linux-kernel - Re: [PATCH v3 12/14] sched/core: uclamp: add system default clamps

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Date:   Mon, 20 Aug 2018 12:18:40 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Patrick Bellasi <patrick.bellasi@....com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v3 12/14] sched/core: uclamp: add system default clamps

On 08/06/2018 06:39 PM, Patrick Bellasi wrote:
> Clamp values cannot be tuned at the root cgroup level. Moreover, because
> of the delegation model requirements and how the parent clamps
> propagation works, if we want to enable subgroups to set a non null
> util.min, we need to be able to configure the root group util.min to the
> allow the maximum utilization (SCHED_CAPACITY_SCALE = 1024).

Why 1024 (100)? Would any non 0 value work here?

[...]

> @@ -1269,6 +1296,75 @@ static inline void uclamp_group_get(struct task_struct *p,
>   	uclamp_group_put(clamp_id, prev_group_id);
>   }
>   
> +int sched_uclamp_handler(struct ctl_table *table, int write,
> +			 void __user *buffer, size_t *lenp,
> +			 loff_t *ppos)
> +{
> +	int group_id[UCLAMP_CNT] = { UCLAMP_NOT_VALID };
> +	struct uclamp_se *uc_se;
> +	int old_min, old_max;
> +	int result;
> +
> +	mutex_lock(&uclamp_mutex);
> +
> +	old_min = sysctl_sched_uclamp_util_min;
> +	old_max = sysctl_sched_uclamp_util_max;
> +
> +	result = proc_dointvec(table, write, buffer, lenp, ppos);
> +	if (result)
> +		goto undo;
> +	if (!write)
> +		goto done;
> +
> +	if (sysctl_sched_uclamp_util_min > sysctl_sched_uclamp_util_max)
> +		goto undo;
> +	if (sysctl_sched_uclamp_util_max > 1024)
> +		goto undo;
> +
> +	/* Find a valid group_id for each required clamp value */
> +	if (old_min != sysctl_sched_uclamp_util_min) {
> +		result = uclamp_group_find(UCLAMP_MIN, sysctl_sched_uclamp_util_min);
> +		if (result == -ENOSPC) {
> +			pr_err("Cannot allocate more than %d UTIL_MIN clamp groups\n",
> +			       CONFIG_UCLAMP_GROUPS_COUNT);
> +			goto undo;
> +		}
> +		group_id[UCLAMP_MIN] = result;
> +	}
> +	if (old_max != sysctl_sched_uclamp_util_max) {
> +		result = uclamp_group_find(UCLAMP_MAX, sysctl_sched_uclamp_util_max);
> +		if (result == -ENOSPC) {
> +			pr_err("Cannot allocate more than %d UTIL_MAX clamp groups\n",
> +			       CONFIG_UCLAMP_GROUPS_COUNT);
> +			goto undo;
> +		}
> +		group_id[UCLAMP_MAX] = result;
> +	}
> +
> +	/* Update each required clamp group */
> +	if (old_min != sysctl_sched_uclamp_util_min) {
> +		uc_se = &uclamp_default[UCLAMP_MIN];
> +		uclamp_group_get(NULL, UCLAMP_MIN, group_id[UCLAMP_MIN],
> +				 uc_se, sysctl_sched_uclamp_util_min);
> +	}
> +	if (old_max != sysctl_sched_uclamp_util_max) {
> +		uc_se = &uclamp_default[UCLAMP_MAX];
> +		uclamp_group_get(NULL, UCLAMP_MAX, group_id[UCLAMP_MAX],
> +				 uc_se, sysctl_sched_uclamp_util_max);
> +	}
> +
> +	if (result) {
> +undo:
> +		sysctl_sched_uclamp_util_min = old_min;
> +		sysctl_sched_uclamp_util_max = old_max;
> +	}

This looks strange! In case uclamp_group_find() returns free_group_id 
instead of -ENOSPC, the sysctl min/max values are reset?

I was under the assumption that I could specify:

sysctl_sched_uclamp_util_min = 40 (for boosting)
sysctl_sched_uclamp_util_max = 80 (for clamping)

with an empty cpu controller hierarchy and then those values become the 
.effective values of (a first level) task group?

[...]