On Tue, May 13, 2025 at 2:13 PM Arnaldo Carvalho de Melo
[off-list ref] wrote:

On Fri, May 02, 2025 at 01:14:32PM +0530, Mukesh Kumar Chaurasiya wrote:

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On Fri, Apr 25, 2025 at 02:46:43PM -0300, Arnaldo Carvalho de Melo wrote:

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Maybe that max() call in perf_cpu_map__intersect() somehow makes the
compiler happy.

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And in perf_cpu_map__alloc() all calls seems to validate it.

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Like:

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+++ b/tools/lib/perf/cpumap.c

@@ -411,7 +411,7 @@ int perf_cpu_map__merge(struct perf_cpu_map **orig, struct perf_cpu_map *other)
        }

        tmp_len = __perf_cpu_map__nr(*orig) + __perf_cpu_map__nr(other);
-       tmp_cpus = malloc(tmp_len * sizeof(struct perf_cpu));
+       tmp_cpus = calloc(tmp_len, sizeof(struct perf_cpu));
        if (!tmp_cpus)
                return -ENOMEM;

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⬢ [acme@toolbx perf-tools-next]$

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And better, do the max size that the compiler is trying to help us
catch?

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Isn't it better to use perf_cpu_map__nr. That should fix this problem.

Maybe, have you tried it?

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One question I have, in perf_cpu_map__nr, the function is returning
1 in case *cpus is NULL. Is it ok to do that? wouldn't it cause problems?

Indeed this better be documented, as by just looking at:

int perf_cpu_map__nr(const struct perf_cpu_map *cpus)
{
        return cpus ? __perf_cpu_map__nr(cpus) : 1;
}

It really doesn't make much sense to say that a NULL map has one entry.

But the next functions are:

bool perf_cpu_map__has_any_cpu_or_is_empty(const struct perf_cpu_map *map)
{
        return map ? __perf_cpu_map__cpu(map, 0).cpu == -1 : true;
}

bool perf_cpu_map__is_any_cpu_or_is_empty(const struct perf_cpu_map *map)
{
        if (!map)
                return true;

        return __perf_cpu_map__nr(map) == 1 && __perf_cpu_map__cpu(map, 0).cpu == -1;
}

bool perf_cpu_map__is_empty(const struct perf_cpu_map *map)
{
        return map == NULL;
}

So it seems that a NULL cpu map means "any/all CPU) and a map with just
one entry would have as its content "-1" that would mean "any/all CPU".

Ian did work on trying to simplify/clarify this, so maybe he can chime
in :-)

So I've tried to improve the naming but not vary the implementation
greatly - initially I was in the code fixing reference count checking
issues. There is an important distinction between "all" meaning a
range of CPUs like 0-15 on a 16 core/hyperthread system, and "any"
meaning the special "-1" value. It is possible to have a perf_cpu_map
to both be "all" and "any", iterating an empty perf_cpu_map has
strangely also meant the "any" and so the code isn't specific but
relies on these odd properties.

Anyway, I'm not sure on the implication of this with
malloc/calloc/unsigned... It would seem reasonable to me for
__perf_cpu_map__nr to return an unsigned number and to propagate that
to fix the new GCC issue.

Thanks,
Ian

- Arnaldo