linux-kernel - Re: [RFC PATCH 9/9] sched/deadline: Allow deeper hierarchies of RT cgroups

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Message-ID: <aEf5bu-50HTXI-wa@jlelli-thinkpadt14gen4.remote.csb>
Date: Tue, 10 Jun 2025 11:22:54 +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 9/9] sched/deadline: Allow deeper hierarchies of RT
 cgroups

Hello,

On 05/06/25 09:14, 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 | 69 ++++++++++++++++++++++++++++++++++-------
>  kernel/sched/rt.c       | 25 +++++++++++++--
>  kernel/sched/sched.h    |  2 +-
>  kernel/sched/syscalls.c |  4 +++
>  5 files changed, 84 insertions(+), 22 deletions(-)

...

> @@ -434,24 +463,40 @@ int dl_init_tg(struct sched_dl_entity *dl_se, u64 rt_runtime, u64 rt_period)
>  	if (rt_period & (1ULL << 63))
>  		return 0;
>  
> +	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;
>  	old_runtime = dl_se->dl_runtime;
>  	dl_se->dl_runtime  = rt_runtime;
>  	dl_se->dl_period   = rt_period;
>  	dl_se->dl_deadline = dl_se->dl_period;
> -	if (is_active) {
> -		sub_running_bw(dl_se, dl_se->dl_rq);
> -	} else if (dl_se->dl_non_contending) {
> -		sub_running_bw(dl_se, dl_se->dl_rq);
> -		dl_se->dl_non_contending = 0;
> -		hrtimer_try_to_cancel(&dl_se->inactive_timer);
> +	if (is_active_group) {
> +		if (is_active) {
> +			sub_running_bw(dl_se, dl_se->dl_rq);
> +		} else if (dl_se->dl_non_contending) {
> +			sub_running_bw(dl_se, dl_se->dl_rq);
> +			dl_se->dl_non_contending = 0;
> +			hrtimer_try_to_cancel(&dl_se->inactive_timer);
> +		}
> +		__sub_rq_bw(dl_se->dl_bw, dl_se->dl_rq);
> +		dl_se->dl_bw = to_ratio(dl_se->dl_period, dl_se->dl_runtime);
> +		__add_rq_bw(dl_se->dl_bw, dl_se->dl_rq);
> +	} else {
> +		dl_se->dl_bw = to_ratio(dl_se->dl_period, dl_se->dl_runtime);
> +	}
> +
> +	// add/remove the parent's bw
> +	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);
> +		}

Don't we need to do something also when rt_runtime changes
({in,de}creases) and old_runtime wasn't zero? Like for example giving a
bit of bandwidth back to the parent if a child bandwidth is reduced, but
not completely set to zero.

Thanks,
Juri