Re: [PATCH v2] sched/uclamp: Align uclamp and util_est and call before freq update

From: Xuewen Yan
Date: Tue Apr 15 2025 - 22:56:01 EST

On Wed, Apr 16, 2025 at 1:05 AM Vincent Guittot
<vincent.guittot@xxxxxxxxxx> wrote:
>
> On Tue, 25 Mar 2025 at 02:48, Xuewen Yan <xuewen.yan@xxxxxxxxxx> 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(),
>
> Strictly speaking, it doesn't happen during enqueue_task but when :
> - attach/detach tasks when migrating
> - update_load_avg decayed
> - io_wait
>
> This often happens during an enqueue but not always ...

Okay, I would make some adjustments to these descriptions.

>
> > so the rq's uclamp needs to be updated before the task is enqueued,
>
> this doesn't ensure that new rq's uclamp will be taken into account

Did I miss something?

As following stack:
enqueue_task_fair()
update_load_avg()
cfs_rq_util_change(cfs_rq, 0);
cpufreq_update_util()
sugov_update_shared()
sugov_next_freq_shared()
sugov_get_util()
effective_cpu_util()
*min = max(irq + cpu_bw_dl(rq), uclamp_rq_get(rq, UCLAMP_MIN));
*max = min(scale, uclamp_rq_get(rq, UCLAMP_MAX));

So, the rq's uclamp value should update before enqueue_task().
>
> > just like util_est.
>
> just like util_est
>
> > So, aline the uclamp and util_est and call before freq update.
>
> nit s/aline/align/ ?
align.
>
> >
> > 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@xxxxxxxxxx>
> > ---
> > v2:
> > - simply the util-est's en/dequeue check;
> > ---
> > Previous discussion:
> > https://lore.kernel.org/all/CAB8ipk8pEvOtCm-d0o1rsekwxPWUHk9iBGtt9TLTWW-iWTQKiA@xxxxxxxxxxxxxx/
> > https://lore.kernel.org/all/84441660bef0a5e67fd09dc3787178d0276dad31.1740664400.git.hongyan.xia2@xxxxxxx/T/#u
> > https://lore.kernel.org/all/CAB8ipk9LpbiUDnbcV6+59+Sa=Ai7tFzO===mpLD3obNdV4=J-A@xxxxxxxxxxxxxx/T/#u
> > https://lore.kernel.org/all/aa8baf67-a8ec-4ad8-a6a8-afdcd7036771@xxxxxxx/
> > ---
> > 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))
>
> commit message doesn't explain why you change util_est condition

Because, the sched_delayed flag is set when dequeue_entity, and clear
after the condition,
so for migrating/prio_change, we could just check the sched_delayed.
And for the wakeup, because the the sched_delayed flag is cleared after this,
so use the ENQUEUE_DELAYED flag to ensure the util_est could enqueue.

>
> > util_est_enqueue(&rq->cfs, p);
> >
> > 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)
>
> same here, you should explain in commit message why it's okay to do so

Same as above, the sched_delayed flag is set when dequeue_entity, so
this place,
the sched_delayed was not set when sleeping, If the flag is set, it
indicates that it
was migrating or prio changing.

By the way, I will kindly update these reasons in the commit message.

Thanks!

BR
---
xuewen