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

[Ian Rogers]

On Thu, Dec 1, 2022 at 3:08 AM Jing Zhang <renyu.zj@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> 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.

Thanks, I see this note in the N2 PMU guide:

"""
For PMU event definitions, some events specifically count
instructions, while other events count micro-operations (which are
referred to as operations). Please be aware of the use of the word
"operations" or "instructions" in the event description.
"""

>From your explanation I wasn't sure if operation was a superset of
instruction that included both retired and speculated ones, or whether
operation had another meaning. I don't see operation being used in the
micro-operation sense elsewhere in the ARM perf json, I think
micro-operation is more consistent and also clearer:
https://git.kernel.org/pub/scm/linux/kernel/git/acme/linux.git/tree/tools/perf/pmu-events/arch/arm64/arm/cortex-a75/pipeline.json?h=perf/core#n27

Perhaps the description can be something like:
Of all the micro-operations issued, what percentage were retired. A
lower number indicates bad speculation.

An alternate way to add documentation is the perf wiki's glossary:
https://perf.wiki.kernel.org/index.php/Glossary

I added the Neoverse N2 PMU Guide to:
https://perf.wiki.kernel.org/index.php/Useful_Links#Manuals

Thanks,
Ian

> >> +        "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
> >>