Re: [PATCH v1] perf header: Fix missing PMU caps

["liwei (GF)" <liwei391@huawei.com>]

Hi Ian:

On 2023/8/19 1:19, Ian Rogers wrote:
> PMU caps are written as HEADER_PMU_CAPS or for the special case of the
> PMU "cpu" as HEADER_CPU_PMU_CAPS. As the PMU "cpu" is special, and not
> any "core" PMU, the logic had become broken and core PMUs not called
> "cpu" were not having their caps written. This affects ARM and s390
> non-hybrid PMUs.
> 
> Simplify the PMU caps writing logic to scan one fewer time and to be
> more explicit in its behavior.
> 
> Reported-by: Wei Li <liwei391@huawei.com>
> Fixes: 178ddf3bad98 ("perf header: Avoid hybrid PMU list in write_pmu_caps")
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/perf/util/header.c | 31 ++++++++++++++++---------------
>  1 file changed, 16 insertions(+), 15 deletions(-)
> 
> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
> index 52fbf526fe74..13c71d28e0eb 100644
> --- a/tools/perf/util/header.c
> +++ b/tools/perf/util/header.c
> @@ -1605,8 +1605,15 @@ static int write_pmu_caps(struct feat_fd *ff,
>  	int ret;
>  
>  	while ((pmu = perf_pmus__scan(pmu))) {
> -		if (!pmu->name || !strcmp(pmu->name, "cpu") ||
> -		    perf_pmu__caps_parse(pmu) <= 0)
> +		if (!strcmp(pmu->name, "cpu")) {

So you removed the check of 'pmu->name', does this check really redundant? since
we can find such checks in many places in the perf code. If not, i think it is
necessary for strcmp().

> +			/*
> +			 * The "cpu" PMU is special and covered by
> +			 * HEADER_CPU_PMU_CAPS. Note, core PMUs are
> +			 * counted/written here for ARM, s390 and Intel hybrid.
> +			 */
> +			continue;
> +		}
> +		if (perf_pmu__caps_parse(pmu) <= 0)
>  			continue;
>  		nr_pmu++;
>  	}
> @@ -1619,23 +1626,17 @@ static int write_pmu_caps(struct feat_fd *ff,
>  		return 0;
>  
>  	/*
> -	 * Write hybrid pmu caps first to maintain compatibility with
> -	 * older perf tool.
> +	 * Note older perf tools assume core PMUs come first, this is a property
> +	 * of perf_pmus__scan.
>  	 */
> -	if (perf_pmus__num_core_pmus() > 1) {
> -		pmu = NULL;
> -		while ((pmu = perf_pmus__scan_core(pmu))) {
> -			ret = __write_pmu_caps(ff, pmu, true);
> -			if (ret < 0)
> -				return ret;
> -		}
> -	}
> -
>  	pmu = NULL;
>  	while ((pmu = perf_pmus__scan(pmu))) {
> -		if (pmu->is_core || !pmu->nr_caps)
> +		if (!strcmp(pmu->name, "cpu")) {

same here

Thanks,
Wei

> +			/* Skip as above. */
> +			continue;
> +		}
> +		if (perf_pmu__caps_parse(pmu) <= 0)
>  			continue;
> -
>  		ret = __write_pmu_caps(ff, pmu, true);
>  		if (ret < 0)
>  			return ret;