cgroup: no need for cgroup_mutex for /proc/cgroups

On the real systems, the cgroups hierarchies are setup early and just
once by the node controller, so, other than number of cgroups, all
information in /proc/cgroups remain same for the system uptime. Let's
remove the cgroup_mutex usage on reading /proc/cgroups. There is a
chance of inconsistent number of cgroups for co-mounted cgroups while
printing the information from /proc/cgroups but that is not a big
issue. In addition /proc/cgroups is a v1 specific interface, so the
dependency on it should reduce over time.

The main motivation for removing the cgroup_mutex from /proc/cgroups is
to reduce the avenues of its contention. On our fleet, we have observed
buggy application hammering on /proc/cgroups and drastically slowing
down the node controller on the system which have many negative
consequences on other workloads running on the system.

Signed-off-by: Shakeel Butt <[email protected]>
Signed-off-by: Tejun Heo <[email protected]>

Author: shakeelb

kernel/cgroup/cgroup-v1.c

@@ -659,19 +659,16 @@ int proc_cgroupstats_show(struct seq_file *m, void *v)
 
 	seq_puts(m, "#subsys_name\thierarchy\tnum_cgroups\tenabled\n");
 	/*
-	 * ideally we don't want subsystems moving around while we do this.
-	 * cgroup_mutex is also necessary to guarantee an atomic snapshot of
-	 * subsys/hierarchy state.
+	 * Grab the subsystems state racily. No need to add avenue to
+	 * cgroup_mutex contention.
 	 */
-	mutex_lock(&cgroup_mutex);
 
 	for_each_subsys(ss, i)
 		seq_printf(m, "%s\t%d\t%d\t%d\n",
 			   ss->legacy_name, ss->root->hierarchy_id,
 			   atomic_read(&ss->root->nr_cgrps),
 			   cgroup_ssid_enabled(i));
 
-	mutex_unlock(&cgroup_mutex);
 	return 0;
 }