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Message-ID: <20230713132306.GA13342@lorien.usersys.redhat.com>
Date: Thu, 13 Jul 2023 09:23:06 -0400
From: Phil Auld <pauld@...hat.com>
To: Benjamin Segall <bsegall@...gle.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 v2 1/2] sched, cgroup: Restore meaning to
hierarchical_quota
On Wed, Jul 12, 2023 at 03:09:31PM -0700 Benjamin Segall wrote:
> 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.
> >
> > Fixes: c53593e5cb69 ("sched, cgroup: Don't reject lower cpu.max on ancestors")
> > Signed-off-by: Phil Auld <pauld@...hat.com>
> > Reviewed-by: Ben Segall <bsegall@...gle.com>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Vincent Guittot <vincent.guittot@...aro.org>
> > Cc: Juri Lelli <juri.lelli@...hat.com>
> > Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> > Cc: Valentin Schneider <vschneid@...hat.com>
> > Cc: Ben Segall <bsegall@...gle.com>
> > Cc: Frederic Weisbecker <frederic@...nel.org>
> > Cc: Tejun Heo <tj@...nel.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?
>
It would, except that the enforcement of child quota <= parent quota
means that cfs_rq->runtime_enabled will be set and we'll hit that first
on cgroup1. So we don't really use it for this determination in cgroup1.
But I could generalize that comment if you want.
Thanks,
Phil
> > 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);
>
--
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