ANDROID: make per-cgroup PSI tracking configurable

With cgroup v2 hierarchy enabled PSI accounts stalls for each cgroup
separately and aggregates at each level of the hierarchy. That causes
additional overhead since psi_avgs_work would be called for each
cgroup in the hierarchy.
In Android we use PSI only at the system level, therefore this overhead
can be avoided.
Introduce CONFIG_PSI_PER_CGROUP_ACCT that controls per-cgroup PSI
tracking and is disabled by default.

Bug: 178872719
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I70a418aba76b46a27eb9e66080434aa870496384
(cherry picked from commit bd3983c8a8)
(cherry picked from commit 5e00dceecb6d26c2d7382045b488fd9d1ced1c11)
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

init/Kconfig

@@ -635,6 +635,21 @@ config PSI_DEFAULT_DISABLED
 
 	  Say N if unsure.
 
+config PSI_PER_CGROUP_ACCT
+	bool "Enable per-cgroup pressure stall information tracking"
+	default n
+	depends on PSI
+	depends on CGROUPS
+	help
+	  If set, pressure stall information will be tracked for each
+	  individual cgroup. Otherwise, pressure stall information will
+	  be tracked only at the system level under /proc/pressure/.
+
+	  This feature generates overhead that depends on the number of
+	  cgroups in the cgroup v2 hierarchy.
+
+	  Say N if unsure.
+
 endmenu # "CPU/Task time and stats accounting"
 
 config CPU_ISOLATION

kernel/sched/psi.c

@@ -743,7 +743,7 @@ static void psi_group_change(struct psi_group *group, int cpu,
 
 static struct psi_group *iterate_groups(struct task_struct *task, void **iter)
 {
-#ifdef CONFIG_CGROUPS
+#if defined CONFIG_CGROUPS && defined CONFIG_PSI_PER_CGROUP_ACCT
 	struct cgroup *cgroup = NULL;
 
 	if (!*iter)