perf pmu: Add CPU map for "cpu" PMUs

A typical "cpu" PMU has no "cpus" or "cpumask" file meaning the CPU
map is set to NULL, which also encodes an empty CPU map. Update
pmu_cpumask so that if the "cpu" PMU fails to load a CPU map, use a
default of all online PMUs.

Remove const from cpu_map__online for the sake of reference counting.

Reviewed-by: Kan Liang <[e-mail]>
Signed-off-by: Ian Rogers <[e-mail]>
Cc: Adrian Hunter <[e-mail]>
Cc: Alexander Shishkin <[e-mail]>
Cc: Ali Saidi <[e-mail]>
Cc: Athira Rajeev <[e-mail]>
Cc: Dmitrii Dolgov <[e-mail]>
Cc: Huacai Chen <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: James Clark <[e-mail]>
Cc: Jing Zhang <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: John Garry <[e-mail]>
Cc: Kajol Jain <[e-mail]>
Cc: Kang Minchul <[e-mail]>
Cc: Leo Yan <[e-mail]>
Cc: Madhavan Srinivasan <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Mike Leach <[e-mail]>
Cc: Ming Wang <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Ravi Bangoria <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: Sandipan Das <[e-mail]>
Cc: Sean Christopherson <[e-mail]>
Cc: Suzuki Poulouse <[e-mail]>
Cc: Thomas Richter <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Xing Zhengjun <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/util/cpumap.c

···
 	return ptr - buf;
 }
 
-const struct perf_cpu_map *cpu_map__online(void) /* thread unsafe */
+struct perf_cpu_map *cpu_map__online(void) /* thread unsafe */
 {
-	static const struct perf_cpu_map *online = NULL;
+	static struct perf_cpu_map *online;
 
 	if (!online)
 		online = perf_cpu_map__new(NULL); /* from /sys/devices/system/cpu/online */

tools/perf/util/cpumap.h

···
 size_t cpu_map__snprint(struct perf_cpu_map *map, char *buf, size_t size);
 size_t cpu_map__snprint_mask(struct perf_cpu_map *map, char *buf, size_t size);
 size_t cpu_map__fprintf(struct perf_cpu_map *map, FILE *fp);
-const struct perf_cpu_map *cpu_map__online(void); /* thread unsafe */
+struct perf_cpu_map *cpu_map__online(void); /* thread unsafe */
 
 int cpu__setup_cpunode_map(void);
 
···
 /**
  * cpu_map__is_dummy - Events associated with a pid, rather than a CPU, use a single dummy map with an entry of -1.
  */
-static inline bool cpu_map__is_dummy(struct perf_cpu_map *cpus)
+static inline bool cpu_map__is_dummy(const struct perf_cpu_map *cpus)
 {
 	return perf_cpu_map__nr(cpus) == 1 && perf_cpu_map__cpu(cpus, 0).cpu == -1;
 }

tools/perf/util/pmu.c

···
 			return cpus;
 	}
 
-	return NULL;
+	return !strcmp(name, "cpu") ? perf_cpu_map__get(cpu_map__online()) : NULL;
 }
 
 static bool pmu_is_uncore(int dirfd, const char *name)