linux-kernel - Re: [PATCH v2 0/8] uclamp sum aggregation

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Message-ID: <13ea9f62-e373-4248-997c-47c15e024c02@arm.com>
Date: Mon, 10 Mar 2025 12:34:38 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Xuewen Yan <xuewen.yan94@...il.com>, Hongyan Xia <hongyan.xia2@....com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Morten Rasmussen <morten.rasmussen@....com>,
 Lukasz Luba <lukasz.luba@....com>,
 Christian Loehle <christian.loehle@....com>,
 Pierre Gondois <pierre.gondois@....com>, linux-kernel@...r.kernel.org,
 Xuewen Yan <xuewen.yan@...soc.com>
Subject: Re: [PATCH v2 0/8] uclamp sum aggregation

On 06/03/2025 12:38, Xuewen Yan wrote:
> On Thu, Mar 6, 2025 at 7:32 PM Hongyan Xia <hongyan.xia2@....com> wrote:
>>
>> Hi Xuewen,
>>
>> On 06/03/2025 11:12, Xuewen Yan wrote:
>>> Hi Hongyan,
>>>
>>> On Tue, Mar 4, 2025 at 10:26 PM Hongyan Xia <hongyan.xia2@....com> wrote:

[...]

>>> Subject: [PATCH] sched/uclamp: Update the rq's uclamp before enqueue task
>>>
>>> When task's uclamp is set, we hope that the CPU frequency
>>> can increase as quickly as possible when the task is enqueued.
>>> Because the cpu frequency updating happens during the enqueue_task(),
>>> so the rq's uclamp needs to be updated before the task is enqueued.
>>> For sched-delayed tasks, the rq uclamp should only be updated
>>> when they are enqueued upon being awakened.
>>>
>>> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
>>> ---
>>>   kernel/sched/core.c | 14 ++++++--------
>>>   1 file changed, 6 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>>> index 67189907214d..b07e78910221 100644
>>> --- a/kernel/sched/core.c
>>> +++ b/kernel/sched/core.c
>>> @@ -1747,7 +1747,7 @@ static inline void uclamp_rq_dec_id(struct rq
>>> *rq, struct task_struct *p,
>>>          }
>>>   }
>>>
>>> -static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p)
>>> +static inline void uclamp_rq_inc(struct rq *rq, struct task_struct
>>> *p, int flags)
>>>   {
>>>          enum uclamp_id clamp_id;
>>>
>>> @@ -1763,7 +1763,8 @@ static inline void uclamp_rq_inc(struct rq *rq,
>>> struct task_struct *p)
>>>          if (unlikely(!p->sched_class->uclamp_enabled))
>>>                  return;
>>>
>>> -       if (p->se.sched_delayed)
>>> +       /* Only inc the delayed task which is being woken up. */
>>> +       if (p->se.sched_delayed && !(flags & ENQUEUE_DELAYED))
>>>                  return;
>>>
>>>          for_each_clamp_id(clamp_id)
>>> @@ -2031,7 +2032,7 @@ static void __init init_uclamp(void)
>>>   }
>>>
>>>   #else /* !CONFIG_UCLAMP_TASK */
>>> -static inline void uclamp_rq_inc(struct rq *rq, struct task_struct *p) { }
>>> +static inline void uclamp_rq_inc(struct rq *rq, struct task_struct
>>> *p, int flags) { }
>>>   static inline void uclamp_rq_dec(struct rq *rq, struct task_struct *p) { }
>>>   static inline void uclamp_fork(struct task_struct *p) { }
>>>   static inline void uclamp_post_fork(struct task_struct *p) { }
>>> @@ -2067,12 +2068,9 @@ void enqueue_task(struct rq *rq, struct
>>> task_struct *p, int flags)
>>>          if (!(flags & ENQUEUE_NOCLOCK))
>>>                  update_rq_clock(rq);
>>>
>>> +       uclamp_rq_inc(rq, p, flags);
>>> +
>>>          p->sched_class->enqueue_task(rq, p, flags);
>>> -       /*
>>> -        * Must be after ->enqueue_task() because ENQUEUE_DELAYED can clear
>>> -        * ->sched_delayed.
>>> -        */
>>> -       uclamp_rq_inc(rq, p);
>>>
>>>          psi_enqueue(p, flags);

Like I mentioned already in the original thread:

https://lkml.kernel.org/r/65365ec7-6a16-4e66-8005-e78788cbedfa@arm.com

I would prefer that uclamp stays in core.c. ENQUEUE_DELAYED among all
the other flags is already used there (ttwu_runnable()).

task_struct contains sched_{,rt_,dl_}entity}. We just have to be
careful when switching policies.

--

Could you also incorporate the changes in {en,de}queue_task_fair()
((task_on_rq_migrating(p) || (flags & {RESTORE,DEQUEUE}_SAVE))) vs.
(!p->se.sched_delayed || (flags & ENQUEUE_DELAYED)) and
(!p->se.sched_delayed) so the uclamp-util_est relation is easier to spot?

[...]