Re: [PATCH 1/2] sched/uclamp: Add a new sysctl to control RT default boost value

[Peter Zijlstra]

On Mon, May 11, 2020 at 04:40:52PM +0100, Qais Yousef wrote:
> +/*
> + * By default RT tasks run at the maximum performance point/capacity of the
> + * system. Uclamp enforces this by always setting UCLAMP_MIN of RT tasks to
> + * SCHED_CAPACITY_SCALE.
> + *
> + * This knob allows admins to change the default behavior when uclamp is being
> + * used. In battery powered devices, particularly, running at the maximum
> + * capacity and frequency will increase energy consumption and shorten the
> + * battery life.
> + *
> + * This knob only affects RT tasks that their uclamp_se->user_defined == false.
> + *
> + * This knob will not override the system default sched_util_clamp_min defined
> + * above.
> + *
> + * Any modification is applied lazily on the next attempt to calculate the
> + * effective value of the task.
> + */
> +unsigned int sysctl_sched_uclamp_util_min_rt_default = SCHED_CAPACITY_SCALE;
> +
>  /* All clamps are required to be less or equal than these values */
>  static struct uclamp_se uclamp_default[UCLAMP_CNT];
>  
> @@ -872,6 +892,28 @@ unsigned int uclamp_rq_max_value(struct rq *rq, enum uclamp_id clamp_id,
>  	return uclamp_idle_value(rq, clamp_id, clamp_value);
>  }
>  
> +static inline void uclamp_sync_util_min_rt_default(struct task_struct *p,
> +						   enum uclamp_id clamp_id)
> +{
> +	unsigned int default_util_min = sysctl_sched_uclamp_util_min_rt_default;
> +	struct uclamp_se *uc_se;
> +
> +	/* Only sync for UCLAMP_MIN and RT tasks */
> +	if (clamp_id != UCLAMP_MIN || !rt_task(p))
> +		return;
> +
> +	uc_se = &p->uclamp_req[UCLAMP_MIN];
> +
> +	/*
> +	 * Only sync if user didn't override the default request and the sysctl
> +	 * knob has changed.
> +	 */
> +	if (uc_se->user_defined || uc_se->value == default_util_min)
> +		return;
> +
> +	uclamp_se_set(uc_se, default_util_min, false);
> +}

So afaict this is directly added to the enqueue/dequeue path, and we've
recently already had complaints that uclamp is too slow.

Is there really no other way?