Re: [RFC PATCH 1/3] sched/fair: Embed sched_entity into cfs_rq

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jun 04, 2025 at 07:58:41PM +0000, Zecheng Li wrote:
> To improve data locality and avoid pointer chasing, embed struct
> sched_entity within struct cfs_rq. This co-locates the runqueue state
> (cfs_rq) and the entity's scheduling state (se).
> 
> This patch implements the following:
> 
> - Adds a struct sched_entity field to struct cfs_rq.
> 
> - Modifies alloc_fair_sched_group() and free_fair_sched_group() to
> remove the separate allocation and freeing logic for sched_entity
> objects themselves.
> 
> - The task_group->se pointer array (struct sched_entity **se) is
> retained. The pointers in this array are updated to point to the
> corresponding embedded &cfs_rq->se for each CPU.
> 
> Signed-off-by: Zecheng Li <zecheng@google.com>
> ---
>  kernel/sched/fair.c  | 10 +---------
>  kernel/sched/sched.h |  4 ++++
>  2 files changed, 5 insertions(+), 9 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 0fb9bf995a47..c2af9896eef4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -13343,8 +13343,6 @@ void free_fair_sched_group(struct task_group *tg)
>  	for_each_possible_cpu(i) {
>  		if (tg->cfs_rq)
>  			kfree(tg->cfs_rq[i]);
> -		if (tg->se)
> -			kfree(tg->se[i]);
>  	}
>  
>  	kfree(tg->cfs_rq);
> @@ -13374,11 +13372,7 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
>  		if (!cfs_rq)
>  			goto err;
>  
> -		se = kzalloc_node(sizeof(struct sched_entity_stats),
> -				  GFP_KERNEL, cpu_to_node(i));
> -		if (!se)
> -			goto err_free_rq;
> -
> +		se = &cfs_rq->se;
>  		init_cfs_rq(cfs_rq);
>  		init_tg_cfs_entry(tg, cfs_rq, se, i, parent->se[i]);
>  		init_entity_runnable_average(se);
> @@ -13386,8 +13380,6 @@ int alloc_fair_sched_group(struct task_group *tg, struct task_group *parent)
>  
>  	return 1;
>  
> -err_free_rq:
> -	kfree(cfs_rq);
>  err:
>  	return 0;
>  }
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 47972f34ea70..6e26b7d59c13 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -738,6 +738,10 @@ struct cfs_rq {
>  	struct list_head	throttled_csd_list;
>  #endif /* CONFIG_CFS_BANDWIDTH */
>  #endif /* CONFIG_FAIR_GROUP_SCHED */
> +#ifdef CONFIG_FAIR_GROUP_SCHED
> +	/* sched_entity on parent runqueue */
> +	struct sched_entity	se ____cacheline_aligned;
> +#endif
>  };

This also blows up struct rq for no reason.

I would much rather you all investigate if you can make the flattened
cgroup stuff from Rik work.