefer the first couple of go-arounds :)

Regards,

- Corey