Re: [PATCH 2/3] perf/x86/pebs: add workaround for broken OVFL status on HSW

[Stephane Eranian]

On Thu, Mar 3, 2016 at 1:43 PM, Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>
> > +             /*
> > +              * There are cases where, even though, the PEBS ovfl bit is set in
> > +              * GLOBAL_OVF_STATUS, the PEBS events may also have their overflow bits
> > +              * set for their counters. We must clear them here because they have
> > +              * been processed as exact samples in the drain_pebs() routine. They
> > +              * must not be processed again in the for_each_bit_set() loop for
> > +              * regular samples below.
> > +              */
> > +             status &= ~cpuc->pebs_enabled;
> > +             status &= x86_pmu.intel_ctrl | GLOBAL_STATUS_TRACE_TOPAPMI;
>
> If you just clear the bits here they will not be acked and stay around
> forever in GLOBAL_STATUS, which causes other problems.
>
> You need a separate ack_status variable that contains all bits and is always
> acked.

I understand that. You mean I need to that has all the bits that were set to
call intel_pmu_ack_status().

But if you look at the code, and where I made the change, there is no more
intel_pmu_ack_status() BEFORE you read the status again via
intel_pmu_get_status().

So why would I need to keep another variable around?

>
>
>
> -Andi
> --
> ak@xxxxxxxxxxxxxxx -- Speaking for myself only