sched_ext: Use sched_clock_cpu() instead of rq_clock_task() in touch_core_sched()

Since 3cf78c5d ("sched_ext: Unpin and repin rq lock from
balance_scx()"), sched_ext's balance path terminates rq_pin in the outermost
function. This is simpler and in line with what other balance functions are
doing but it loses control over rq->clock_update_flags which makes
assert_clock_udpated() trigger if other CPUs pins the rq lock.

The only place this matters is touch_core_sched() which uses the timestamp
to order tasks from sibling rq's. Switch to sched_clock_cpu(). Later, it may
be better to use per-core dispatch sequence number.

v2: Use sched_clock_cpu() instead of ktime_get_ns() per David.
Signed-off-by: Tejun Heo <tj@kernel.org>
Fixes: 3cf78c5d ("sched_ext: Unpin and repin rq lock from balance_scx()")
Acked-by: David Vernet <void@manifault.com>
Cc: Peter Zijlstra <peterz@infradead.org>

kernel/sched/ext.c

@@ -1453,13 +1453,18 @@ static void schedule_deferred(struct rq *rq)
  */
 static void touch_core_sched(struct rq *rq, struct task_struct *p)
 {
+	lockdep_assert_rq_held(rq);
+
 #ifdef CONFIG_SCHED_CORE
 	/*
 	 * It's okay to update the timestamp spuriously. Use
 	 * sched_core_disabled() which is cheaper than enabled().
+	 *
+	 * As this is used to determine ordering between tasks of sibling CPUs,
+	 * it may be better to use per-core dispatch sequence instead.
 	 */
 	if (!sched_core_disabled())
-		p->scx.core_sched_at = rq_clock_task(rq);
+		p->scx.core_sched_at = sched_clock_cpu(cpu_of(rq));
 #endif
 }
 
@@ -1476,7 +1481,6 @@ static void touch_core_sched(struct rq *rq, struct task_struct *p)
 static void touch_core_sched_dispatch(struct rq *rq, struct task_struct *p)
 {
 	lockdep_assert_rq_held(rq);
-	assert_clock_updated(rq);
 
 #ifdef CONFIG_SCHED_CORE
 	if (SCX_HAS_OP(core_sched_before))