sched/psi: Zero the memory of struct psi_group

After commit 5f69a657 ("psi: dont alloc memory for psi by default"),
the memory used by struct psi_group is no longer allocated and zeroed
in cgroup_create().

Since the memory of struct psi_group is not zeroed, the data in this
memory is random, which will lead to inaccurate psi statistics when
creating a new cgroup.

So we use kzlloc() to allocate and zero the struct psi_group and
remove the redundant zeroing in group_init().

Steps to reproduce:
1. Use cgroup v2 and enable CONFIG_PSI
2. Create a new cgroup, and query psi statistics
mkdir /sys/fs/cgroup/test
cat /sys/fs/cgroup/test/cpu.pressure
some avg10=0.00 avg60=0.00 avg300=47927752200.00 total=12884901
full avg10=561815124.00 avg60=125835394188.00 avg300=1077090462000.00 total=10273561772

cat /sys/fs/cgroup/test/io.pressure
some avg10=1040093132823.95 avg60=1203770351379.21 avg300=3862252669559.46 total=4294967296
full avg10=921884564601.39 avg60=0.00 avg300=1984507298.35 total=442381631

cat /sys/fs/cgroup/test/memory.pressure
some avg10=232476085778.11 avg60=0.00 avg300=0.00 total=0
full avg10=0.00 avg60=0.00 avg300=2585658472280.57 total=12884901

Fixes: commit 5f69a657 ("psi: dont alloc memory for psi by default")
Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
Reviewed-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/sched/psi.c

@@ -190,12 +190,8 @@ static void group_init(struct psi_group *group)
 	/* Init trigger-related members */
 	mutex_init(&group->trigger_lock);
 	INIT_LIST_HEAD(&group->triggers);
-	memset(group->nr_triggers, 0, sizeof(group->nr_triggers));
-	group->poll_states = 0;
 	group->poll_min_period = U32_MAX;
-	memset(group->polling_total, 0, sizeof(group->polling_total));
 	group->polling_next_update = ULLONG_MAX;
-	group->polling_until = 0;
 	init_waitqueue_head(&group->poll_wait);
 	timer_setup(&group->poll_timer, poll_timer_fn, 0);
 	rcu_assign_pointer(group->poll_task, NULL);
@@ -957,7 +953,7 @@ int psi_cgroup_alloc(struct cgroup *cgroup)
 	if (static_branch_likely(&psi_disabled))
 		return 0;
 
-	cgroup->psi = kmalloc(sizeof(struct psi_group), GFP_KERNEL);
+	cgroup->psi = kzalloc(sizeof(struct psi_group), GFP_KERNEL);
 	if (!cgroup->psi)
 		return -ENOMEM;