Re: [PATCH v3 5/6] perf vendor events arm64: Add PE utilization metrics for neoverse-n2

[Jing Zhang]

在 2022/12/1 上午2:58, Ian Rogers 写道:
> On Thu, Nov 24, 2022 at 9:15 AM Jing Zhang <renyu.zj@xxxxxxxxxxxxxxxxx> wrote:
>>
>> Add PE utilization related metrics.
>>
>> Signed-off-by: Jing Zhang <renyu.zj@xxxxxxxxxxxxxxxxx>
>> ---
>>  .../arch/arm64/arm/neoverse-n2/metrics.json        | 45 ++++++++++++++++++++++
>>  1 file changed, 45 insertions(+)
>>
>> diff --git a/tools/perf/pmu-events/arch/arm64/arm/neoverse-n2/metrics.json b/tools/perf/pmu-events/arch/arm64/arm/neoverse-n2/metrics.json
>> index 23c7d62..7b54819 100644
>> --- a/tools/perf/pmu-events/arch/arm64/arm/neoverse-n2/metrics.json
>> +++ b/tools/perf/pmu-events/arch/arm64/arm/neoverse-n2/metrics.json
>> @@ -189,5 +189,50 @@
>>          "MetricGroup": "Branch",
>>          "MetricName": "branch_miss_pred_rate",
>>          "ScaleUnit": "100%"
>> +    },
>> +    {
>> +        "MetricExpr": "instructions / CPU_CYCLES",
>> +        "PublicDescription": "The average number of instructions executed for each cycle.",
>> +        "BriefDescription": "Instructions per cycle",
>> +        "MetricGroup": "PEutilization",
>> +        "MetricName": "ipc"
>> +    },
> 
> A related useful metric is percentage of peak, so if the peak IPC is 8
> (usually a constant related to the number of functional units) then
> you can just compute the ratio of IPC with this.
> 

Glad to discuss these with you.
The peak ipc value of neoverse-n2 is 5. Maybe I should add an ipc_rate metric?

>> +    {
>> +        "MetricExpr": "INST_RETIRED / CPU_CYCLES",
>> +        "PublicDescription": "Architecturally executed Instructions Per Cycle (IPC)",
>> +        "BriefDescription": "Architecturally executed Instructions Per Cycle (IPC)",
> 
> 
> The duplicated descriptions are unnecessary. Drop the public one for
> consistency with what we do for Intel:
> https://github.com/intel/perfmon/blob/main/scripts/create_perf_json.py#L299
> 

Sounds good, will do.

>> +        "MetricGroup": "PEutilization",
>> +        "MetricName": "retired_ipc"
>> +    },
>> +    {
>> +        "MetricExpr": "INST_SPEC / CPU_CYCLES",
>> +        "PublicDescription": "Speculatively executed Instructions Per Cycle (IPC)",
>> +        "BriefDescription": "Speculatively executed Instructions Per Cycle (IPC)",
>> +        "MetricGroup": "PEutilization",
>> +        "MetricName": "spec_ipc"
>> +    },
>> +    {
>> +        "MetricExpr": "OP_RETIRED / OP_SPEC",
>> +        "PublicDescription": "Fraction of operations retired",
>> +        "BriefDescription": "Fraction of operations retired",
> 
> Would instructions be clearer than operations here?
> 

operation and instruction are different. OP_RETIRED counts any operation (not instruction)
that has been architecturally executed, For example, speculatively executed operations that
have been abandoned for a branch mispredict will not be counted. So I think operation might
be more accurate.

>> +        "MetricGroup": "PEutilization",
>> +        "MetricName": "retired_rate",
>> +        "ScaleUnit": "100%"
>> +    },
>> +    {
>> +        "MetricExpr": "1 - OP_RETIRED / OP_SPEC",
> 
> Should OP_RETIRED be greater than OP_SPEC? In which case won't this
> metric be negative?
> 

OP_RETIRED will not be greater than OP_SPEC. OP_SPEC counts any operation that has been
speculatively executed. OP_SPEC is a superset of the OP_RETIRED event. There is a
description about OP_SPEC and OP_RETIRED in this neoverse-n2 document.
Link: https://documentation-service.arm.com/static/62cfe21e31ea212bb6627393?token=

>> +        "PublicDescription": "Fraction of operations wasted",
>> +        "BriefDescription": "Fraction of operations wasted",
>> +        "MetricGroup": "PEutilization",
>> +        "MetricName": "wasted_rate",
>> +        "ScaleUnit": "100%"
>> +    },
>> +    {
>> +        "MetricExpr": "OP_RETIRED / OP_SPEC * (1 - (STALL_SLOT - CPU_CYCLES) / (CPU_CYCLES * 5))",
>> +        "PublicDescription": "Utilization of CPU",
>> +        "BriefDescription": "Utilization of CPU",
> 
> Some more detail in the description would be useful.
> 

Ok, I'll describe it in more detail. CPU_utilization reflects the truly effective ratio of operation
executed by the CPU, which means that misprediction and stall are not included. Note that stall_slot
minus cpu_cycles is a correction to the stall_slot error count.

>> +        "MetricGroup": "PEutilization",
>> +        "MetricName": "cpu_utilization",
>> +        "ScaleUnit": "100%"
>>      }
>>  ]
>> --
>> 1.8.3.1
>>