Messages in this thread | | | Date | Wed, 29 Nov 2023 13:23:20 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH RFC 2/3] perf/x86/intel/pt: Add support for pause_resume() |
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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?
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