linux-kernel - Re: [RFC PATCH v3 18/24] sched/deadline: Allow deeper hierarchies of RT cgroups

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Message-ID: <aO-uqIZRS3qqsuN6@jlelli-thinkpadt14gen4.remote.csb>
Date: Wed, 15 Oct 2025 16:24:40 +0200
From: Juri Lelli <juri.lelli@...hat.com>
To: Yuri Andriaccio <yurand2000@...il.com>
Cc: Ingo Molnar <mingo@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
	Valentin Schneider <vschneid@...hat.com>,
	linux-kernel@...r.kernel.org,
	Luca Abeni <luca.abeni@...tannapisa.it>,
	Yuri Andriaccio <yuri.andriaccio@...tannapisa.it>
Subject: Re: [RFC PATCH v3 18/24] sched/deadline: Allow deeper hierarchies of
 RT cgroups

Hello,

On 29/09/25 11:22, Yuri Andriaccio wrote:
> From: luca abeni <luca.abeni@...tannapisa.it>
> 
> Allow creation of cgroup hierachies with depth greater than two.
> Add check to prevent attaching tasks to a child cgroup of an active cgroup (i.e.
> with a running FIFO/RR task).
> Add check to prevent attaching tasks to cgroups which have children with
> non-zero runtime.
> Update rt-cgroups allocated bandwidth accounting for nested cgroup hierachies.
> 
> Co-developed-by: Yuri Andriaccio <yurand2000@...il.com>
> Signed-off-by: Yuri Andriaccio <yurand2000@...il.com>
> Signed-off-by: luca abeni <luca.abeni@...tannapisa.it>
> ---
>  kernel/sched/core.c     |  6 -----
>  kernel/sched/deadline.c | 51 +++++++++++++++++++++++++++++++++++++----
>  kernel/sched/rt.c       | 16 ++++++++++---
>  kernel/sched/sched.h    |  3 ++-
>  4 files changed, 62 insertions(+), 14 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 6f516cdc7bb..d1d7215c4a2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -9281,12 +9281,6 @@ cpu_cgroup_css_alloc(struct cgroup_subsys_state *parent_css)
>  		return &root_task_group.css;
>  	}
>  
> -	/* Do not allow cpu_cgroup hierachies with depth greater than 2. */
> -#ifdef CONFIG_RT_GROUP_SCHED
> -	if (parent != &root_task_group)
> -		return ERR_PTR(-EINVAL);
> -#endif
> -
>  	tg = sched_create_group(parent);
>  	if (IS_ERR(tg))
>  		return ERR_PTR(-ENOMEM);
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 5d93b3ca030..abe11985c41 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -388,11 +388,42 @@ int dl_check_tg(unsigned long total)
>  	return 1;
>  }
>  
> -void dl_init_tg(struct sched_dl_entity *dl_se, u64 rt_runtime, u64 rt_period)
> +bool is_active_sched_group(struct task_group *tg)

I wonder if the function name could be misleading, as this checks runtime
and not if there are tasks in the group.

>  {
> +	struct task_group *child;
> +	bool is_active = 1;
> +
> +	// if there are no children, this is a leaf group, thus it is active
> +	list_for_each_entry_rcu(child, &tg->children, siblings) {
> +		if (child->dl_bandwidth.dl_runtime > 0) {
> +			is_active = 0;
> +		}
> +	}
> +	return is_active;
> +}
> +
> +static inline bool sched_group_has_active_siblings(struct task_group *tg)
> +{
> +	struct task_group *child;
> +	bool has_active_siblings = 0;
> +
> +	// if there are no children, this is a leaf group, thus it is active

Copy-pasta from above? :) Also not the correct comment style.

> +	list_for_each_entry_rcu(child, &tg->parent->children, siblings) {
> +		if (child != tg && child->dl_bandwidth.dl_runtime > 0) {
> +			has_active_siblings = 1;
> +		}
> +	}
> +	return has_active_siblings;
> +}
> +
> +void dl_init_tg(struct task_group *tg, int cpu, u64 rt_runtime, u64 rt_period)
> +{
> +	struct sched_dl_entity *dl_se = tg->dl_se[cpu];
>  	struct rq *rq = container_of(dl_se->dl_rq, struct rq, dl);
> -	int is_active;
> -	u64 new_bw;
> +	int is_active, is_active_group;
> +	u64 old_runtime, new_bw;
> +
> +	is_active_group = is_active_sched_group(tg);
>  
>  	raw_spin_rq_lock_irq(rq);
>  	is_active = dl_se->my_q->rt.rt_nr_running > 0;
> @@ -400,8 +431,10 @@ void dl_init_tg(struct sched_dl_entity *dl_se, u64 rt_runtime, u64 rt_period)
>  	update_rq_clock(rq);
>  	dl_server_stop(dl_se);
>  
> +	old_runtime = dl_se->dl_runtime;
>  	new_bw = to_ratio(dl_se->dl_period, dl_se->dl_runtime);
> -	dl_rq_change_utilization(rq, dl_se, new_bw);
> +	if (is_active_group)
> +		dl_rq_change_utilization(rq, dl_se, new_bw);
>  
>  	dl_se->dl_runtime  = rt_runtime;
>  	dl_se->dl_deadline = rt_period;
> @@ -413,6 +446,16 @@ void dl_init_tg(struct sched_dl_entity *dl_se, u64 rt_runtime, u64 rt_period)
>  	dl_se->dl_bw = new_bw;
>  	dl_se->dl_density = new_bw;
>  
> +	// add/remove the parent's bw

Comment style is not correct. Also the comment itself is not very much
informative. What about something like (IIUC)

 /*
  * Handle parent bandwidth accounting when child runtime changes:
  * - Disabling the last active child: parent becomes a leaf group,
  *   so add the parent's bandwidth back to active accounting
  * - Enabling the first child: parent becomes a non-leaf group,
  *   so remove the parent's bandwidth from active accounting
  * Only leaf groups (those without active children) should have
  * non-zero bandwidth.
  */

> +	if (tg->parent && tg->parent != &root_task_group)
> +	{
> +		if (rt_runtime == 0 && old_runtime != 0 && !sched_group_has_active_siblings(tg)) {
> +			__add_rq_bw(tg->parent->dl_se[cpu]->dl_bw, dl_se->dl_rq);
> +		} else if (rt_runtime != 0 && old_runtime == 0 && !sched_group_has_active_siblings(tg)) {
> +			__sub_rq_bw(tg->parent->dl_se[cpu]->dl_bw, dl_se->dl_rq);
> +		}
> +	}
> +

Thanks,
Juri