[PATCH REPOST 2/2] cgroup, perf_event: make perf_event controller work on cgroup2 hierarchy

[Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>]

perf_event is a utility controller whose primary role is identifying
cgroup membership to filter perf events; however, because it also
tracks some per-css state, it can't be replaced by pure cgroup
membership test.  Mark the controller as implicitly enabled on the
default hierarchy so that perf events can always be filtered based on
cgroup v2 path as long as the controller is not mounted on a legacy
hierarchy.

"perf record" is updated accordingly so that it searches for both v1
and v2 hierarchies.  A v1 hierarchy is used if perf_event is mounted
on it; otherwise, it uses the v2 hierarchy.

v2: Doc updated to reflect more flexible rebinding behavior.

Signed-off-by: Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
Acked-by: Peter Zijlstra <a.p.zijlstra-/NLkJaSkS4VmR6Xm/wNWPw@public.gmane.org>
Cc: Ingo Molnar <mingo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Cc: Arnaldo Carvalho de Melo <acme-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
---
Hello,

This was posted months ago and acked by Peter.  I thought it was
applied but apparently wasn't.  Peter asked Arnaldo whether the
userspace part looked which didn't get replied and that probably was
how it slipped through the crack.

Peter, Arnanldo, how should we proceed?  Should I route this through
the cgroup tree?

Thanks.

 Documentation/cgroup-v2.txt |   12 ++++++++++++
 kernel/events/core.c        |    6 ++++++
 tools/perf/util/cgroup.c    |   26 +++++++++++++++++++-------
 3 files changed, 37 insertions(+), 7 deletions(-)

--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -49,6 +49,8 @@ CONTENTS
     5-3-2. Writeback
   5-4. PID
     5-4-1. PID Interface Files
+  5-5. Misc
+    5-5-1. perf_event
 6. Namespace
   6-1. Basics
   6-2. The Root and Views
@@ -1160,6 +1162,16 @@ through fork() or clone(). These will re
 of a new process would cause a cgroup policy to be violated.
 
 
+5-5. Misc
+
+5-5-1. perf_event
+
+perf_event controller, if not mounted on a legacy hierarchy, is
+automatically enabled on the v2 hierarchy so that perf events can
+always be filtered by cgroup v2 path.  The controller can still be
+moved to a legacy hierarchy after v2 hierarchy is populated.
+
+
 6. Namespace
 
 6-1. Basics
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -10792,5 +10792,11 @@ struct cgroup_subsys perf_event_cgrp_sub
 	.css_alloc	= perf_cgroup_css_alloc,
 	.css_free	= perf_cgroup_css_free,
 	.attach		= perf_cgroup_attach,
+	/*
+	 * Implicitly enable on dfl hierarchy so that perf events can
+	 * always be filtered by cgroup2 path as long as perf_event
+	 * controller is not mounted on a legacy hierarchy.
+	 */
+	.implicit_on_dfl = true,
 };
 #endif /* CONFIG_CGROUP_PERF */
--- a/tools/perf/util/cgroup.c
+++ b/tools/perf/util/cgroup.c
@@ -12,8 +12,8 @@ cgroupfs_find_mountpoint(char *buf, size
 {
 	FILE *fp;
 	char mountpoint[PATH_MAX + 1], tokens[PATH_MAX + 1], type[PATH_MAX + 1];
+	char path_v1[PATH_MAX + 1], path_v2[PATH_MAX + 2], *path;
 	char *token, *saved_ptr = NULL;
-	int found = 0;
 
 	fp = fopen("/proc/mounts", "r");
 	if (!fp)
@@ -24,31 +24,43 @@ cgroupfs_find_mountpoint(char *buf, size
 	 * and inspect every cgroupfs mount point to find one that has
 	 * perf_event subsystem
 	 */
+	path_v1[0] = '\0';
+	path_v2[0] = '\0';
+
 	while (fscanf(fp, "%*s %"STR(PATH_MAX)"s %"STR(PATH_MAX)"s %"
 				STR(PATH_MAX)"s %*d %*d\n",
 				mountpoint, type, tokens) == 3) {
 
-		if (!strcmp(type, "cgroup")) {
+		if (!path_v1[0] && !strcmp(type, "cgroup")) {
 
 			token = strtok_r(tokens, ",", &saved_ptr);
 
 			while (token != NULL) {
 				if (!strcmp(token, "perf_event")) {
-					found = 1;
+					strcpy(path_v1, mountpoint);
 					break;
 				}
 				token = strtok_r(NULL, ",", &saved_ptr);
 			}
 		}
-		if (found)
+
+		if (!path_v2[0] && !strcmp(type, "cgroup2"))
+			strcpy(path_v2, mountpoint);
+
+		if (path_v1[0] && path_v2[0])
 			break;
 	}
 	fclose(fp);
-	if (!found)
+
+	if (path_v1[0])
+		path = path_v1;
+	else if (path_v2[0])
+		path = path_v2;
+	else
 		return -1;
 
-	if (strlen(mountpoint) < maxlen) {
-		strcpy(buf, mountpoint);
+	if (strlen(path) < maxlen) {
+		strcpy(buf, path);
 		return 0;
 	}
 	return -1;