linux-kernel - Re: [PATCH v5 10/15] perf jevents: Generate metrics and events as separate tables

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Message-ID: <d885941e-a22b-4bda-cf56-968e6d6dc069@oracle.com>
Date:   Wed, 1 Feb 2023 18:29:31 +0000
From:   John Garry <john.g.garry@...cle.com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     Will Deacon <will@...nel.org>, James Clark <james.clark@....com>,
        Mike Leach <mike.leach@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Kim Phillips <kim.phillips@....com>,
        Florian Fischer <florian.fischer@...q.space>,
        Ravi Bangoria <ravi.bangoria@....com>,
        Xing Zhengjun <zhengjun.xing@...ux.intel.com>,
        Rob Herring <robh@...nel.org>,
        Kang Minchul <tegongkang@...il.com>,
        linux-arm-kernel@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        Sandipan Das <sandipan.das@....com>,
        Jing Zhang <renyu.zj@...ux.alibaba.com>,
        linuxppc-dev@...ts.ozlabs.org, Kajol Jain <kjain@...ux.ibm.com>,
        Stephane Eranian <eranian@...gle.com>,
        Perry Taylor <perry.taylor@...el.com>,
        Caleb Biggers <caleb.biggers@...el.com>
Subject: Re: [PATCH v5 10/15] perf jevents: Generate metrics and events as
 separate tables

On 30/01/2023 22:54, Ian Rogers wrote:
>> This is almost identical to generated perf_pmu__find_events_table(),
>> except we return a pmu_metrics_table * (instead of a pmu_events_table *)
>> and also return the metric table member (instead of event table). But
>> the definitions are:
>>
>> /* Struct used to make the PMU event table implementation opaque to
>> callers. */
>> struct pmu_events_table {
>>           const struct compact_pmu_event *entries;
>>           size_t length;
>> };
>>
>> /* Struct used to make the PMU metric table implementation opaque to
>> callers. */
>> struct pmu_metrics_table {
>>           const struct compact_pmu_event *entries;
>>           size_t length;
>> };
>>
>> Those structs are defined to be the same thing, so I am failing to see
>> the point in a) separate structure types b) why so much duplication
>>
>> As for b), I know that they are generated and the python code may be
>> simpler this way (is it?), but still...
> Agreed. The point is to separate the two tables for the typing at the
> API layer, internally the representation is the same. When we decode
> one we get a pmu_event and the other we get a pmu_metric, so we don't
> want to allow the tables to be switched - hence two types.

I do see the advantage of stronger types but it does seem a bit odd to 
achieve it like this.

Thanks,
John