Re: [PATCH 1/2] perf tool AMD: Use non-precise cycles as default event on certain Zen2 processors

[Ravi Bangoria]

Hi Ian,

> We read max_precise from sysfs:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/pmu.c?h=perf-tools-next#n984
> 
> But it appears not to be used:
> https://git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/evsel.c?h=perf-tools-next#n1323
> 
> I think:
> ```
> if (evsel->precise_max)
>     attr->precise_ip = 3;
> ```
> should be:
> ```
> if (evsel->precise_max)
>     attr->precise_ip = evsel->pmu->max_precise;
> ```

This segfaults as evsel->pmu is NULL. But I get your point.

If we go with this change, I'll need to switch to cycles:p as default
event on AMD. Otherwise perf will use non-precise event as default event
on AMD.

> Presumably this is misreporting as some non-zero value on the buggy
> systems - if it doesn't we can declare victory here. If not we can
> modify max_precise's calculation on perf's struct pmu for cpu for AMD
> so that we just hardwire it to zero in arch code.

max_precise for AMD core pmu is 0, which is correct as AMD core pmu does
not support precise mode. However, cycles:p, r76:p and rC1:p are special
cases and allowed via core pmu.

Thanks,
Ravi