tools api fs: Cache cgroupfs mount point

Currently it parses the /proc file everytime it opens a file in the
cgroupfs.  Save the last result to avoid it (assuming it won't be
changed between the accesses).
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20201216090556.813996-3-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/api/fs/cgroup.c

@@ -8,6 +8,14 @@
 #include <string.h>
 #include "fs.h"
 
+struct cgroupfs_cache_entry {
+	char	subsys[32];
+	char	mountpoint[PATH_MAX];
+};
+
+/* just cache last used one */
+static struct cgroupfs_cache_entry cached;
+
 int cgroupfs_find_mountpoint(char *buf, size_t maxlen, const char *subsys)
 {
 	FILE *fp;
@@ -16,6 +24,14 @@ int cgroupfs_find_mountpoint(char *buf, size_t maxlen, const char *subsys)
 	char *p, *path;
 	char mountpoint[PATH_MAX];
 
+	if (!strcmp(cached.subsys, subsys)) {
+		if (strlen(cached.mountpoint) < maxlen) {
+			strcpy(buf, cached.mountpoint);
+			return 0;
+		}
+		return -1;
+	}
+
 	fp = fopen("/proc/mounts", "r");
 	if (!fp)
 		return -1;
@@ -75,6 +91,9 @@ int cgroupfs_find_mountpoint(char *buf, size_t maxlen, const char *subsys)
 	free(line);
 	fclose(fp);
 
+	strncpy(cached.subsys, subsys, sizeof(cached.subsys) - 1);
+	strcpy(cached.mountpoint, mountpoint);
+
 	if (mountpoint[0] && strlen(mountpoint) < maxlen) {
 		strcpy(buf, mountpoint);
 		return 0;