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Date:   Tue, 12 Dec 2023 15:20:47 +0000
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Subject: Re: [PATCH v1 14/14] libperf cpumap: Document perf_cpu_map__nr's
 behavior
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To:     Ian Rogers <irogers@google.com>
References: <20231129060211.1890454-1-irogers@google.com>
 <20231129060211.1890454-15-irogers@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
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        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@kernel.org>,
        Namhyung Kim <namhyung@kernel.org>,
        Adrian Hunter <adrian.hunter@intel.com>,
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        Changbin Du <changbin.du@huawei.com>,
        Sandipan Das <sandipan.das@amd.com>,
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From:   James Clark <james.clark@arm.com>
In-Reply-To: <20231129060211.1890454-15-irogers@google.com>
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On 29/11/2023 06:02, Ian Rogers wrote:
> perf_cpu_map__nr's behavior around an empty CPU map is strange as it
> returns that there is 1 CPU. Changing code that may rely on this
> behavior is hard, we can at least document the behavior.
> 
> Signed-off-by: Ian Rogers <irogers@google.com>
> ---
>  tools/lib/perf/include/perf/cpumap.h | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 
> diff --git a/tools/lib/perf/include/perf/cpumap.h b/tools/lib/perf/include/perf/cpumap.h
> index 523e4348fc96..90457d17fb2f 100644
> --- a/tools/lib/perf/include/perf/cpumap.h
> +++ b/tools/lib/perf/include/perf/cpumap.h
> @@ -44,7 +44,18 @@ LIBPERF_API struct perf_cpu_map *perf_cpu_map__merge(struct perf_cpu_map *orig,
>  LIBPERF_API struct perf_cpu_map *perf_cpu_map__intersect(struct perf_cpu_map *orig,
>  							 struct perf_cpu_map *other);
>  LIBPERF_API void perf_cpu_map__put(struct perf_cpu_map *map);
> +/**
> + * perf_cpu_map__cpu - get the CPU value at the given index. Returns -1 if index
> + *                     is invalid.
> + */
>  LIBPERF_API struct perf_cpu perf_cpu_map__cpu(const struct perf_cpu_map *cpus, int idx);
> +/**
> + * perf_cpu_map__nr - for an empty map returns 1, as perf_cpu_map__cpu returns a
> + *                    cpu of -1 for an invalid index, this makes an empty map
> + *                    look like it contains the "any CPU"/dummy value. Otherwise
> + *                    the result is the number CPUs in the map plus one if the
> + *                    "any CPU"/dummy value is present.

Hmmm... I'm not sure whether to laugh or cry at that API.

Reviewed-by: James Clark <james.clark@arm.com>

> + */
>  LIBPERF_API int perf_cpu_map__nr(const struct perf_cpu_map *cpus);
>  /**
>   * perf_cpu_map__has_any_cpu_or_is_empty - is map either empty or has the "any CPU"/dummy value.