Re: [PATCH v4 2/5] perf header: Parse non-cpu pmu capabilities

[Ravi Bangoria]

Hi Kan,

[...]

>> +static int write_pmu_caps(struct feat_fd *ff, struct evlist *evlist __maybe_unused)
>> +{
>> +    struct perf_pmu_caps *caps = NULL;
>> +    struct perf_pmu *pmu = NULL;
>> +    u32 nr_pmus = 0;
>> +    int ret;
>> +
>> +    while ((pmu = perf_pmu__scan(pmu))) {
>> +        if (!pmu->name || !strncmp(pmu->name, "cpu", 3) ||
>> +            perf_pmu__caps_parse(pmu) <= 0)
>> +            continue;
>> +        nr_pmus++;
>> +    }
>> +
>> +    ret = do_write(ff, &nr_pmus, sizeof(nr_pmus));
>> +    if (ret < 0)
>> +        return ret;
>> +
>> +    if (!nr_pmus)
>> +        return 0;
>> +
>> +    while ((pmu = perf_pmu__scan(pmu))) {
>> +        if (!pmu->name || !strncmp(pmu->name, "cpu", 3) || !pmu->nr_caps)
>> +            continue;
>> +
>> +        ret = do_write_string(ff, pmu->name);
>> +        if (ret < 0)
>> +            return ret;
>> +
>> +        ret = do_write(ff, &pmu->nr_caps, sizeof(pmu->nr_caps));
>> +        if (ret < 0)
>> +            return ret;
>> +
>> +        list_for_each_entry(caps, &pmu->caps, list) {
>> +            ret = do_write_string(ff, caps->name);
>> +            if (ret < 0)
>> +                return ret;
>> +
>> +            ret = do_write_string(ff, caps->value);
>> +            if (ret < 0)
>> +                return ret;
>> +        }
>> +    }
> 
> The write_per_cpu_pmu_caps() also does a similar thing. Can we factor out a generic write_pmu_caps() which works for both cpu and non-cpu pmu capabilities?

I might be able to do this but..

> It seems the print_pmu_caps()/process_pmu_caps() can also does similar factor out.

not this, see below..

> Actually, more aggressively, why not use the HEADER_PMU_CAPS to replace the HEADER_HYBRID_CPU_PMU_CAPS? The HEADER_HYBRID_CPU_PMU_CAPS is the last header feature. It seems doable. We can always write/print the "cpu_" kind of PMU first to be compatible with the old tools.

There are some differences in how capabilities are stored in perf.data header
as well as perf_env. In case of HEADER_CPU_PMU_CAPS or
HEADER_HYBRID_CPU_PMU_CAPS, all capabilities are stored in a single string
separated by NULL character. Whereas, in case of HEADER_PMU_CAPS, they are
stored as an array of strings. The reason for this difference is, searching
in an array is far easier compared to searching in a NULL separated string.
So, I don't think I can extend HEADER_HYBRID_CPU_PMU_CAPS as HEADER_PMU_CAPS
without adding complexity in perf_env__find_pmu_cap().

Thanks for the review,
Ravi