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Message-ID: <03344e80-4ed2-41f1-9d2b-f7ed2e201eba@amd.com>
Date: Tue, 25 Mar 2025 22:24:21 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Xuewen Yan <xuewen.yan@...soc.com>, <dietmar.eggemann@....com>,
<mingo@...hat.com>, <peterz@...radead.org>, <juri.lelli@...hat.com>,
<vincent.guittot@...aro.org>
CC: <rostedt@...dmis.org>, <bsegall@...gle.com>, <mgorman@...e.de>,
<vschneid@...hat.com>, <hongyan.xia2@....com>, <qyousef@...alina.io>,
<ke.wang@...soc.com>, <di.shen@...soc.com>, <linux-kernel@...r.kernel.org>,
<xuewen.yan94@...il.com>
Subject: Re: [PATCH v2] sched/uclamp: Align uclamp and util_est and call
before freq update
Hello Xuewen,
On 3/25/2025 7:17 AM, Xuewen Yan wrote:
> 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,
> just like util_est.
> So, aline the uclamp and util_est and call before freq update.
>
> For sched-delayed tasks, the rq uclamp/util_est should only be updated
> when they are enqueued upon being awakened.
> So simply the logic of util_est's enqueue/dequeue check.
>
> Signed-off-by: Xuewen Yan <xuewen.yan@...soc.com>
> ---
> v2:
> - simply the util-est's en/dequeue check;
> ---
> Previous discussion:
> https://lore.kernel.org/all/CAB8ipk8pEvOtCm-d0o1rsekwxPWUHk9iBGtt9TLTWW-iWTQKiA@mail.gmail.com/
> https://lore.kernel.org/all/84441660bef0a5e67fd09dc3787178d0276dad31.1740664400.git.hongyan.xia2@arm.com/T/#u
> https://lore.kernel.org/all/CAB8ipk9LpbiUDnbcV6+59+Sa=Ai7tFzO===mpLD3obNdV4=J-A@mail.gmail.com/T/#u
> https://lore.kernel.org/all/aa8baf67-a8ec-4ad8-a6a8-afdcd7036771@arm.com/
> ---
> kernel/sched/core.c | 17 ++++++++++-------
> kernel/sched/fair.c | 4 ++--
> 2 files changed, 12 insertions(+), 9 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 042351c7afce..72fbe2031e54 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 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 +2068,14 @@ void enqueue_task(struct rq *rq, struct task_struct *p, int flags)
> if (!(flags & ENQUEUE_NOCLOCK))
> update_rq_clock(rq);
>
> - p->sched_class->enqueue_task(rq, p, flags);
> /*
> - * Must be after ->enqueue_task() because ENQUEUE_DELAYED can clear
> - * ->sched_delayed.
> + * Can be before ->enqueue_task() because uclamp considers the
> + * ENQUEUE_DELAYED task before its ->sched_delayed gets cleared
> + * in ->enqueue_task().
> */
> - uclamp_rq_inc(rq, p);
> + uclamp_rq_inc(rq, p, flags);
> +
> + p->sched_class->enqueue_task(rq, p, flags);
>
> psi_enqueue(p, flags);
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c798d2795243..c92fee07fb7b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6930,7 +6930,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> * Let's add the task's estimated utilization to the cfs_rq's
> * estimated utilization, before we update schedutil.
> */
> - if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & ENQUEUE_RESTORE))))
> + if (!p->se.sched_delayed || (flags & ENQUEUE_DELAYED))
> util_est_enqueue(&rq->cfs, p);
Wouldn't this do a util_est_{dequeue,enqueue}() for a save restore
operation too of a non-delayed task? Is that desired?
On a larger note ...
An enqueue of a delayed task will call requeue_delayed_entity() which
will only enqueue p->se on its cfs_rq and do an update_load_avg() for
that cfs_rq alone.
With cgroups enabled, this cfs_rq might not be the root cfs_rq and
cfs_rq_util_change() will not call cpufreq_update_util() leaving the
CPU running at the older frequency despite the updated uclamp
constraints.
If think cfs_rq_util_change() should be called for the root cfs_rq
when a task is delayed or when it is re-enqueued to re-evaluate
the uclamp constraints.
Please let me know if I missed something.
>
> if (flags & ENQUEUE_DELAYED) {
> @@ -7168,7 +7168,7 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags)
> */
> static bool dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
> {
> - if (!(p->se.sched_delayed && (task_on_rq_migrating(p) || (flags & DEQUEUE_SAVE))))
> + if (!p->se.sched_delayed)
> util_est_dequeue(&rq->cfs, p);
>
> util_est_update(&rq->cfs, p, flags & DEQUEUE_SLEEP);
--
Thanks and Regards,
Prateek
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