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

[Liang, Kan]

On 2023-10-03 6:27 a.m., Peter Zijlstra wrote:
> On Mon, Oct 02, 2023 at 08:57:57PM -0400, Liang, Kan wrote:
> 
>>> Did I already say that the ISE document raises more questions than it
>>> provides answers?
>>
>> Yes. Would an improved CPUID enumeration can address the questions? For
>> example, the CPUID enumeration can give the maximum number of counters
>> and supported width? I think we can discuss it with the architect.
> 
> So.. no. Suppose another arch goes and does the same, but with a
> different number and width of counters. They won't have CPUID.
> 
> 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.

Sure. I will expose them under the "caps" folder.

> 
> 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.
> 
> 
> So then we end up with:
> 
>          *      { u64                   nr;
>          *        { u64 hw_idx; } && PERF_SAMPLE_BRANCH_HW_INDEX
>          *        { u64 from, to, flags } lbr[nr];
> +        *        { u64 counters; } cntr[nr] && PERF_SAMPLE_BRANCH_COUNTERS
>          *      } && PERF_SAMPLE_BRANCH_STACK
> 
> Have it explicitly named counters, have only the one flag and have sysfs
> files describe how to decode it.
> 
> Then for this Intel thing we have 4 counters of 2 bits, but if someone
> else were to do something different (both Power and ARM64 have this
> branch stack stuff now) they can describe it.
> 
> It is a bit wasteful on bits... but at least its clear I suppose.

Yes. I will send a V4 with the above suggestions.

Thanks,
Kan