perf tools: Ignore deleted cgroups

On large systems, cgroups can be created and deleted often.  That means
there's a race between perf tools and cgroups when it gets the cgroup
name and opens the cgroup.

I got a report that 'perf stat' with many cgroups failed quite often due
to the missing cgroups on such a large machine.

I think we can ignore such cgroups when expanding events and use id 0 if
it fails to read the cgroup id.  IIUC 0 is not a vaild cgroup id so it
won't update event counts for the failed cgroups.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20240509182235.2319599-1-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/bpf_counter_cgroup.c

@@ -136,9 +136,8 @@ static int bperf_load_program(struct evlist *evlist)
 		cgrp = evsel->cgrp;
 
 		if (read_cgroup_id(cgrp) < 0) {
-			pr_err("Failed to get cgroup id\n");
-			err = -1;
-			goto out;
+			pr_debug("Failed to get cgroup id for %s\n", cgrp->name);
+			cgrp->id = 0;
 		}
 
 		map_fd = bpf_map__fd(skel->maps.cgrp_idx);

tools/perf/util/cgroup.c

@@ -465,9 +465,11 @@ int evlist__expand_cgroup(struct evlist *evlist, const char *str,
 		name = cn->name + prefix_len;
 		if (name[0] == '/' && name[1])
 			name++;
+
+		/* the cgroup can go away in the meantime */
 		cgrp = cgroup__new(name, open_cgroup);
 		if (cgrp == NULL)
-			goto out_err;
+			continue;
 
 		leader = NULL;
 		evlist__for_each_entry(orig_list, pos) {