linux-kernel - Re: [PATCH v5 1/2] sched, cgroup: Restore meaning to hierarchical

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Date:   Mon, 10 Jul 2023 17:00:11 -0700
From:   Benjamin Segall <bsegall@...gle.com>
To:     Phil Auld <pauld@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Juri Lelli <juri.lelli@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mel Gorman <mgorman@...e.de>,
        Frederic Weisbecker <frederic@...nel.org>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH v5 1/2] sched, cgroup: Restore meaning to
 hierarchical_quota

Phil Auld <pauld@...hat.com> writes:

> In cgroupv2 cfs_b->hierarchical_quota is set to -1 for all task
> groups due to the previous fix simply taking the min.  It should
> reflect a limit imposed at that level or by an ancestor. Even
> though cgroupv2 does not require child quota to be less than or
> equal to that of its ancestors the task group will still be
> constrained by such a quota so this should be shown here. Cgroupv1
> continues to set this correctly.
>
> In both cases, add initialization when a new task group is created
> based on the current parent's value (or RUNTIME_INF in the case of
> root_task_group). Otherwise, the field is wrong until a quota is
> changed after creation and __cfs_schedulable() is called.

Reviewed-by: Ben Segall <bsegall@...gle.com>

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a68d1276bab0..1b214e10c25d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -11038,11 +11038,14 @@ static int tg_cfs_schedulable_down(struct task_group *tg, void *data)
>  
>  		/*
>  		 * Ensure max(child_quota) <= parent_quota.  On cgroup2,
> -		 * always take the min.  On cgroup1, only inherit when no
> -		 * limit is set:
> +		 * always take the non-RUNTIME_INF min.  On cgroup1, only
> +		 * inherit when no limit is set:
>  		 */
>  		if (cgroup_subsys_on_dfl(cpu_cgrp_subsys)) {
> -			quota = min(quota, parent_quota);
> +			if (quota == RUNTIME_INF)
> +				quota = parent_quota;
> +			else if (parent_quota != RUNTIME_INF)
> +				quota = min(quota, parent_quota);
>  		} else {
>  			if (quota == RUNTIME_INF)
>  				quota = parent_quota;

I suppose you could also set RUNTIME_INF to be a positive value or
better yet just compare at unsigned, but it's not like config needs to
be fast, so no need to mess with that.

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 373ff5f55884..92381f9ecf37 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6005,13 +6005,14 @@ static enum hrtimer_restart sched_cfs_period_timer(struct hrtimer *timer)
>  	return idle ? HRTIMER_NORESTART : HRTIMER_RESTART;
>  }
>  
> -void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b)
> +void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth *parent)
>  {
>  	raw_spin_lock_init(&cfs_b->lock);
>  	cfs_b->runtime = 0;
>  	cfs_b->quota = RUNTIME_INF;
>  	cfs_b->period = ns_to_ktime(default_cfs_period());
>  	cfs_b->burst = 0;
> +	cfs_b->hierarchical_quota = ((parent) ? parent->hierarchical_quota : RUNTIME_INF);

Minor style nit: don't need any of these parens here.