[PATCH v2 2/2] powerpc/perf: Return regs->nip as instruction pointer value when SIAR is 0

[kajoljain]

On 8/14/21 6:14 PM, Michael Ellerman wrote:
> Christophe Leroy <christophe.leroy at csgroup.eu> writes:
>> Le 13/08/2021 à 10:24, Kajol Jain a écrit :
>>> Incase of random sampling, there can be scenarios where SIAR is not
>>> latching sample address and results in 0 value. Since current code
>>> directly returning the siar value, we could see multiple instruction
>>> pointer values as 0 in perf report.
> 
> Can you please give more detail on that? What scenarios? On what CPUs?
> 

Hi Michael,
    Sure I will update these details in my next patch-set.

>>> Patch resolves this issue by adding a ternary condition to return
>>> regs->nip incase SIAR is 0.
>>
>> Your description seems rather similar to 
>> https://github.com/linuxppc/linux/commit/2ca13a4cc56c920a6c9fc8ee45d02bccacd7f46c
>>
>> Does it mean that the problem occurs on more than the power10 DD1 ?
>>
>> In that case, can the solution be common instead of doing something for power10 DD1 and something 
>> for others ?
> 
> Agreed.
> 
> This change would seem to make that P10 DD1 logic superfluous.
> 
> Also we already have a fallback to regs->nip in the else case of the if,
> so we should just use that rather than adding a ternary condition.
> 
> eg.
> 
> 	if (use_siar && siar_valid(regs) && siar)
> 		return siar + perf_ip_adjust(regs);
> 	else if (use_siar)
> 		return 0;		// no valid instruction pointer
> 	else
> 		return regs->nip;
> 
> 
> I'm also not sure why we have that return 0 case, I can't think of why
> we'd ever want to do that rather than using nip. So maybe we should do
> another patch to drop that case.

Yeah make sense. I will remove return 0 case in my next version.

Thanks,
Kajol Jain
> 
> cheers
>