linux-kernel - Re: [PATCH] v2 sched: Fix tg->load when offlining a CPU

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Message-ID: <dbd36b16-4acb-415a-9bbd-258b2e022475@arm.com>
Date: Sat, 30 Dec 2023 18:56:58 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Vincent Guittot <vincent.guittot@...aro.org>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, rostedt@...dmis.org,
 bsegall@...gle.com, mgorman@...e.de, bristot@...hat.com,
 vschneid@...hat.com, imran.f.khan@...cle.com, aaron.lu@...el.com,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] v2 sched: Fix tg->load when offlining a CPU

On 23/12/2023 12:15, Vincent Guittot wrote:
> When a CPU taken offline, the contribution of its cfs_rqs to task_groups'
> load may remain and impact the calculation of the share of the online
> CPUs.
> Clear the contribution of an offlining CPU to task groups' load and skip
> its contribution while it is inactive.

The patch got applied to tip/sched/core a couple of hours ago so this
might be too late ...

Isn't this fix in rq_offline_fair() also affecting all the other cpus
via cpuset_hotplug_workfn() -> rebuild_sched_domains_locked() ->
partition_sched_domains_locked() -> cpu_attach_domain() ->
rq_attach_root() -> set_rq_offline() ?

> 
> Reported-by: Imran Khan <imran.f.khan@...cle.com>
> Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
> Reviewed-and-tested-by: Imran Khan <imran.f.khan@...cle.com>
> ---
> 
> - Fix !CONFIG_FAIR_GROUP_SCHED case
> 
>  kernel/sched/fair.c | 52 +++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 52 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bcea3d55d95d..4b09237d24b9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4100,6 +4100,10 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
>  	if (cfs_rq->tg == &root_task_group)
>  		return;
>  
> +	/* rq has been offline and doesn't contribute anymore to the share */
> +	if (!cpu_active(cpu_of(rq_of(cfs_rq))))
> +		return;
> +
>  	/*
>  	 * For migration heavy workloads, access to tg->load_avg can be
>  	 * unbound. Limit the update rate to at most once per ms.
> @@ -4116,6 +4120,49 @@ static inline void update_tg_load_avg(struct cfs_rq *cfs_rq)
>  	}
>  }
>  
> +static inline void clear_tg_load_avg(struct cfs_rq *cfs_rq)
> +{
> +	long delta;
> +	u64 now;
> +
> +	/*
> +	 * No need to update load_avg for root_task_group as it is not used.
> +	 */
> +	if (cfs_rq->tg == &root_task_group)
> +		return;
> +
> +	now = sched_clock_cpu(cpu_of(rq_of(cfs_rq)));
> +	delta = 0 - cfs_rq->tg_load_avg_contrib;
> +	atomic_long_add(delta, &cfs_rq->tg->load_avg);

Why not:

atomic_long_sub(cfs_rq->tg_load_avg_contrib, &cfs_rq->tg->load_avg) w/o
the local `long delta`?

> +	cfs_rq->tg_load_avg_contrib = 0;
> +	cfs_rq->last_update_tg_load_avg = now;
> +}
> +
> +/* cpu offline callback */
> +static void __maybe_unused clear_tg_offline_cfs_rqs(struct rq *rq)
> +{
> +	struct task_group *tg;
> +
> +	lockdep_assert_rq_held(rq);
> +
> +	/*
> +	 * The rq clock has already been updated in the
> +	 * set_rq_offline(), so we should skip updating
> +	 * the rq clock again in unthrottle_cfs_rq().

This comment seems misleading here since it looks that it is tailored to
unthrottle_offline_cfs_rqs(), the other cpu offline callback.

> +	 */
> +	rq_clock_start_loop_update(rq);
> +
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(tg, &task_groups, list) {
> +		struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
> +
> +		clear_tg_load_avg(cfs_rq);
> +	}
> +	rcu_read_unlock();
> +
> +	rq_clock_stop_loop_update(rq);
> +}
> +
>  /*
>   * Called within set_task_rq() right before setting a task's CPU. The
>   * caller only guarantees p->pi_lock is held; no other assumptions,
> @@ -4412,6 +4459,8 @@ static inline bool skip_blocked_update(struct sched_entity *se)
>  
>  static inline void update_tg_load_avg(struct cfs_rq *cfs_rq) {}
>  
> +static inline void clear_tg_offline_cfs_rqs(struct rq *rq) {}
> +
>  static inline int propagate_entity_load_avg(struct sched_entity *se)
>  {
>  	return 0;
> @@ -12446,6 +12495,9 @@ static void rq_offline_fair(struct rq *rq)
>  
>  	/* Ensure any throttled groups are reachable by pick_next_task */
>  	unthrottle_offline_cfs_rqs(rq);
> +
> +	/* Ensure that we remove rq contribution to group share */
> +	clear_tg_offline_cfs_rqs(rq);
>  }
>  
>  #endif /* CONFIG_SMP */