Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
From:   Benjamin Segall <bsegall@google.com>
To:     Phil Auld <pauld@redhat.com>
Cc:     linux-kernel@vger.kernel.org, Juri Lelli <juri.lelli@redhat.com>,
        Ingo Molnar <mingo@redhat.com>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Frederic Weisbecker <frederic@kernel.org>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH v2 1/2] sched, cgroup: Restore meaning to
 hierarchical_quota
References: <20230712133357.381137-1-pauld@redhat.com>
        <20230712133357.381137-2-pauld@redhat.com>
Date:   Wed, 12 Jul 2023 15:09:31 -0700
In-Reply-To: <20230712133357.381137-2-pauld@redhat.com> (Phil Auld's message
        of "Wed, 12 Jul 2023 09:33:56 -0400")
Message-ID: <xm268rbkg4tg.fsf@google.com>
User-Agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux)
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Phil Auld <pauld@redhat.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.
>
> Fixes: c53593e5cb69 ("sched, cgroup: Don't reject lower cpu.max on ancestors")
> Signed-off-by: Phil Auld <pauld@redhat.com>
> Reviewed-by: Ben Segall <bsegall@google.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Vincent Guittot <vincent.guittot@linaro.org>
> Cc: Juri Lelli <juri.lelli@redhat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
> Cc: Valentin Schneider <vschneid@redhat.com>
> Cc: Ben Segall <bsegall@google.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Tejun Heo <tj@kernel.org>
> ---
>
> v2: Improve comment about how setting hierarchical_quota correctly
>
> helps the scheduler. Remove extra parens.
>  kernel/sched/core.c  | 13 +++++++++----
>  kernel/sched/fair.c  |  7 ++++---
>  kernel/sched/sched.h |  2 +-
>  3 files changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index a68d1276bab0..f80697a79baf 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -9904,7 +9904,7 @@ void __init sched_init(void)
>  		ptr += nr_cpu_ids * sizeof(void **);
>  
>  		root_task_group.shares = ROOT_TASK_GROUP_LOAD;
> -		init_cfs_bandwidth(&root_task_group.cfs_bandwidth);
> +		init_cfs_bandwidth(&root_task_group.cfs_bandwidth, NULL);
>  #endif /* CONFIG_FAIR_GROUP_SCHED */
>  #ifdef CONFIG_RT_GROUP_SCHED
>  		root_task_group.rt_se = (struct sched_rt_entity **)ptr;
> @@ -11038,11 +11038,16 @@ 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. In cgroup2 this is used
> +		 * by the scheduler to determine if a given CFS task has a
> +		 * bandwidth constraint at some higher level.
>  		 */

It's still used for determining this on cgroup1 (and the cgroup1 code
still works for that), right?

>  		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;
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 373ff5f55884..d9b3d4617e16 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;
>  
>  	INIT_LIST_HEAD(&cfs_b->throttled_cfs_rq);
>  	hrtimer_init(&cfs_b->period_timer, CLOCK_MONOTONIC, HRTIMER_MODE_ABS_PINNED);
> @@ -6168,7 +6169,7 @@ static inline int throttled_lb_pair(struct task_group *tg,
>  	return 0;
>  }
>  
> -void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b) {}
> +void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth *parent) {}
>  
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>  static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq) {}
> @@ -12373,7 +12374,7 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
>  
>  	tg->shares = NICE_0_LOAD;
>  
> -	init_cfs_bandwidth(tg_cfs_bandwidth(tg));
> +	init_cfs_bandwidth(tg_cfs_bandwidth(tg), tg_cfs_bandwidth(parent));
>  
>  	for_each_possible_cpu(i) {
>  		cfs_rq = kzalloc_node(sizeof(struct cfs_rq),
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index ec7b3e0a2b20..63822c9238cc 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -460,7 +460,7 @@ extern void unregister_fair_sched_group(struct task_group *tg);
>  extern void init_tg_cfs_entry(struct task_group *tg, struct cfs_rq *cfs_rq,
>  			struct sched_entity *se, int cpu,
>  			struct sched_entity *parent);
> -extern void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b);
> +extern void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth *parent);
>  
>  extern void __refill_cfs_bandwidth_runtime(struct cfs_bandwidth *cfs_b);
>  extern void start_cfs_bandwidth(struct cfs_bandwidth *cfs_b);