linux-kernel - Re: [PATCH v4 1/2] sched: Fix UCLAMP_FLAG

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Date:   Wed, 21 Jul 2021 12:07:04 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Quentin Perret <qperret@...gle.com>, mingo@...hat.com,
        peterz@...radead.org, vincent.guittot@...aro.org,
        qais.yousef@....com, rickyiu@...gle.com, wvw@...gle.com,
        patrick.bellasi@...bug.net, xuewen.yan94@...il.com
Cc:     linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v4 1/2] sched: Fix UCLAMP_FLAG_IDLE setting

On 19/07/2021 18:16, Quentin Perret wrote:
> The UCLAMP_FLAG_IDLE flag is set on a runqueue when dequeueing the last
> active task to maintain the last uclamp.max and prevent blocked util

s/active/runnable ?

> from suddenly becoming visible.
> 

[...]

IMHO, the main argument in v3 to do the clearing outside
uclamp_rq_inc_id() was a possible order change in `for_each_clamp_id()`.
So setting/clearing `rq->uclamp_flags` (UCLAMP_FLAG_IDLE) on UCLAMP_MAX
(currently the highest Uclamp constraint (UCLAMP_CNT-1)) could be
incorrect when UCLAMP_MIN and UCLAMP_MAX change place because the
same `rq->uclamp_flags` value is needed for both Uclamp constraint
values.

What about decoupling rq->uclamp_flags` handling from UCLAMP_MAX and
doing this for 'UCLAMP_CNT - 1', i.e. always on the highest Uclamp
constraint?

#define for_each_clamp_id(clamp_id) \
    for ((clamp_id) = 0; (clamp_id) < UCLAMP_CNT; (clamp_id)++)

In this case the code change can be as easy as in your original v3.

Setting UCLAMP_FLAG_IDLE in uclamp_idle_value():

  uclamp_rq_dec_id() -> uclamp_rq_max_value() -> *uclamp_idle_value()*

Resetting UCLAMP_FLAG_IDLE in uclamp_idle_reset():

  uclamp_rq_inc_id()                          -> *uclamp_idle_reset()*  

This would be more symmetrical then uclamp_idle_value() and
uclamp_rq_inc()/uclamp_rq_reinc_id().

--8<--

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0c22cd026440..600f68f3378c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1300,15 +1300,16 @@ static inline unsigned int
 uclamp_idle_value(struct rq *rq, enum uclamp_id clamp_id,
 		  unsigned int clamp_value)
 {
+	if (clamp_id == UCLAMP_CNT - 1)
+		rq->uclamp_flags |= UCLAMP_FLAG_IDLE;
+
 	/*
 	 * Avoid blocked utilization pushing up the frequency when we go
 	 * idle (which drops the max-clamp) by retaining the last known
 	 * max-clamp.
 	 */
-	if (clamp_id == UCLAMP_MAX) {
-		rq->uclamp_flags |= UCLAMP_FLAG_IDLE;
+	if (clamp_id == UCLAMP_MAX)
 		return clamp_value;
-	}
 
 	return uclamp_none(UCLAMP_MIN);
 }
@@ -1320,6 +1321,9 @@ static inline void uclamp_idle_reset(struct rq *rq, enum uclamp_id clamp_id,
 	if (!(rq->uclamp_flags & UCLAMP_FLAG_IDLE))
 		return;
 
+	if ((clamp_id == UCLAMP_CNT - 1) && (rq->uclamp_flags & UCLAMP_FLAG_IDLE))
+		rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
+
 	WRITE_ONCE(rq->uclamp[clamp_id].value, clamp_value);
 }
 
@@ -1595,10 +1599,6 @@ static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
 
 	for_each_clamp_id(clamp_id)
 		uclamp_rq_inc_id(rq, p, clamp_id);
-
-	/* Reset clamp idle holding when there is one RUNNABLE task */
-	if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
-		rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
 }