linux-kernel - Re: [PATCH 2/3] cgroup: make css_rstat

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Message-ID: <9cf00007-f068-4ced-8977-f39a792eef6a@gmail.com>
Date: Tue, 10 Jun 2025 22:23:51 -0700
From: JP Kobryn <inwardvessel@...il.com>
To: Shakeel Butt <shakeel.butt@...ux.dev>
Cc: Johannes Weiner <hannes@...xchg.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Tejun Heo <tj@...nel.org>,
 Michal Hocko <mhocko@...nel.org>, Roman Gushchin <roman.gushchin@...ux.dev>,
 Muchun Song <muchun.song@...ux.dev>, Vlastimil Babka <vbabka@...e.cz>,
 Alexei Starovoitov <ast@...nel.org>,
 Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
 Michal Koutný <mkoutny@...e.com>,
 Harry Yoo <harry.yoo@...cle.com>, Yosry Ahmed <yosry.ahmed@...ux.dev>,
 bpf@...r.kernel.org, linux-mm@...ck.org, cgroups@...r.kernel.org,
 linux-kernel@...r.kernel.org, Meta kernel team <kernel-team@...a.com>
Subject: Re: [PATCH 2/3] cgroup: make css_rstat_updated nmi safe

On 6/9/25 3:56 PM, Shakeel Butt wrote:
[..]
> diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
> index a5608ae2be27..4fabd7973067 100644
> --- a/kernel/cgroup/rstat.c
> +++ b/kernel/cgroup/rstat.c
> @@ -138,13 +138,15 @@ void _css_rstat_cpu_unlock(struct cgroup_subsys_state *css, int cpu,
>    * @css: target cgroup subsystem state
>    * @cpu: cpu on which rstat_cpu was updated
>    *
> - * @css's rstat_cpu on @cpu was updated. Put it on the parent's matching
> - * rstat_cpu->updated_children list. See the comment on top of
> - * css_rstat_cpu definition for details.
> + * Atomically inserts the css in the ss's llist for the given cpu. This is nmi
> + * safe. The ss's llist will be processed at the flush time to create the update
> + * tree.
>    */
>   __bpf_kfunc void css_rstat_updated(struct cgroup_subsys_state *css, int cpu)
>   {
> -	unsigned long flags;
> +	struct llist_head *lhead = ss_lhead_cpu(css->ss, cpu);
> +	struct css_rstat_cpu *rstatc = css_rstat_cpu(css, cpu);
> +	struct llist_node *self;
>   
>   	/*

> -	flags = _css_rstat_cpu_lock(css, cpu, true);
> +	llist_add(&rstatc->lnode, lhead);
> +}
[..]
> +
> +static void css_process_update_tree(struct cgroup_subsys *ss, int cpu)
> +{
> +	struct llist_head *lhead = ss_lhead_cpu(ss, cpu);
> +	struct llist_node *lnode;
> +
> +	while ((lnode = llist_del_first_init(lhead))) {
> +		struct css_rstat_cpu *rstatc;
>   
> -	_css_rstat_cpu_unlock(css, cpu, flags, true);
> +		rstatc = container_of(lnode, struct css_rstat_cpu, lnode);
> +		__css_process_update_tree(rstatc->owner, cpu);
> +	}
>   }
>   
>   /**
> @@ -300,6 +331,8 @@ static struct cgroup_subsys_state *css_rstat_updated_list(
>   
>   	flags = _css_rstat_cpu_lock(root, cpu, false);

The subsystem per-cpu locks were used to synchronize updater and flusher
on a given cpu. Since you no longer use them on the updater side, it
seems these locks can be removed altogether.