Re: [RFC] perf/x86: Fix a warning on x86_pmu_stop()

[Peter Zijlstra]

On Tue, Nov 24, 2020 at 12:19:34AM -0800, Stephane Eranian wrote:
> Hi,
> 
> Another remark on the PEBS drainage code, it seems to me like a test
> is not quite correct:
> intel_pmu_drain_pebs_nhm()
> {
> ...
>                if (p->status != (1ULL << bit)) {
>                         for_each_set_bit(i, (unsigned long *)&pebs_status, size)
>                                 error[i]++;
>                         continue;
>                 }
> 
> The kernel cannot disambiguate when 2+ PEBS counters overflow at the
> same time. This is what the comment for this code suggests.
> However, I see the comparison is done with the unfiltered p->status
> which is a copy of  IA32_PERF_GLOBAL_STATUS at the time of
> the sample. This register contains more than the PEBS counter overflow
> bits. It also includes many other bits which could also be set.
> 
> Shouldn't this test use pebs_status instead (which covers only the
> PEBS counters)?

Hmm, yes, think so.