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

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Message-ID: <ea7ff9b-b91c-d158-dfde-4569784a4720@os.amperecomputing.com>
Date: Fri, 6 Sep 2024 11:28:03 -0700 (PDT)
From: Ilkka Koskinen <ilkka@...amperecomputing.com>
To: Anshuman Khandual <anshuman.khandual@....com>
cc: Ilkka Koskinen <ilkka@...amperecomputing.com>, 
    Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>, 
    linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] perf: arm_pmuv3: Use BR_RETIRED for HW branch event if
 enabled


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@...amperecomputing.com>
>> ---
>>  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