Re: [PATCH V9 3/3] perf/x86/intel: Support PEBS counters snapshotting

Peter Zijlstra
2025-01-16T20:57:06Z

On Thu, Jan 16, 2025 at 09:42:25PM +0100, Peter Zijlstra wrote:
> On Thu, Jan 16, 2025 at 10:55:46AM -0500, Liang, Kan wrote:
>
> > > Also, I think I found you another bug... Consider what happens to the
> > > counter value when we reschedule a HES_STOPPED counter, then we skip
> > > x86_pmu_start(RELOAD) on step2, which leave the counter value with
> > > 'random' crap from whatever was there last.
> > >
> > > But meanwhile you do program PEBS to sample it. That will happily sample
> > > this garbage.
> > >
> > > Hmm?
> >
> > I'm not quite sure I understand the issue.
> >
> > The HES_STOPPED counter should be a pre-existing counter. Just for some
> > reason, it's stopped, right? So perf doesn't need to re-configure the
> > PEBS__DATA_CFG, since the idx is not changed.
>
> Suppose you have your group {A, B, C} and lets suppose A is the PEBS
> event, further suppose that B is also a sampling event. Lets say they
> get hardware counters 1,2 and 3 respectively.
>
> Then lets say B gets throttled.
>
> While it is throttled, we get a new event D scheduled, and D gets placed
> on counter 2 -- where B lives, which gets moved over to counter 4.
>
> Then our loops will update and remove B from 2, but because
> throttled/HES_STOPPED it will not start it on counter 4.
>
> Meanwhile, we do have the PEBS_DATA_CFG thing updated to sample counter
> 1,3 and 4.
>
> PEBS assist happens, and samples the uninitialized counter 4.

Also, by skipping x86_pmu_start() we miss the assignment of
cpuc->events[] so PEBS buffer decode can't even find the dodgy event.