linux-kernel - Re: [PATCH v3 1/3] sched: Fix UCLAMP_FLAG

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Date:   Wed, 30 Jun 2021 15:45:14 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     mingo@...hat.com, peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com, rickyiu@...gle.com, wvw@...gle.com,
        patrick.bellasi@...bug.net, xuewen.yan94@...il.com,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v3 1/3] sched: Fix UCLAMP_FLAG_IDLE setting

Hi Qais,

On Wednesday 30 Jun 2021 at 15:58:48 (+0100), Qais Yousef wrote:
> I just realized this needs
> 
> 	if (clamp_id == UCLAMP_MAX)
> 		rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
> 
> The code is only set for UCLAMP_MAX, so should be cleared for UCLAMP_MAX too.
> 
> Though there's ugly overload here:
> 
> 	if (!(rq->uclamp_flags & UCLAMP_FLAG_IDLE))
> 		return;
> 
> This check would fail prematurely if UCLAMP_MAX was reset before UCLAMP_MIN.
> The code before your change would reset both then do the clear. But now when we
> do it from here, we need to be more careful about that.

Right, although this should all work fine as-is, I agree that relying on
the calling order is a bit dodgy and might cause issues in the long run.

What do you think of this instead?

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index b094da4c5fea..c0b999a8062a 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -980,7 +980,6 @@ static inline void uclamp_idle_reset(struct rq *rq, enum uclamp_id clamp_id,
        if (!(rq->uclamp_flags & UCLAMP_FLAG_IDLE))
                return;

-       rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
        WRITE_ONCE(rq->uclamp[clamp_id].value, clamp_value);
 }

@@ -1253,6 +1252,10 @@ 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;
 }

 static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p)
@@ -1300,6 +1303,13 @@ uclamp_update_active(struct task_struct *p, enum uclamp_id clamp_id)
        if (p->uclamp[clamp_id].active) {
                uclamp_rq_dec_id(rq, p, clamp_id);
                uclamp_rq_inc_id(rq, p, clamp_id);
+
+               /*
+                * Make sure to clear the idle flag if we've transiently reached
+                * 0 uclamp active tasks on the rq.
+                */
+               if (rq->uclamp_flags & UCLAMP_FLAG_IDLE)
+                       rq->uclamp_flags &= ~UCLAMP_FLAG_IDLE;
        }

        task_rq_unlock(rq, p, &rf);