Re: [PATCH] Revert "perf/x86: Allow zero PEBS status with only single active event"

[Liang, Kan]

On 3/16/2021 3:22 AM, Namhyung Kim wrote:
> Hi Peter and Kan,
>
> On Thu, Mar 4, 2021 at 5:22 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > On Wed, Mar 03, 2021 at 02:53:00PM -0500, Liang, Kan wrote:
> > > On 3/3/2021 1:59 PM, Peter Zijlstra wrote:
> > > > On Wed, Mar 03, 2021 at 05:42:18AM -0800, kan.liang@xxxxxxxxxxxxxxx wrote:
> >
> > > > > +++ b/arch/x86/events/intel/ds.c
> > > > > @@ -2000,18 +2000,6 @@ static void intel_pmu_drain_pebs_nhm(struct pt_regs *iregs, struct perf_sample_d
> > > > >                            continue;
> > > > >                    }
> > > > > -         /*
> > > > > -          * On some CPUs the PEBS status can be zero when PEBS is
> > > > > -          * racing with clearing of GLOBAL_STATUS.
> > > > > -          *
> > > > > -          * Normally we would drop that record, but in the
> > > > > -          * case when there is only a single active PEBS event
> > > > > -          * we can assume it's for that event.
> > > > > -          */
> > > > > -         if (!pebs_status && cpuc->pebs_enabled &&
> > > > > -                 !(cpuc->pebs_enabled & (cpuc->pebs_enabled-1)))
> > > > > -                 pebs_status = cpuc->pebs_enabled;
> > > >
> > > > Wouldn't something like:
> > > >
> > > >                      pebs_status = p->status = cpus->pebs_enabled;
> > >
> > > I didn't consider it as a potential solution in this patch because I don't
> > > think it's a proper way that SW modifies the buffer, which is supposed to be
> > > manipulated by the HW.
> >
> > Right, but then HW was supposed to write sane values and it doesn't do
> > that either ;-)
> >
> > > It's just a personal preference. I don't see any issue here. We may try it.
> >
> > So I mostly agree with you, but I think it's a shame to unsupport such
> > chips, HSW is still a plenty useable chip today.
>
> I got a similar issue on ivybridge machines which caused kernel crash.
> My case it's related to the branch stack with PEBS events but I think
> it's the same issue.  And I can confirm that the above approach of
> updating p->status fixed the problem.
>
> I've talked to Stephane about this, and he wants to make it more
> robust when we see stale (or invalid) PEBS records.  I'll send the
> patch soon.

Hi Namhyung,

In case you didn't see it, I've already submitted a patch to fix the 
issue last Friday.
https://lore.kernel.org/lkml/1615555298-140216-1-git-send-email-kan.liang@xxxxxxxxxxxxxxx/
But if you have a more robust proposal, please feel free to submit it.

BTW: The patch set from last Friday also fixed another bug found by the 
perf_fuzzer test. You may be interested.

Thanks,
Kan