sched/psi: Fix psi_seq initialization

[ Upstream commit 99b773d720aeea1ef2170dce5fcfa80649e26b78 ]

With the seqcount moved out of the group into a global psi_seq,
re-initializing the seqcount on group creation is causing seqcount
corruption.

Fixes: 570c8efd5eb7 ("sched/psi: Optimize psi_group_change() cpu_clock() usage")
Reported-by: Chris Mason <[email protected]>
Suggested-by: Beata Michalska <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>
Signed-off-by: Sasha Levin <[email protected]>
(cherry picked from commit 9184a2bb522c7dee7a823cb61d8efd89e443968f)

Author: Peter Zijlstra

kernel/sched/psi.c

@@ -172,7 +172,7 @@ struct psi_group psi_system = {
 	.pcpu = &system_group_pcpu,
 };
 
-static DEFINE_PER_CPU(seqcount_t, psi_seq);
+static DEFINE_PER_CPU(seqcount_t, psi_seq) = SEQCNT_ZERO(psi_seq);
 
 static inline void psi_write_begin(int cpu)
 {
@@ -200,11 +200,7 @@ static void poll_timer_fn(struct timer_list *t);
 
 static void group_init(struct psi_group *group)
 {
-	int cpu;
-
 	group->enabled = true;
-	for_each_possible_cpu(cpu)
-		seqcount_init(per_cpu_ptr(&psi_seq, cpu));
 	group->avg_last_update = sched_clock();
 	group->avg_next_update = group->avg_last_update + psi_period;
 	mutex_init(&group->avgs_lock);