linux-kernel - Re: [PATCH 3/3] cgroup: use nodes

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Message-ID: <51675d7c-5c9d-4596-8e5c-692c90b79e06@redhat.com>
Date: Wed, 14 Jan 2026 17:33:06 -0500
From: Waiman Long <llong@...hat.com>
To: Yury Norov <ynorov@...dia.com>, Andrew Morton
 <akpm@...ux-foundation.org>, Alistair Popple <apopple@...dia.com>,
 Byungchul Park <byungchul@...com>, David Hildenbrand <david@...nel.org>,
 Gregory Price <gourry@...rry.net>, Johannes Weiner <hannes@...xchg.org>,
 Joshua Hahn <joshua.hahnjy@...il.com>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Matthew Brost <matthew.brost@...el.com>, Michal Hocko <mhocko@...e.com>,
 Michal Koutný <mkoutny@...e.com>,
 Mike Rapoport <rppt@...nel.org>, Rakie Kim <rakie.kim@...com>,
 Suren Baghdasaryan <surenb@...gle.com>, Tejun Heo <tj@...nel.org>,
 Vlastimil Babka <vbabka@...e.cz>, Ying Huang <ying.huang@...ux.alibaba.com>,
 Zi Yan <ziy@...dia.com>, cgroups@...r.kernel.org
Cc: Yury Norov <yury.norov@...il.com>,
 Rasmus Villemoes <linux@...musvillemoes.dk>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] cgroup: use nodes_and() output where appropriate

On 1/14/26 12:22 PM, Yury Norov wrote:
> Now that nodes_and() returns true if the result nodemask is not empty,
> drop useless nodes_intersects() in guarantee_online_mems() and
> nodes_empty() in update_nodemasks_hier(), which both are O(N).
>
> Signed-off-by: Yury Norov <ynorov@...dia.com>
> ---
>   kernel/cgroup/cpuset.c | 7 +++----
>   1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 3e8cc34d8d50..e962efbb300d 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -456,9 +456,8 @@ static void guarantee_active_cpus(struct task_struct *tsk,
>    */
>   static void guarantee_online_mems(struct cpuset *cs, nodemask_t *pmask)
>   {
> -	while (!nodes_intersects(cs->effective_mems, node_states[N_MEMORY]))
> +	while (!nodes_and(*pmask, cs->effective_mems, node_states[N_MEMORY]))
>   		cs = parent_cs(cs);
> -	nodes_and(*pmask, cs->effective_mems, node_states[N_MEMORY]);
>   }
>   
>   /**
> @@ -2862,13 +2861,13 @@ static void update_nodemasks_hier(struct cpuset *cs, nodemask_t *new_mems)
>   	cpuset_for_each_descendant_pre(cp, pos_css, cs) {
>   		struct cpuset *parent = parent_cs(cp);
>   
> -		nodes_and(*new_mems, cp->mems_allowed, parent->effective_mems);
> +		bool has_mems = nodes_and(*new_mems, cp->mems_allowed, parent->effective_mems);
>   
>   		/*
>   		 * If it becomes empty, inherit the effective mask of the
>   		 * parent, which is guaranteed to have some MEMs.
>   		 */
> -		if (is_in_v2_mode() && nodes_empty(*new_mems))
> +		if (is_in_v2_mode() && !has_mems)
>   			*new_mems = parent->effective_mems;
>   
>   		/* Skip the whole subtree if the nodemask remains the same. */
Reviewed-by: Waiman Long <longman@...hat.com>