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

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