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Message-ID: <57cf1fbe-8356-4c5d-864a-20b07d63de72@huaweicloud.com>
Date: Mon, 20 Oct 2025 14:57:38 +0800
From: Chen Ridong <chenridong@...weicloud.com>
To: Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
Johannes Weiner <hannes@...xchg.org>, Michal Koutný
<mkoutny@...e.com>
Cc: cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
Chen Ridong <chenridong@...wei.com>
Subject: Re: [PATCH 1/2] cgroup/cpuset: Don't track # of local child
partitions
On 2025/10/20 10:32, Waiman Long wrote:
> The cpuset structure has a nr_subparts field which tracks the number
> of child local partitions underneath a particular cpuset. Right now,
> nr_subparts is only used in partition_is_populated() to avoid iteration
> of child cpusets if the condition is right. So by always performing the
> child iteration, we can avoid tracking the number of child partitions
> and simplify the code a bit.
>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
> kernel/cgroup/cpuset-internal.h | 3 ---
> kernel/cgroup/cpuset.c | 41 +++++++++++----------------------
> 2 files changed, 13 insertions(+), 31 deletions(-)
>
> diff --git a/kernel/cgroup/cpuset-internal.h b/kernel/cgroup/cpuset-internal.h
> index 337608f408ce..5cac42c5fd97 100644
> --- a/kernel/cgroup/cpuset-internal.h
> +++ b/kernel/cgroup/cpuset-internal.h
> @@ -155,9 +155,6 @@ struct cpuset {
> /* for custom sched domain */
> int relax_domain_level;
>
> - /* number of valid local child partitions */
> - int nr_subparts;
> -
> /* partition root state */
> int partition_root_state;
>
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 52468d2c178a..7aef59ea9627 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -358,8 +358,13 @@ static inline bool is_in_v2_mode(void)
> * @excluded_child: a child cpuset to be excluded in task checking
> * Return: true if there are tasks, false otherwise
> *
> - * It is assumed that @cs is a valid partition root. @excluded_child should
> - * be non-NULL when this cpuset is going to become a partition itself.
> + * @cs should be a valid partition root or going to become a partition root.
> + * @excluded_child should be non-NULL when this cpuset is going to become a
> + * partition itself.
> + *
> + * Note that a remote partition is not allowed underneath a valid local
> + * or remote partition. So if a non-partition root child is populated,
> + * the whole partition is considered populated.
> */
> static inline bool partition_is_populated(struct cpuset *cs,
> struct cpuset *excluded_child)
> @@ -369,8 +374,6 @@ static inline bool partition_is_populated(struct cpuset *cs,
>
> if (cs->css.cgroup->nr_populated_csets)
> return true;
> - if (!excluded_child && !cs->nr_subparts)
> - return cgroup_is_populated(cs->css.cgroup);
>
> rcu_read_lock();
> cpuset_for_each_child(child, css, cs) {
> @@ -1302,7 +1305,6 @@ static void reset_partition_data(struct cpuset *cs)
>
> lockdep_assert_held(&callback_lock);
>
> - cs->nr_subparts = 0;
> if (cpumask_empty(cs->exclusive_cpus)) {
> cpumask_clear(cs->effective_xcpus);
> if (is_cpu_exclusive(cs))
> @@ -1746,7 +1748,6 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
> int deleting; /* Deleting cpus from parent's effective_cpus */
> int old_prs, new_prs;
> int part_error = PERR_NONE; /* Partition error? */
> - int subparts_delta = 0;
> int isolcpus_updated = 0;
> struct cpumask *xcpus = user_xcpus(cs);
> bool nocpu;
> @@ -1771,10 +1772,9 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
> if (is_partition_valid(parent))
> adding = cpumask_and(tmp->addmask,
> xcpus, parent->effective_xcpus);
> - if (old_prs > 0) {
> + if (old_prs > 0)
> new_prs = -old_prs;
> - subparts_delta--;
> - }
> +
> goto write_error;
> }
>
> @@ -1829,7 +1829,6 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
> WARN_ON_ONCE(!cpumask_subset(tmp->new_cpus, parent->effective_cpus));
>
> deleting = true;
> - subparts_delta++;
> } else if (cmd == partcmd_disable) {
> /*
> * May need to add cpus back to parent's effective_cpus
> @@ -1840,7 +1839,6 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
> if (is_partition_valid(cs)) {
> cpumask_copy(tmp->addmask, cs->effective_xcpus);
> adding = true;
> - subparts_delta--;
> }
> new_prs = PRS_MEMBER;
> } else if (newmask) {
> @@ -1963,17 +1961,13 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
> switch (cs->partition_root_state) {
> case PRS_ROOT:
> case PRS_ISOLATED:
> - if (part_error) {
> + if (part_error)
> new_prs = -old_prs;
> - subparts_delta--;
> - }
> break;
> case PRS_INVALID_ROOT:
> case PRS_INVALID_ISOLATED:
> - if (!part_error) {
> + if (!part_error)
> new_prs = -old_prs;
> - subparts_delta++;
> - }
> break;
> }
> }
> @@ -2002,11 +1996,9 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
> * newly deleted ones will be added back to effective_cpus.
> */
> spin_lock_irq(&callback_lock);
> - if (old_prs != new_prs) {
> + if (old_prs != new_prs)
> cs->partition_root_state = new_prs;
> - if (new_prs <= 0)
> - cs->nr_subparts = 0;
> - }
> +
> /*
> * Adding to parent's effective_cpus means deletion CPUs from cs
> * and vice versa.
> @@ -2018,10 +2010,6 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
> isolcpus_updated += partition_xcpus_add(new_prs, parent,
> tmp->delmask);
>
> - if (is_partition_valid(parent)) {
> - parent->nr_subparts += subparts_delta;
> - WARN_ON_ONCE(parent->nr_subparts < 0);
> - }
> spin_unlock_irq(&callback_lock);
> update_unbound_workqueue_cpumask(isolcpus_updated);
>
> @@ -2105,8 +2093,6 @@ static void compute_partition_effective_cpumask(struct cpuset *cs,
> */
> spin_lock_irq(&callback_lock);
> make_partition_invalid(child);
> - cs->nr_subparts--;
> - child->nr_subparts = 0;
> spin_unlock_irq(&callback_lock);
> notify_partition_change(child, old_prs);
> continue;
> @@ -4021,7 +4007,6 @@ static void cpuset_handle_hotplug(void)
> */
> if (!cpumask_empty(subpartitions_cpus)) {
> if (cpumask_subset(&new_cpus, subpartitions_cpus)) {
> - top_cpuset.nr_subparts = 0;
> cpumask_clear(subpartitions_cpus);
> } else {
> cpumask_andnot(&new_cpus, &new_cpus,
LGTM
--
Best regards,
Ridong
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