Re: [PATCH RFC 2/3] perf/x86/intel/pt: Add support for pause_resume()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Nov 29, 2023 at 01:15:43PM +0200, Adrian Hunter wrote:
> On 29/11/23 12:58, Peter Zijlstra wrote:
> > On Wed, Nov 29, 2023 at 09:53:39AM +0000, James Clark wrote:
> >> On 23/11/2023 12:18, Adrian Hunter wrote:
> > 
> >>> +static void pt_event_pause_resume(struct perf_event *event)
> >>> +{
> >>> +	if (event->aux_paused)
> >>> +		pt_config_stop(event);
> >>> +	else if (!event->hw.state)
> >>> +		pt_config_start(event);
> >>> +}
> >>
> >> It seems like having a single pause/resume callback rather than separate
> >> pause and resume ones pushes some of the event state management into the
> >> individual drivers and would be prone to code duplication and divergent
> >> behavior.
> >>
> >> Would it be possible to move the conditions from here into the core code
> >> and call separate functions instead?
> >>
> >>> +
> >>>  static void pt_event_start(struct perf_event *event, int mode)
> >>>  {
> >>>  	struct hw_perf_event *hwc = &event->hw;
> >>> @@ -1798,6 +1809,7 @@ static __init int pt_init(void)
> >>>  	pt_pmu.pmu.del			 = pt_event_del;
> >>>  	pt_pmu.pmu.start		 = pt_event_start;
> >>>  	pt_pmu.pmu.stop			 = pt_event_stop;
> >>> +	pt_pmu.pmu.pause_resume		 = pt_event_pause_resume;
> >>
> >> The general idea seems ok to me. Is there a reason to not use the
> >> existing start() stop() callbacks, rather than adding a new one?
> >>
> >> I assume it's intended to be something like an optimisation where you
> >> can turn it on and off without having to do the full setup, teardown and
> >> emit an AUX record because you know the process being traced never gets
> >> switched out?
> > 
> > So the actual scheduling uses ->add() / ->del(), the ->start() /
> > ->stop() methods are something that can be used after ->add() and before
> > ->del() to 'temporarily' pause things.
> > 
> > Pretty much exactly what is required here I think. We currently use this
> > for PMI throttling and adaptive frequency stuff, but there is no reason
> > it could not also be used for this.
> > 
> > As is, we don't track the paused state across ->del() / ->add(), but
> > perhaps that can be fixed. We can easily add more PERF_EF_ / PERF_HES_
> > bits to manage things.
> > 
> > 
> 
> I am not sure stop / start play nice with NMI's from other events e.g.
> 
> PMC NMI wants to pause or resume AUX but what if AUX event is currently
> being processed in ->stop() or ->start()?  Or maybe that can't happen?

I think that can happen, and pt_event_stop() can actually handle some of
that, while your pause_resume() thing, which uses pt_config_stop() does
not.

But yes, I think that if you add pt_event_{stop,start}() calls from
*other* events their PMI, then you get to deal with more 'fun'.

Something like:

  perf_addr_filters_adjust()
    __perf_addr_filters_adjust()
      perf_event_stop()
        __perf_event_stop()
	  event->pmu->stop()
	  <NMI>
	    ...
	    perf_event_overflow()
	      pt_event->pmu->stop()
	  </NMI>
	  event->pmu->start() // whoopsie!

Should now be possible.

I think what you want to do is rename pt->handle_nmi into pt->stop_count
and make it a counter, then ->stop() increments it, and ->start()
decrements it and everybody ensures the thing doesn't get restart while
!0 etc..

I suspect you need to guard the generic part of this feature with a new
PERF_PMU_CAP_ flag and then have the coresight/etc. people opt-in once
they've audited things.

James, does that work for you?