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

[Pavan Kondeti <pkondeti@codeaurora.org>]

On Mon, Aug 06, 2018 at 05:39:44PM +0100, 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).
> 
> Unfortunately this setup will also mean that all tasks running in the
> root group, will always get a maximum util.min clamp, unless they have a
> lower task specific clamp which is definitively not a desirable default
> configuration.
> 
> Let's fix this by explicitly adding a system default configuration
> (sysctl_sched_uclamp_util_{min,max}) which works as a restrictive clamp
> for all tasks running on the root group.
> 
> This interface is available independently from cgroups, thus providing a
> complete solution for system wide utilization clamping configuration.
> 
> Signed-off-by: Patrick Bellasi <patrick.bellasi@arm.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Paul Turner <pjt@google.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Todd Kjos <tkjos@google.com>
> Cc: Joel Fernandes <joelaf@google.com>
> Cc: Steve Muckle <smuckle@google.com>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Morten Rasmussen <morten.rasmussen@arm.com>
> Cc: linux-kernel@vger.kernel.org
> Cc: linux-pm@vger.kernel.org

<snip>

> +/*
> + * Minimum utilization for tasks in the root cgroup
> + * default: 0
> + */
> +unsigned int sysctl_sched_uclamp_util_min;
> +
> +/*
> + * Maximum utilization for tasks in the root cgroup
> + * default: 1024
> + */
> +unsigned int sysctl_sched_uclamp_util_max = 1024;
> +
> +static struct uclamp_se uclamp_default[UCLAMP_CNT];
> +

The default group id for un-clamped root tasks is 0 because of
this declaration, correct?

>  /**
>   * uclamp_map: reference counts a utilization "clamp value"
>   * @value:    the utilization "clamp value" required
> @@ -957,12 +971,25 @@ static inline int uclamp_task_group_id(struct task_struct *p, int clamp_id)
>  	group_id = uc_se->group_id;
>  
>  #ifdef CONFIG_UCLAMP_TASK_GROUP
> +	/*
> +	 * Tasks in the root group, which do not have a task specific clamp
> +	 * value, get the system default calmp value.
> +	 */
> +	if (group_id == UCLAMP_NOT_VALID &&
> +	    task_group(p) == &root_task_group) {
> +		return uclamp_default[clamp_id].group_id;
> +	}
> +
>  	/* Use TG's clamp value to limit task specific values */
>  	uc_se = &task_group(p)->uclamp[clamp_id];
>  	if (group_id == UCLAMP_NOT_VALID ||
>  	    clamp_value > uc_se->effective.value) {
>  		group_id = uc_se->effective.group_id;
>  	}
> +#else
> +	/* By default, all tasks get the system default clamp value */
> +	if (group_id == UCLAMP_NOT_VALID)
> +		return uclamp_default[clamp_id].group_id;
>  #endif
>  
>  	return group_id;
> @@ -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);
> +	}

uclamp_group_get() also drops the reference on the previous group id.
The initial group id for uclamp_default[] i.e 0 is never claimed by
us. so we end up releasing it. But root group still points to group#0.
is this a problem?

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