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

[Patrick Bellasi <patrick.bellasi@arm.com>]

On 06-Aug 17:39, 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.
> 
> 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.

[...]

> @@ -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)
>  		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;
> +	}
> +
>  	rq->uclamp.value[clamp_id] = max_value;
>  }
>  
> @@ -953,13 +967,26 @@ static inline void uclamp_cpu_get_id(struct task_struct *p,
>  	uc_grp = &rq->uclamp.group[clamp_id][0];
>  	uc_grp[group_id].tasks += 1;
>  
> +	/* Force clamp update on idle exit */
> +	uc_cpu = &rq->uclamp;
> +	clamp_value = p->uclamp[clamp_id].value;
> +	if (unlikely(uc_cpu->flags & UCLAMP_FLAG_IDLE)) {
> +		/*
> +		 * This function is called for both UCLAMP_MIN (before) and
> +		 * UCLAMP_MAX (after). Let's reset the flag only the second
> +		 * once we know that UCLAMP_MIN has been already updated.
> +		 */
> +		if (clamp_id == UCLAMP_MAX)
> +			uc_cpu->flags &= ~UCLAMP_FLAG_IDLE;
> +		uc_cpu->value[clamp_id] = clamp_value;
> +		return;
> +	}

Just notice that the code block above is not reached when we enqueue a task
without a valid clamp group, i.e. an un-clamped task, which is the
default for all tasks.

The fix should be as simple as moving this block at the beginning of
uclamp_cpu_update() so that we always un-conditionally release the
clamp holding as soon as we enqueue the first task after a CPU wakeup,
i.e. when the UCLAMP_FLAG_IDLE flag is set.

Will fix this on v4.

-- 
#include <best/regards.h>

Patrick Bellasi