Re: [PATCH v3 07/14] sched/core: uclamp: enforce last task UCLAMP_MAX

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 08/06/2018 06:39 PM, Patrick Bellasi wrote:
> When a util_max clamped task sleeps, its clamp constraints are removed
> from the CPU. However, the blocked utilization on that CPU can still be
> higher than the max clamp value enforced while that task was running.
> This max clamp removal when a CPU is going to be idle could thus allow
> unwanted CPU frequency increases, right while the task is not running.

So 'rq->uclamp.flags == UCLAMP_FLAG_IDLE' means CPU is IDLE because
non-clamped tasks are tracked as well ((group_id = 0)).

Maybe this is worth mentioning here?

> This can happen, for example, where there is another (smaller) task
> running on a different CPU of the same frequency domain.
> In this case, when we aggregate the utilization of all the CPUs in a
> shared frequency domain, schedutil can still see the full non clamped
> blocked utilization of all the CPUs and thus eventually increase the
> frequency.
> 
> Let's fix this by using:
> 
>     uclamp_cpu_put_id(UCLAMP_MAX)
>        uclamp_cpu_update(last_clamp_value)
> 
> to detect when a CPU has no more RUNNABLE clamped tasks and to flag this
> condition. Thus, while a CPU is idle, we can still enforce the last used
> clamp value for it.
> 
> To the contrary, we do not track any UCLAMP_MIN since, while a CPU is
> idle, we don't want to enforce any minimum frequency
> Indeed, we rely just on blocked load decay to smoothly reduce the
> frequency.

[...]

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bc2beedec7bf..ff76b000bbe8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -906,7 +906,8 @@ uclamp_group_find(int clamp_id, unsigned int clamp_value)
>    * For the specified clamp index, this method computes the new CPU utilization
>    * clamp to use until the next change on the set of RUNNABLE tasks on that CPU.
>    */
> -static inline void uclamp_cpu_update(struct rq *rq, int clamp_id)
> +static inline void uclamp_cpu_update(struct rq *rq, int clamp_id,
> +				     unsigned int last_clamp_value)
>   {
>   	struct uclamp_group *uc_grp = &rq->uclamp.group[clamp_id][0];
>   	int max_value = UCLAMP_NOT_VALID;
> @@ -924,6 +925,19 @@ static inline void uclamp_cpu_update(struct rq *rq, int clamp_id)

The condition:

     if (!uclamp_group_active(uc_grp, group_id))
         continue;

in 'for (group_id = 0; group_id <= CONFIG_UCLAMP_GROUPS_COUNT; 
++group_id) {}' makes sure that 'max_value == UCLAMP_NOT_VALID' is true 
for the if condition (*):


>   		if (max_value >= SCHED_CAPACITY_SCALE)
>   			break;
>   	}
> +
> +	/*
> +	 * Just for the UCLAMP_MAX value, in case there are no RUNNABLE
> +	 * task, we keep the CPU clamped to the last task's clamp value.
> +	 * This avoids frequency spikes to MAX when one CPU, with an high
> +	 * blocked utilization, sleeps and another CPU, in the same frequency
> +	 * domain, do not see anymore the clamp on the first CPU.
> +	 */
> +	if (clamp_id == UCLAMP_MAX && max_value == UCLAMP_NOT_VALID) {
> +		rq->uclamp.flags |= UCLAMP_FLAG_IDLE;
> +		max_value = last_clamp_value;
> +	}
> +

(*): So the uc_grp[group_id].value stays last_clamp_value?

What do you do when the blocked utilization decays below this enforced 
last_clamp_value on that CPU?

I assume there are plenty of this kind of corner cases because we have 
blocked signals (including all tasks) and clamping (including runnable 
tasks).