perf tools: Add read_cgroup_id() function

The read_cgroup_id() is to read a cgroup id from a file handle using
name_to_handle_at(2) for the given cgroup.  It'll be used by bperf
cgroup stat later.

Committer notes:

  -int read_cgroup_id(struct cgroup *cgrp)
  +static inline int read_cgroup_id(struct cgroup *cgrp __maybe_unused)

To fix the build when HAVE_FILE_HANDLE is not defined.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20210625071826.608504-2-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/cgroup.c

@@ -45,6 +45,31 @@ static int open_cgroup(const char *name)
 	return fd;
 }
 
+#ifdef HAVE_FILE_HANDLE
+int read_cgroup_id(struct cgroup *cgrp)
+{
+	char path[PATH_MAX + 1];
+	char mnt[PATH_MAX + 1];
+	struct {
+		struct file_handle fh;
+		uint64_t cgroup_id;
+	} handle;
+	int mount_id;
+
+	if (cgroupfs_find_mountpoint(mnt, PATH_MAX + 1, "perf_event"))
+		return -1;
+
+	scnprintf(path, PATH_MAX, "%s/%s", mnt, cgrp->name);
+
+	handle.fh.handle_bytes = sizeof(handle.cgroup_id);
+	if (name_to_handle_at(AT_FDCWD, path, &handle.fh, &mount_id, 0) < 0)
+		return -1;
+
+	cgrp->id = handle.cgroup_id;
+	return 0;
+}
+#endif  /* HAVE_FILE_HANDLE */
+
 static struct cgroup *evlist__find_cgroup(struct evlist *evlist, const char *str)
 {
 	struct evsel *counter;

tools/perf/util/cgroup.h

@@ -2,6 +2,7 @@
 #ifndef __CGROUP_H__
 #define __CGROUP_H__
 
+#include <linux/compiler.h>
 #include <linux/refcount.h>
 #include <linux/rbtree.h>
 #include "util/env.h"
@@ -38,4 +39,13 @@ struct cgroup *cgroup__find(struct perf_env *env, uint64_t id);
 
 void perf_env__purge_cgroups(struct perf_env *env);
 
+#ifdef HAVE_FILE_HANDLE
+int read_cgroup_id(struct cgroup *cgrp);
+#else
+static inline int read_cgroup_id(struct cgroup *cgrp __maybe_unused)
+{
+	return -1;
+}
+#endif  /* HAVE_FILE_HANDLE */
+
 #endif /* __CGROUP_H__ */