perf cgroup: Maintain cgroup hierarchy

Each cgroup is kept in the perf_env's cgroup_tree sorted by the cgroup
id.  Hist entries have cgroup id can compare it directly and later it
can be used to find a group name using this tree.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200325124536.2800725-5-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/env.h b/tools/perf/util/env.h
index 11d05ae..7632075 100644
--- a/tools/perf/util/env.h
+++ b/tools/perf/util/env.h
@@ -88,6 +88,12 @@ struct perf_env {
 		u32			btfs_cnt;
 	} bpf_progs;
 
+	/* same reason as above (for perf-top) */
+	struct {
+		struct rw_semaphore	lock;
+		struct rb_root		tree;
+	} cgroups;
+
 	/* For fast cpu to numa node lookup via perf_env__numa_node */
 	int			*numa_map;
 	int			 nr_numa_map;