perf jevents: fix breakage when do perf stat on system metric

When do perf stat on sys metric, perf tool output nothing now:

  $ perf stat -a -M imx95_ddr_read.all -I 1000
  $

This command runs on an arm64 machine and the Soc has one DDR hw pmu
except one armv8_cortex_a55 pmu. Their maps show as follows:

const struct pmu_events_map pmu_events_map[] = {
{
	.arch = "arm64",
	.cpuid = "0x00000000410fd050",
	.event_table = {
		.pmus = pmu_events__arm_cortex_a55,
		.num_pmus = ARRAY_SIZE(pmu_events__arm_cortex_a55)
	},
	.metric_table = {
		.pmus = NULL,
		.num_pmus = 0
	}
},

static const struct pmu_sys_events pmu_sys_event_tables[] = {
{
	.event_table = {
		.pmus = pmu_events__freescale_imx95_sys,
		.num_pmus = ARRAY_SIZE(pmu_events__freescale_imx95_sys)
	},
	.metric_table = {
		.pmus = pmu_metrics__freescale_imx95_sys,
		.num_pmus = ARRAY_SIZE(pmu_metrics__freescale_imx95_sys)
	},
	.name = "pmu_events__freescale_imx95_sys",
},

Currently, pmu_metrics_table__find() will return NULL when only do perf
stat on sys metric. Then parse_groups() will never be called to parse
sys metric_name, finally perf tool will exit directly. This should be a
common problem.

To fix the issue, this will keep the logic before commit f20c15d
("perf pmu-events: Remember the perf_events_map for a PMU") to return a
empty metric table rather than a NULL pointer.

This should be fine since the removed part just check if the table match
provided metric_name.  Without these code, the code in parse_groups()
will also check the validity of metrci_name too.

Fixes: f20c15d ("perf pmu-events: Remember the perf_events_map for a PMU")
Reviewed-by: James Clark <james.clark@linaro.org>
Signed-off-by: Xu Yang <xu.yang_2@nxp.com>
Tested-by: Xu Yang <xu.yang_2@nxp.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Cc: Benjamin Gray <bgray@linux.ibm.com>
Cc: Ben Zong-You Xie <ben717@andestech.com>
Cc: Bibo Mao <maobibo@loongson.cn>
Cc: Clément Le Goffic <clement.legoffic@foss.st.com>
Cc: Dima Kogan <dima@secretsauce.net>
Cc: Dr. David Alan Gilbert <linux@treblig.org>
Cc: Huacai Chen <chenhuacai@kernel.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Leo Yan <leo.yan@linux.dev>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mike Leach <mike.leach@linaro.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: Sandipan Das <sandipan.das@amd.com>
Cc: Will Deacon <will@kernel.org>
Cc: Yicong Yang <yangyicong@hisilicon.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-riscv@lists.infradead.org
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20241107162035.52206-2-irogers@google.com
Signed-off-by: Ian Rogers <irogers@google.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Author: Xu Yang

tools/perf/pmu-events/empty-pmu-events.c

@@ -585,17 +585,7 @@ const struct pmu_metrics_table *perf_pmu__find_metrics_table(struct perf_pmu *pm
         if (!map)
                 return NULL;
 
-        if (!pmu)
-                return &map->metric_table;
-
-        for (size_t i = 0; i < map->metric_table.num_pmus; i++) {
-                const struct pmu_table_entry *table_pmu = &map->metric_table.pmus[i];
-                const char *pmu_name = &big_c_string[table_pmu->pmu_name.offset];
-
-                if (pmu__name_match(pmu, pmu_name))
-                           return &map->metric_table;
-        }
-        return NULL;
+	return &map->metric_table;
 }
 
 const struct pmu_events_table *find_core_events_table(const char *arch, const char *cpuid)

tools/perf/pmu-events/jevents.py

@@ -1101,17 +1101,7 @@ def print_system_mapping_table() -> None:
         if (!map)
                 return NULL;
 
-        if (!pmu)
-                return &map->metric_table;
-
-        for (size_t i = 0; i < map->metric_table.num_pmus; i++) {
-                const struct pmu_table_entry *table_pmu = &map->metric_table.pmus[i];
-                const char *pmu_name = &big_c_string[table_pmu->pmu_name.offset];
-
-                if (pmu__name_match(pmu, pmu_name))
-                           return &map->metric_table;
-        }
-        return NULL;
+	return &map->metric_table;
 }
 
 const struct pmu_events_table *find_core_events_table(const char *arch, const char *cpuid)