linux-kernel - Re: [PATCH v3 12/12] perf jevents: Add load event json to verify and allow fallbacks

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Date: Fri, 19 Apr 2024 15:37:11 -0400
From: "Liang, Kan" <kan.liang@...ux.intel.com>
To: Ian Rogers <irogers@...gle.com>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
 Jiri Olsa <jolsa@...nel.org>, Adrian Hunter <adrian.hunter@...el.com>,
 John Garry <john.g.garry@...cle.com>, Jing Zhang
 <renyu.zj@...ux.alibaba.com>, Thomas Richter <tmricht@...ux.ibm.com>,
 James Clark <james.clark@....com>, linux-kernel@...r.kernel.org,
 linux-perf-users@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
 Kajol Jain <kjain@...ux.ibm.com>, Sandipan Das <sandipan.das@....com>,
 Ravi Bangoria <ravi.bangoria@....com>, Perry Taylor
 <perry.taylor@...el.com>, Samantha Alt <samantha.alt@...el.com>,
 Caleb Biggers <caleb.biggers@...el.com>, Weilin Wang
 <weilin.wang@...el.com>, Edward Baker <edward.baker@...el.com>,
 Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH v3 12/12] perf jevents: Add load event json to verify and
 allow fallbacks



On 2024-03-14 1:50 a.m., Ian Rogers wrote:
> Add a LoadEvents function that loads all event json files in a
> directory. In the Event constructor ensure all events are defined in
> the event json except for legacy events like "cycles". If the initial
> event isn't found then legacy_event1 is used, and if that isn't found
> legacy_event2 is used. This allows a single Event to have multiple
> event names as models will often rename the same event over time. If
> the event doesn't exist an exception is raised.
> 
> So that references to metrics can be added, add the MetricRef
> class. This doesn't validate as an event name and so provides an
> escape hatch for metrics to refer to each other.
> 
> Signed-off-by: Ian Rogers <irogers@...gle.com>
> ---
>  tools/perf/pmu-events/Build            | 12 ++--
>  tools/perf/pmu-events/amd_metrics.py   |  6 +-
>  tools/perf/pmu-events/arm64_metrics.py |  6 +-
>  tools/perf/pmu-events/intel_metrics.py |  6 +-
>  tools/perf/pmu-events/metric.py        | 77 +++++++++++++++++++++++++-
>  5 files changed, 95 insertions(+), 12 deletions(-)
>

make fails.

  GEN     pmu-events/arch/arm64/arm/cortex-a710/extra-metricgroups.json
Traceback (most recent call last):
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 43, in <module>
    main()
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 35, in main
    LoadEvents(directory)
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/metric.py",
line 26, in LoadEvents
    for x in json.load(open(f"{directory}/{filename}")):
  File "/usr/lib64/python3.10/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib64/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
  GEN     pmu-events/arch/arm64/arm/cortex-a73/extra-metrics.json
make[3]: Circular pmu-events/arch/arm64/arm/cortex-a75/branch.json <-
pmu-events/arch/arm64/arm/cortex-a75/branch.json dependency dropped.
make[3]: Circular pmu-events/arch/arm64/arm/cortex-a75/bus.json <-
pmu-events/arch/arm64/arm/cortex-a75/bus.json dependency dropped.
make[3]: Circular pmu-events/arch/arm64/arm/cortex-a75/cache.json <-
pmu-events/arch/arm64/arm/cortex-a75/cache.json dependency dropped.
make[3]: Circular pmu-events/arch/arm64/arm/cortex-a75/etm.json <-
pmu-events/arch/arm64/arm/cortex-a75/etm.json dependency dropped.
make[3]: Circular pmu-events/arch/arm64/arm/cortex-a75/exception.json <-
pmu-events/arch/arm64/arm/cortex-a75/exception.json dependency dropped.
make[3]: Circular pmu-events/arch/arm64/arm/cortex-a75/instruction.json
<- pmu-events/arch/arm64/arm/cortex-a75/instruction.json dependency dropped.
make[3]: Circular pmu-events/arch/arm64/arm/cortex-a75/memory.json <-
pmu-events/arch/arm64/arm/cortex-a75/memory.json dependency dropped.
make[3]: Circular pmu-events/arch/arm64/arm/cortex-a75/mmu.json <-
pmu-events/arch/arm64/arm/cortex-a75/mmu.json dependency dropped.
make[3]: Circular pmu-events/arch/arm64/arm/cortex-a75/pipeline.json <-
pmu-events/arch/arm64/arm/cortex-a75/pipeline.json dependency dropped.
  GEN     pmu-events/arch/arm64/arm/cortex-a73/extra-metricgroups.json
make[3]: *** [pmu-events/Build:58:
pmu-events/arch/arm64/arm/cortex-a57-a72/extra-metrics.json] Error 1
make[3]: *** Deleting file
'pmu-events/arch/arm64/arm/cortex-a57-a72/extra-metrics.json'
make[3]: *** Waiting for unfinished jobs....
Traceback (most recent call last):
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 43, in <module>
    main()
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 35, in main
    LoadEvents(directory)
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/metric.py",
line 26, in LoadEvents
    for x in json.load(open(f"{directory}/{filename}")):
  File "/usr/lib64/python3.10/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib64/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
make[3]: *** [pmu-events/Build:58:
pmu-events/arch/arm64/arm/cortex-a65-e1/extra-metrics.json] Error 1
make[3]: *** Deleting file
'pmu-events/arch/arm64/arm/cortex-a65-e1/extra-metrics.json'
Traceback (most recent call last):
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 43, in <module>
    main()
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 35, in main
    LoadEvents(directory)
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/metric.py",
line 26, in LoadEvents
    for x in json.load(open(f"{directory}/{filename}")):
  File "/usr/lib64/python3.10/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib64/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
make[3]: *** [pmu-events/Build:62:
pmu-events/arch/arm64/arm/cortex-a57-a72/extra-metricgroups.json] Error 1
make[3]: *** Deleting file
'pmu-events/arch/arm64/arm/cortex-a57-a72/extra-metricgroups.json'
Traceback (most recent call last):
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 43, in <module>
    main()
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 35, in main
    LoadEvents(directory)
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/metric.py",
line 26, in LoadEvents
    for x in json.load(open(f"{directory}/{filename}")):
  File "/usr/lib64/python3.10/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib64/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
make[3]: *** [pmu-events/Build:62:
pmu-events/arch/arm64/arm/cortex-a65-e1/extra-metricgroups.json] Error 1
make[3]: *** Deleting file
'pmu-events/arch/arm64/arm/cortex-a65-e1/extra-metricgroups.json'
Traceback (most recent call last):
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 43, in <module>
    main()
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 35, in main
    LoadEvents(directory)
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/metric.py",
line 26, in LoadEvents
    for x in json.load(open(f"{directory}/{filename}")):
  File "/usr/lib64/python3.10/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib64/python3.10/json/__init__.py", line 346, in loads
Traceback (most recent call last):
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 43, in <module>
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.10/json/decoder.py", line 355, in raw_decode
    main()
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 35, in main
    LoadEvents(directory)
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/metric.py",
line 26, in LoadEvents
    raise JSONDecodeError("Expecting value", s, err.value) from None
    for x in json.load(open(f"{directory}/{filename}")):
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
  File "/usr/lib64/python3.10/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib64/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
Traceback (most recent call last):
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 43, in <module>
    main()
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 35, in main
    LoadEvents(directory)
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/metric.py",
line 26, in LoadEvents
    for x in json.load(open(f"{directory}/{filename}")):
  File "/usr/lib64/python3.10/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib64/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
Traceback (most recent call last):
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 43, in <module>
    main()
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/arm64_metrics.py",
line 35, in main
    LoadEvents(directory)
  File
"/home/kan/tmp/perf-tools-next-test/tools/perf/pmu-events/metric.py",
line 26, in LoadEvents
    for x in json.load(open(f"{directory}/{filename}")):
  File "/usr/lib64/python3.10/json/__init__.py", line 293, in load
    return loads(fp.read(),
  File "/usr/lib64/python3.10/json/__init__.py", line 346, in loads
    return _default_decoder.decode(s)
  File "/usr/lib64/python3.10/json/decoder.py", line 337, in decode
    obj, end = self.raw_decode(s, idx=_w(s, 0).end())
  File "/usr/lib64/python3.10/json/decoder.py", line 355, in raw_decode
    raise JSONDecodeError("Expecting value", s, err.value) from None
json.decoder.JSONDecodeError: Expecting value: line 1 column 1 (char 0)
make[3]: *** [pmu-events/Build:62:
pmu-events/arch/arm64/arm/cortex-a710/extra-metricgroups.json] Error 1
make[3]: *** Deleting file
'pmu-events/arch/arm64/arm/cortex-a710/extra-metricgroups.json'
make[3]: *** [pmu-events/Build:58:
pmu-events/arch/arm64/arm/cortex-a710/extra-metrics.json] Error 1
make[3]: *** Deleting file
'pmu-events/arch/arm64/arm/cortex-a710/extra-metrics.json'
make[3]: *** [pmu-events/Build:62:
pmu-events/arch/arm64/arm/cortex-a73/extra-metricgroups.json] Error 1
make[3]: *** Deleting file
'pmu-events/arch/arm64/arm/cortex-a73/extra-metricgroups.json'
make[2]: *** [Makefile.perf:730: pmu-events/pmu-events-in.o] Error 2
make[1]: *** [Makefile.perf:264: sub-make] Error 2
make: *** [Makefile:70: all] Error 2


Thanks,
Kan

> diff --git a/tools/perf/pmu-events/Build b/tools/perf/pmu-events/Build
> index e2db33577707..a2c5c04e5c46 100644
> --- a/tools/perf/pmu-events/Build
> +++ b/tools/perf/pmu-events/Build
> @@ -42,11 +42,11 @@ ZEN_METRICGROUPS = $(foreach x,$(ZENS),$(OUTPUT)$(x)/extra-metricgroups.json)
>  
>  $(ZEN_METRICS): pmu-events/amd_metrics.py
>  	$(call rule_mkdir)
> -	$(Q)$(call echo-cmd,gen)$(PYTHON) $< $(call model_name,$@) arch > $@
> +	$(Q)$(call echo-cmd,gen)$(PYTHON) $< $(call model_name,$@) pmu-events/arch > $@
>  
>  $(ZEN_METRICGROUPS): pmu-events/amd_metrics.py
>  	$(call rule_mkdir)
> -	$(Q)$(call echo-cmd,gen)$(PYTHON) $< -metricgroups $(call model_name,$@) arch > $@
> +	$(Q)$(call echo-cmd,gen)$(PYTHON) $< -metricgroups $(call model_name,$@) pmu-events/arch > $@
>  
>  # Generate ARM Json
>  ARMS = $(shell ls -d pmu-events/arch/arm64/arm/*)
> @@ -55,11 +55,11 @@ ARM_METRICGROUPS = $(foreach x,$(ARMS),$(OUTPUT)$(x)/extra-metricgroups.json)
>  
>  $(ARM_METRICS): pmu-events/arm64_metrics.py
>  	$(call rule_mkdir)
> -	$(Q)$(call echo-cmd,gen)$(PYTHON) $< $(call vendor_name,$@) $(call model_name,$@) arch > $@
> +	$(Q)$(call echo-cmd,gen)$(PYTHON) $< $(call vendor_name,$@) $(call model_name,$@) pmu-events/arch > $@
>  
>  $(ARM_METRICGROUPS): pmu-events/arm64_metrics.py
>  	$(call rule_mkdir)
> -	$(Q)$(call echo-cmd,gen)$(PYTHON) $< -metricgroups $(call vendor_name,$@) $(call model_name,$@) arch > $@
> +	$(Q)$(call echo-cmd,gen)$(PYTHON) $< -metricgroups $(call vendor_name,$@) $(call model_name,$@) pmu-events/arch > $@
>  
>  # Generate Intel Json
>  INTELS = $(shell ls -d pmu-events/arch/x86/*|grep -v amdzen|grep -v mapfile.csv)
> @@ -68,11 +68,11 @@ INTEL_METRICGROUPS = $(foreach x,$(INTELS),$(OUTPUT)$(x)/extra-metricgroups.json
>  
>  $(INTEL_METRICS): pmu-events/intel_metrics.py
>  	$(call rule_mkdir)
> -	$(Q)$(call echo-cmd,gen)$(PYTHON) $< $(call model_name,$@) arch > $@
> +	$(Q)$(call echo-cmd,gen)$(PYTHON) $< $(call model_name,$@) pmu-events/arch > $@
>  
>  $(INTEL_METRICGROUPS): pmu-events/intel_metrics.py
>  	$(call rule_mkdir)
> -	$(Q)$(call echo-cmd,gen)$(PYTHON) $< -metricgroups $(call model_name,$@) arch > $@
> +	$(Q)$(call echo-cmd,gen)$(PYTHON) $< -metricgroups $(call model_name,$@) pmu-events/arch > $@
>  
>  GEN_JSON = $(patsubst %,$(OUTPUT)%,$(JSON)) \
>              $(ZEN_METRICS) $(ZEN_METRICGROUPS) \
> diff --git a/tools/perf/pmu-events/amd_metrics.py b/tools/perf/pmu-events/amd_metrics.py
> index 7ab2ee4fdb17..4f728e7aae4a 100755
> --- a/tools/perf/pmu-events/amd_metrics.py
> +++ b/tools/perf/pmu-events/amd_metrics.py
> @@ -1,6 +1,7 @@
>  #!/usr/bin/env python3
>  # SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
> -from metric import (JsonEncodeMetric, JsonEncodeMetricGroupDescriptions, MetricGroup)
> +from metric import (JsonEncodeMetric, JsonEncodeMetricGroupDescriptions, LoadEvents,
> +                    MetricGroup)
>  import argparse
>  import json
>  import os
> @@ -27,6 +28,9 @@ def main() -> None:
>    )
>    _args = parser.parse_args()
>  
> +  directory = f"{_args.events_path}/x86/{_args.model}/"
> +  LoadEvents(directory)
> +
>    all_metrics = MetricGroup("",[])
>  
>    if _args.metricgroups:
> diff --git a/tools/perf/pmu-events/arm64_metrics.py b/tools/perf/pmu-events/arm64_metrics.py
> index a9f0e6bc751b..c9aa2d827a82 100755
> --- a/tools/perf/pmu-events/arm64_metrics.py
> +++ b/tools/perf/pmu-events/arm64_metrics.py
> @@ -1,6 +1,7 @@
>  #!/usr/bin/env python3
>  # SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
> -from metric import (JsonEncodeMetric, JsonEncodeMetricGroupDescriptions, MetricGroup)
> +from metric import (JsonEncodeMetric, JsonEncodeMetricGroupDescriptions, LoadEvents,
> +                    MetricGroup)
>  import argparse
>  import json
>  import os
> @@ -30,6 +31,9 @@ def main() -> None:
>  
>    all_metrics = MetricGroup("",[])
>  
> +  directory = f"{_args.events_path}/arm64/{_args.vendor}/{_args.model}/"
> +  LoadEvents(directory)
> +
>    if _args.metricgroups:
>      print(JsonEncodeMetricGroupDescriptions(all_metrics))
>    else:
> diff --git a/tools/perf/pmu-events/intel_metrics.py b/tools/perf/pmu-events/intel_metrics.py
> index f004c27640d2..04a19d05c6c1 100755
> --- a/tools/perf/pmu-events/intel_metrics.py
> +++ b/tools/perf/pmu-events/intel_metrics.py
> @@ -1,6 +1,7 @@
>  #!/usr/bin/env python3
>  # SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
> -from metric import (JsonEncodeMetric, JsonEncodeMetricGroupDescriptions, MetricGroup)
> +from metric import (JsonEncodeMetric, JsonEncodeMetricGroupDescriptions, LoadEvents,
> +                    MetricGroup)
>  import argparse
>  import json
>  import os
> @@ -27,6 +28,9 @@ def main() -> None:
>    )
>    _args = parser.parse_args()
>  
> +  directory = f"{_args.events_path}/x86/{_args.model}/"
> +  LoadEvents(directory)
> +
>    all_metrics = MetricGroup("",[])
>  
>    if _args.metricgroups:
> diff --git a/tools/perf/pmu-events/metric.py b/tools/perf/pmu-events/metric.py
> index dd8fd06940e6..03312cd6d491 100644
> --- a/tools/perf/pmu-events/metric.py
> +++ b/tools/perf/pmu-events/metric.py
> @@ -3,10 +3,50 @@
>  import ast
>  import decimal
>  import json
> +import os
>  import re
>  from enum import Enum
>  from typing import Dict, List, Optional, Set, Tuple, Union
>  
> +all_events = set()
> +
> +def LoadEvents(directory: str) -> None:
> +  """Populate a global set of all known events for the purpose of validating Event names"""
> +  global all_events
> +  all_events = {
> +      "context\-switches",
> +      "cycles",
> +      "duration_time",
> +      "instructions",
> +      "l2_itlb_misses",
> +  }
> +  for file in os.listdir(os.fsencode(directory)):
> +    filename = os.fsdecode(file)
> +    if filename.endswith(".json"):
> +      for x in json.load(open(f"{directory}/{filename}")):
> +        if "EventName" in x:
> +          all_events.add(x["EventName"])
> +        elif "ArchStdEvent" in x:
> +          all_events.add(x["ArchStdEvent"])
> +
> +
> +def CheckEvent(name: str) -> bool:
> +  """Check the event name exists in the set of all loaded events"""
> +  global all_events
> +  if len(all_events) == 0:
> +    # No events loaded so assume any event is good.
> +    return True
> +
> +  if ':' in name:
> +    # Remove trailing modifier.
> +    name = name[:name.find(':')]
> +  elif '/' in name:
> +    # Name could begin with a PMU or an event, for now assume it is good.
> +    return True
> +
> +  return name in all_events
> +
> +
>  class MetricConstraint(Enum):
>    GROUPED_EVENTS = 0
>    NO_GROUP_EVENTS = 1
> @@ -317,9 +357,18 @@ def _FixEscapes(s: str) -> str:
>  class Event(Expression):
>    """An event in an expression."""
>  
> -  def __init__(self, name: str, legacy_name: str = ''):
> -    self.name = _FixEscapes(name)
> -    self.legacy_name = _FixEscapes(legacy_name)
> +  def __init__(self, *args: str):
> +    error = ""
> +    for name in args:
> +      if CheckEvent(name):
> +        self.name = _FixEscapes(name)
> +        return
> +      if error:
> +        error += " or " + name
> +      else:
> +        error = name
> +    global all_events
> +    raise Exception(f"No event {error} in:\n{all_events}")
>  
>    def ToPerfJson(self):
>      result = re.sub('/', '@', self.name)
> @@ -338,6 +387,28 @@ class Event(Expression):
>      return self
>  
>  
> +class MetricRef(Expression):
> +  """A metric reference in an expression."""
> +
> +  def __init__(self, name: str):
> +    self.name = _FixEscapes(name)
> +
> +  def ToPerfJson(self):
> +    return self.name
> +
> +  def ToPython(self):
> +    return f'MetricRef(r"{self.name}")'
> +
> +  def Simplify(self) -> Expression:
> +    return self
> +
> +  def Equals(self, other: Expression) -> bool:
> +    return isinstance(other, MetricRef) and self.name == other.name
> +
> +  def Substitute(self, name: str, expression: Expression) -> Expression:
> +    return self
> +
> +
>  class Constant(Expression):
>    """A constant within the expression tree."""
>