Re: [PATCH] perf: arm_pmuv3: Use BR_RETIRED for HW branch event if enabled

[Ilkka Koskinen]

On Fri, 6 Sep 2024, Anshuman Khandual wrote:
> On 9/6/24 02:17, Ilkka Koskinen wrote:
> > The PMU driver attempts to use PC_WRITE_RETIRED for the HW branch event,
> > if enabled. However, PC_WRITE_RETIRED counts only taken branches,
> > whereas BR_RETIRED counts also non-taken ones.
> >
> > Furthermore, perf uses HW branch event to calculate branch misses ratio,
> > implying BR_RETIRED is the correct event to count.
> But is the event BR_RETIRED always guaranteed to be available. Should not
> armpmu->pmceid_bitmap be checked first ?
>
> > Signed-off-by: Ilkka Koskinen <ilkka@xxxxxxxxxxxxxxxxxxxxxx>
> > ---
> >  drivers/perf/arm_pmuv3.c | 27 ++++-----------------------
> >  1 file changed, 4 insertions(+), 23 deletions(-)
> >
> > diff --git a/drivers/perf/arm_pmuv3.c b/drivers/perf/arm_pmuv3.c
> > index d246840797b6..a8ed08df1411 100644
> > --- a/drivers/perf/arm_pmuv3.c
> > +++ b/drivers/perf/arm_pmuv3.c
> > @@ -46,6 +46,7 @@ static const unsigned armv8_pmuv3_perf_map[PERF_COUNT_HW_MAX] = {
> >  	[PERF_COUNT_HW_INSTRUCTIONS]		= ARMV8_PMUV3_PERFCTR_INST_RETIRED,
> >  	[PERF_COUNT_HW_CACHE_REFERENCES]	= ARMV8_PMUV3_PERFCTR_L1D_CACHE,
> >  	[PERF_COUNT_HW_CACHE_MISSES]		= ARMV8_PMUV3_PERFCTR_L1D_CACHE_REFILL,
> > +	[PERF_COUNT_HW_BRANCH_INSTRUCTIONS]	= ARMV8_PMUV3_PERFCTR_BR_RETIRED,
> >  	[PERF_COUNT_HW_BRANCH_MISSES]		= ARMV8_PMUV3_PERFCTR_BR_MIS_PRED,
> >  	[PERF_COUNT_HW_BUS_CYCLES]		= ARMV8_PMUV3_PERFCTR_BUS_CYCLES,
> >  	[PERF_COUNT_HW_STALLED_CYCLES_FRONTEND]	= ARMV8_PMUV3_PERFCTR_STALL_FRONTEND,
> > @@ -1083,28 +1084,6 @@ static void armv8pmu_reset(void *info)
> >  	armv8pmu_pmcr_write(pmcr);
> >  }
> >
> > -static int __armv8_pmuv3_map_event_id(struct arm_pmu *armpmu,
> > -				      struct perf_event *event)
> > -{
> > -	if (event->attr.type == PERF_TYPE_HARDWARE &&
> > -	    event->attr.config == PERF_COUNT_HW_BRANCH_INSTRUCTIONS) {
> > -
> > -		if (test_bit(ARMV8_PMUV3_PERFCTR_PC_WRITE_RETIRED,
> > -			     armpmu->pmceid_bitmap))
> > -			return ARMV8_PMUV3_PERFCTR_PC_WRITE_RETIRED;
> > -
> > -		if (test_bit(ARMV8_PMUV3_PERFCTR_BR_RETIRED,
> > -			     armpmu->pmceid_bitmap))
> > -			return ARMV8_PMUV3_PERFCTR_BR_RETIRED;
>
> If BR_RETIRED event is absent on the platform, PC_WRITE_RETIRED still remains
> a good alternative to fallback on. Hence wondering if the above order could
> just be changed to use BR_RETIRED first when available.

If PC_WRITE_RETIRED is a good alternative, then I guess, it's just better 
change the order of those two. I send a new version just doing that.

Cheers, Ilkka