[PATCH v2] sched/uclamp: Avoid getting unreasonable ucalmp value when rq is idle

[Xuewen Yan]

From: Xuewen Yan <xuewen.yan@xxxxxxxxxx>

Now in uclamp_rq_util_with(), when the task != NULL, the uclamp_max as following:
uc_rq_max = rq->uclamp[UCLAMP_MAX].value;
uc_eff_max = uclamp_eff_value(p, UCLAMP_MAX);
uclamp_max = max{uc_rq_max, uc_eff_max};

Consider the following scenario:
(1)the rq is idle, the uc_rq_max is last runnable task's UCLAMP_MAX;
(2)the p's uc_eff_max < uc_rq_max.

As a result, the uclamp_max = uc_rq_max instead of uc_eff_max, it is unreasonable.

The scenario often happens in find_energy_efficient_cpu(), when the task has smaller UCLAMP_MAX.

When rq has UCLAMP_FLAG_IDLE flag, enqueuing the task will lift UCLAMP_FLAG_IDLE
and set the rq clamp as the task's via uclamp_idle_reset(). It doesn't need
to read the rq clamp. And it can also avoid the problems described above.

Fixes: 9d20ad7dfc9a ("sched/uclamp: Add uclamp_util_with()")

Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>

---
change v2:
	*add Fixes(Valentin Schneider);
	*ignore all rq clamp when idle (Valentin Schneider)
---
 kernel/sched/sched.h | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c80d42e9589b..14a41a243f7b 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2818,20 +2818,27 @@ static __always_inline
 unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
 				  struct task_struct *p)
 {
-	unsigned long min_util;
-	unsigned long max_util;
+	unsigned long min_util = 0;
+	unsigned long max_util = 0;
 
 	if (!static_branch_likely(&sched_uclamp_used))
 		return util;
 
-	min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
-	max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
-
 	if (p) {
-		min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
-		max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
+		min_util = uclamp_eff_value(p, UCLAMP_MIN);
+		max_util = uclamp_eff_value(p, UCLAMP_MAX);
+
+		/*
+		 * Ignore last runnable task's max clamp, as this task will
+		 * reset it. Similarly, no need to read the rq's min clamp.
+		 */
+		if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
+			goto out;
 	}
 
+	min_util = max_t(unsigned long, min_util, READ_ONCE(rq->uclamp[UCLAMP_MIN].value));
+	max_util = max_t(unsigned long, max_util, READ_ONCE(rq->uclamp[UCLAMP_MAX].value));
+out:
 	/*
 	 * Since CPU's {min,max}_util clamps are MAX aggregated considering
 	 * RUNNABLE tasks with _different_ clamps, we can end up with an
-- 
2.25.1