Re: [PATCH 0/7] Rewrite jevents program in python

[John Garry]

On 11/05/2022 08:01, Ian Rogers wrote:
> New architectures bring new complexity, such as Intel's hybrid
> models. jevents provides an alternative to specifying events in the
> kernel and exposing them through sysfs, however, it is difficult to
> work with. For example, an error in the json input would yield an
> error message but no json file or location. It is also a challenge to
> update jsmn.c given its forked nature.
>
> The changes here switch from jevents.c to a rewrite in python called
> jevents.py. This means there is a build time dependency on python, but
> such a dependency already exists for asciidoc (used to generate perf's
> man pages).

Hi Ian,

This does not build for me:

Auto-detecting system features:
...                         dwarf: [ on  ]
...            dwarf_getlocations: [ on  ]
...                         glibc: [ on  ]
...                        libbfd: [ OFF ]
...                libbfd-buildid: [ OFF ]
...                        libcap: [ on  ]
...                        libelf: [ on  ]
...                       libnuma: [ on  ]
...        numa_num_possible_cpus: [ on  ]
...                       libperl: [ on  ]
...                     libpython: [ on  ]
...                     libcrypto: [ on  ]
...                     libunwind: [ on  ]
...            libdw-dwarf-unwind: [ on  ]
...                          zlib: [ on  ]
...                          lzma: [ on  ]
...                     get_cpuid: [ on  ]
...                           bpf: [ on  ]
...                        libaio: [ on  ]
...                       libzstd: [ on  ]
...        disassembler-four-args: [ on  ]


make[3]: Nothing to be done for 'install_headers'.
 GEN     pmu-events/pmu-events.c
 CC      /home/john/acme/tools/perf/libbpf/staticobjs/libbpf.o
 LINK    dlfilters/dlfilter-test-api-v0.so
 CC      dlfilters/dlfilter-show-cycles.o
 CC      builtin-bench.o
Traceback (most recent call last):
 File "pmu-events/jevents.py", line 23, in <module>
   def file_name_to_table_name(parents: list[str], dirname: str) -> str:
TypeError: 'type' object is not subscriptable
make[3]: *** [pmu-events/Build:15: pmu-events/pmu-events.c] Error 1
make[2]: *** [Makefile.perf:662: pmu-events/pmu-events-in.o] Error 2
make[2]: *** Waiting for unfinished jobs....

What baseline do you use? It's always good to mention it I think.

> A challenge with this code is in avoiding regressions. For this reason
> the jevents.py produces identical output to jevents.c, validated with a
> test script and build target.

As you know (and have done), verifying no regression should be 
straightforward by diff'ing. For avoidance of doubt, which archs did you 
test? We also need to ensure those which don't use pmu-events (like 
arm32) work as before.

> A difference in the python to the C approach is that the python loads
> an entire json file in to memory, while the C code works from token to
> token. In some cases the C approach was sensitive to the order of
> dictionary items in the json file. To ensure matching output there are
> two changes made to jevents.c to cause it to read all values before
> creating output.
>
> The changes also found a bug in Ivytown's UNC_M_ACT_COUNT.RD event
> encoding, as well as unnecessary whitespace introduced in Alderlake's
> metrics. In these cases the json input is fixed.

Thanks,
John