Message-ID: <ffe1d7ec-70fc-44cd-879c-23902929a24a@huaweicloud.com>
Date: Mon, 3 Nov 2025 14:46:23 +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>, Gabriele Monaco <gmonaco@...hat.com>,
 Frederic Weisbecker <frederic@...nel.org>
Subject: Re: [cgroup/for-6.19 PATCH 3/3] cgroup/cpuset: Globally track
 isolated_cpus update



On 2025/11/3 9:34, Waiman Long wrote:
> The current cpuset code passes a local isolcpus_updated flag around in a
> number of functions to determine if external isolation related cpumasks
> like wq_unbound_cpumask should be updated. It is a bit cumbersome and
> makes the code more complex. Simplify the code by using a global boolean

Agree.

> flag "isolated_cpus_updating" to track this. This flag will be set in
> isolated_cpus_update() and cleared in update_isolation_cpumasks().
> 
> No functional change is expected.
> 
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>  kernel/cgroup/cpuset.c | 74 ++++++++++++++++++++----------------------
>  1 file changed, 35 insertions(+), 39 deletions(-)
> 
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index d6d459c95d82..406a1c3789f5 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -81,6 +81,13 @@ static cpumask_var_t	subpartitions_cpus;
>   */
>  static cpumask_var_t	isolated_cpus;
>  

Is isolated_cpus protected by cpuset_mutex or callback_lock?

If isolated_cpus is indeed protected by cpuset_mutex, perhaps we can move the update of
isolated_cpus outside the critical section of callback_lock. This would allow us to call
update_isolation_cpumasks in isolated_cpus_update, making the isolated_cpus_updating variable
unnecessary. Reducing a global variable would be beneficial.

> +/*
> + * isolated_cpus updating flag (protected by cpuset_mutex)
> + * Set if isolated_cpus is going to be updated in the current
> + * cpuset_mutex crtical section.
> + */
> +static bool isolated_cpus_updating;
> +
>  /*
>   * Housekeeping (HK_TYPE_DOMAIN) CPUs at boot
>   */
> @@ -1327,13 +1334,14 @@ static void isolated_cpus_update(int old_prs, int new_prs, struct cpumask *xcpus
>  		cpumask_or(isolated_cpus, isolated_cpus, xcpus);
>  	else
>  		cpumask_andnot(isolated_cpus, isolated_cpus, xcpus);
> +
> +	isolated_cpus_updating = true;
>  }
>  
>  /*
>   * isolated_cpus_should_update - Returns if the isolated_cpus mask needs update
>   * @prs: new or old partition_root_state
>   * @parent: parent cpuset
> - * Return: true if isolated_cpus needs modification, false otherwise
>   */
>  static bool isolated_cpus_should_update(int prs, struct cpuset *parent)
>  {
> @@ -1347,15 +1355,12 @@ static bool isolated_cpus_should_update(int prs, struct cpuset *parent)
>   * @new_prs: new partition_root_state
>   * @parent: parent cpuset
>   * @xcpus: exclusive CPUs to be added
> - * Return: true if isolated_cpus modified, false otherwise
>   *
>   * Remote partition if parent == NULL
>   */
> -static bool partition_xcpus_add(int new_prs, struct cpuset *parent,
> +static void partition_xcpus_add(int new_prs, struct cpuset *parent,
>  				struct cpumask *xcpus)
>  {
> -	bool isolcpus_updated;
> -
>  	WARN_ON_ONCE(new_prs < 0);
>  	lockdep_assert_held(&callback_lock);
>  	if (!parent)
> @@ -1365,13 +1370,11 @@ static bool partition_xcpus_add(int new_prs, struct cpuset *parent,
>  	if (parent == &top_cpuset)
>  		cpumask_or(subpartitions_cpus, subpartitions_cpus, xcpus);
>  
> -	isolcpus_updated = (new_prs != parent->partition_root_state);
> -	if (isolcpus_updated)
> +	if (new_prs != parent->partition_root_state)

Can this if statement be replaced with new helper isolated_cpus_should_update？

>  		isolated_cpus_update(parent->partition_root_state, new_prs,
>  				     xcpus);
>  
>  	cpumask_andnot(parent->effective_cpus, parent->effective_cpus, xcpus);
> -	return isolcpus_updated;
>  }
>  
>  /*
> @@ -1379,15 +1382,12 @@ static bool partition_xcpus_add(int new_prs, struct cpuset *parent,
>   * @old_prs: old partition_root_state
>   * @parent: parent cpuset
>   * @xcpus: exclusive CPUs to be removed
> - * Return: true if isolated_cpus modified, false otherwise
>   *
>   * Remote partition if parent == NULL
>   */
> -static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
> +static void partition_xcpus_del(int old_prs, struct cpuset *parent,
>  				struct cpumask *xcpus)
>  {
> -	bool isolcpus_updated;
> -
>  	WARN_ON_ONCE(old_prs < 0);
>  	lockdep_assert_held(&callback_lock);
>  	if (!parent)
> @@ -1396,14 +1396,12 @@ static bool partition_xcpus_del(int old_prs, struct cpuset *parent,
>  	if (parent == &top_cpuset)
>  		cpumask_andnot(subpartitions_cpus, subpartitions_cpus, xcpus);
>  
> -	isolcpus_updated = (old_prs != parent->partition_root_state);
> -	if (isolcpus_updated)
> +	if (old_prs != parent->partition_root_state)
>  		isolated_cpus_update(old_prs, parent->partition_root_state,
>  				     xcpus);
>  

Ditto

>  	cpumask_and(xcpus, xcpus, cpu_active_mask);
>  	cpumask_or(parent->effective_cpus, parent->effective_cpus, xcpus);
> -	return isolcpus_updated;
>  }
>  
>  /*
> @@ -1442,17 +1440,24 @@ static bool isolated_cpus_can_update(struct cpumask *add_cpus,
>  	return res;
>  }
>  
> -static void update_isolation_cpumasks(bool isolcpus_updated)
> +/*
> + * update_isolation_cpumasks - Update external isolation related CPU masks
> + *
> + * The following external CPU masks will be updated if necessary:
> + * - workqueue unbound cpumask
> + */
> +static void update_isolation_cpumasks(void)
>  {
>  	int ret;
>  
> -	lockdep_assert_cpus_held();
> -
> -	if (!isolcpus_updated)
> +	if (!isolated_cpus_updating)
>  		return;
>  
> +	lockdep_assert_cpus_held();
> +
>  	ret = workqueue_unbound_exclude_cpumask(isolated_cpus);
>  	WARN_ON_ONCE(ret < 0);
> +	isolated_cpus_updating = false;
>  }
>  
>  /**
> @@ -1577,8 +1582,6 @@ static inline bool is_local_partition(struct cpuset *cs)
>  static int remote_partition_enable(struct cpuset *cs, int new_prs,
>  				   struct tmpmasks *tmp)
>  {
> -	bool isolcpus_updated;
> -
>  	/*
>  	 * The user must have sysadmin privilege.
>  	 */
> @@ -1605,11 +1608,11 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
>  		return PERR_HKEEPING;
>  
>  	spin_lock_irq(&callback_lock);
> -	isolcpus_updated = partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
> +	partition_xcpus_add(new_prs, NULL, tmp->new_cpus);
>  	list_add(&cs->remote_sibling, &remote_children);
>  	cpumask_copy(cs->effective_xcpus, tmp->new_cpus);
>  	spin_unlock_irq(&callback_lock);
> -	update_isolation_cpumasks(isolcpus_updated);
> +	update_isolation_cpumasks();
>  	cpuset_force_rebuild();
>  	cs->prs_err = 0;
>  
> @@ -1632,15 +1635,12 @@ static int remote_partition_enable(struct cpuset *cs, int new_prs,
>   */
>  static void remote_partition_disable(struct cpuset *cs, struct tmpmasks *tmp)
>  {
> -	bool isolcpus_updated;
> -
>  	WARN_ON_ONCE(!is_remote_partition(cs));
>  	WARN_ON_ONCE(!cpumask_subset(cs->effective_xcpus, subpartitions_cpus));
>  
>  	spin_lock_irq(&callback_lock);
>  	list_del_init(&cs->remote_sibling);
> -	isolcpus_updated = partition_xcpus_del(cs->partition_root_state,
> -					       NULL, cs->effective_xcpus);
> +	partition_xcpus_del(cs->partition_root_state, NULL, cs->effective_xcpus);
>  	if (cs->prs_err)
>  		cs->partition_root_state = -cs->partition_root_state;
>  	else
> @@ -1650,7 +1650,7 @@ static void remote_partition_disable(struct cpuset *cs, struct tmpmasks *tmp)
>  	compute_excpus(cs, cs->effective_xcpus);
>  	reset_partition_data(cs);
>  	spin_unlock_irq(&callback_lock);
> -	update_isolation_cpumasks(isolcpus_updated);
> +	update_isolation_cpumasks();
>  	cpuset_force_rebuild();
>  
>  	/*
> @@ -1675,7 +1675,6 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
>  {
>  	bool adding, deleting;
>  	int prs = cs->partition_root_state;
> -	int isolcpus_updated = 0;
>  
>  	if (WARN_ON_ONCE(!is_remote_partition(cs)))
>  		return;
> @@ -1711,9 +1710,9 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
>  
>  	spin_lock_irq(&callback_lock);
>  	if (adding)
> -		isolcpus_updated += partition_xcpus_add(prs, NULL, tmp->addmask);
> +		partition_xcpus_add(prs, NULL, tmp->addmask);
>  	if (deleting)
> -		isolcpus_updated += partition_xcpus_del(prs, NULL, tmp->delmask);
> +		partition_xcpus_del(prs, NULL, tmp->delmask);
>  	/*
>  	 * Need to update effective_xcpus and exclusive_cpus now as
>  	 * update_sibling_cpumasks() below may iterate back to the same cs.
> @@ -1722,7 +1721,7 @@ static void remote_cpus_update(struct cpuset *cs, struct cpumask *xcpus,
>  	if (xcpus)
>  		cpumask_copy(cs->exclusive_cpus, xcpus);
>  	spin_unlock_irq(&callback_lock);
> -	update_isolation_cpumasks(isolcpus_updated);
> +	update_isolation_cpumasks();
>  	if (adding || deleting)
>  		cpuset_force_rebuild();
>  
> @@ -1803,7 +1802,6 @@ 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 isolcpus_updated = 0;
>  	struct cpumask *xcpus = user_xcpus(cs);
>  	bool nocpu;
>  
> @@ -2065,14 +2063,12 @@ static int update_parent_effective_cpumask(struct cpuset *cs, int cmd,
>  	 * and vice versa.
>  	 */
>  	if (adding)
> -		isolcpus_updated += partition_xcpus_del(old_prs, parent,
> -							tmp->addmask);
> +		partition_xcpus_del(old_prs, parent, tmp->addmask);
>  	if (deleting)
> -		isolcpus_updated += partition_xcpus_add(new_prs, parent,
> -							tmp->delmask);
> +		partition_xcpus_add(new_prs, parent, tmp->delmask);
>  
>  	spin_unlock_irq(&callback_lock);
> -	update_isolation_cpumasks(isolcpus_updated);
> +	update_isolation_cpumasks();
>  
>  	if ((old_prs != new_prs) && (cmd == partcmd_update))
>  		update_partition_exclusive_flag(cs, new_prs);
> @@ -3094,7 +3090,7 @@ static int update_prstate(struct cpuset *cs, int new_prs)
>  	else if (isolcpus_updated)
>  		isolated_cpus_update(old_prs, new_prs, cs->effective_xcpus);
>  	spin_unlock_irq(&callback_lock);
> -	update_isolation_cpumasks(isolcpus_updated);
> +	update_isolation_cpumasks();
>  
>  	/* Force update if switching back to member & update effective_xcpus */
>  	update_cpumasks_hier(cs, &tmpmask, !new_prs);

-- 
Best regards,
Ridong

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