Re: [RESEND PATCH V3 1/6] perf: Add branch stack extra

[Liang, Kan]

On 2023-10-03 11:06 a.m., Andi Kleen wrote:
>> I'm thinking we should do something like expose branch_counter_nr and
>> branch_counter_width in the sysfs node, and then rename this extra field
>> to counters.
>>
>> Then userspace can do something like:
>>
>> 	for (i = 0; i < branch_counter_nr; i++) {
>> 		counter[i] = counters & ((1 << branch_counter_width) - 1);
>> 		counters >>= branch_counter_width;
>> 	}
>>
>> to extract the actual counter values.
> 
> perf script/report won't necessarily have access to the sysfs
> values if they run on a different system
> 
> It would need extra PT style metadata written by perf record to
> perf.data and read by the user tools.
> 
> Seems complicated. It would be better if it just parsed on its own.
> 

If so, perf probably have to save the information in the
perf_branch_entry. At least, perf has to be able to support 64 counters
and 64 width. That will use at least 12 bits of the info field of the
perf_branch_entry. (We only have 31 spare bits now. Almost half.) That
seems a waste.

Perf tool already supports "branches" in the caps folder. It has been
saved in the header. We can use a similar way to support
"branch_counter_nr" and "branch_counter_width". It doesn't seem very
complex. I think I will try the sysfs method first.

Thanks,
Kan