sched, psi: Don't account irq time if sched_clock_irqtime is disabled

sched_clock_irqtime may be disabled due to the clock source. When disabled,
irq_time_read() won't change over time, so there is nothing to account. We
can save iterating the whole hierarchy on every tick and context switch.

Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20250103022409.2544-4-laoar.shao@gmail.com

kernel/sched/psi.c

@@ -998,7 +998,7 @@ void psi_account_irqtime(struct rq *rq, struct task_struct *curr, struct task_st
 	s64 delta;
 	u64 irq;
 
-	if (static_branch_likely(&psi_disabled))
+	if (static_branch_likely(&psi_disabled) || !irqtime_enabled())
 		return;
 
 	if (!curr->pid)
@@ -1240,6 +1240,11 @@ int psi_show(struct seq_file *m, struct psi_group *group, enum psi_res res)
 	if (static_branch_likely(&psi_disabled))
 		return -EOPNOTSUPP;
 
+#ifdef CONFIG_IRQ_TIME_ACCOUNTING
+	if (!irqtime_enabled() && res == PSI_IRQ)
+		return -EOPNOTSUPP;
+#endif
+
 	/* Update averages before reporting them */
 	mutex_lock(&group->avgs_lock);
 	now = sched_clock();