sched/debug: Provide slice length for fair tasks

[ Upstream commit 9065ce69754dece78606c8bbb3821449272e56bf ]

Since commit:

  857b158dc5 ("sched/eevdf: Use sched_attr::sched_runtime to set request/slice suggestion")

... we have the userspace per-task tunable slice length, which is
a key parameter that is otherwise difficult to obtain, so provide
it in /proc/$PID/sched.

[ mingo: Clarified the changelog. ]

Signed-off-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/453349b1-1637-42f5-a7b2-2385392b5956@arm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

kernel/sched/debug.c

@@ -1264,6 +1264,8 @@ void proc_sched_show_task(struct task_struct *p, struct pid_namespace *ns,
 	if (task_has_dl_policy(p)) {
 		P(dl.runtime);
 		P(dl.deadline);
+	} else if (fair_policy(p->policy)) {
+		P(se.slice);
 	}
 #ifdef CONFIG_SCHED_CLASS_EXT
 	__PS("ext.enabled", task_on_scx(p));