[PATCHv6 1/2] cgroup/cpuset: Introduce cpuset_cpus_allowed_locked()

[PATCHv6 1/2] cgroup/cpuset: Introduce cpuset_cpus_allowed_locked()

[Pingfan Liu]

cpuset_cpus_allowed() uses a reader lock that is sleepable under RT,
which means it cannot be called inside raw_spin_lock_t context.

Introduce a new cpuset_cpus_allowed_locked() helper that performs the
same function as cpuset_cpus_allowed() except that the caller must have
acquired the cpuset_mutex so that no further locking will be needed.

Suggested-by: Waiman Long <longman@redhat.com>
Signed-off-by: Pingfan Liu <piliu@redhat.com>
Cc: Waiman Long <longman@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: "Michal Koutný" <mkoutny@suse.com>
Cc: linux-kernel@vger.kernel.org
To: cgroups@vger.kernel.org
---
 include/linux/cpuset.h |  1 +
 kernel/cgroup/cpuset.c | 51 +++++++++++++++++++++++++++++-------------
 2 files changed, 37 insertions(+), 15 deletions(-)

diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
index 2ddb256187b51..e057a3123791e 100644
--- a/include/linux/cpuset.h
+++ b/include/linux/cpuset.h
@@ -75,6 +75,7 @@ extern void dec_dl_tasks_cs(struct task_struct *task);
 extern void cpuset_lock(void);
 extern void cpuset_unlock(void);
 extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
+extern void cpuset_cpus_allowed_locked(struct task_struct *p, struct cpumask *mask);
 extern bool cpuset_cpus_allowed_fallback(struct task_struct *p);
 extern bool cpuset_cpu_is_isolated(int cpu);
 extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index 52468d2c178a3..7a179a1a2e30a 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -4116,24 +4116,13 @@ void __init cpuset_init_smp(void)
 	BUG_ON(!cpuset_migrate_mm_wq);
 }
 
-/**
- * cpuset_cpus_allowed - return cpus_allowed mask from a tasks cpuset.
- * @tsk: pointer to task_struct from which to obtain cpuset->cpus_allowed.
- * @pmask: pointer to struct cpumask variable to receive cpus_allowed set.
- *
- * Description: Returns the cpumask_var_t cpus_allowed of the cpuset
- * attached to the specified @tsk.  Guaranteed to return some non-empty
- * subset of cpu_active_mask, even if this means going outside the
- * tasks cpuset, except when the task is in the top cpuset.
- **/
-
-void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
+/*
+ * Return cpus_allowed mask from a task's cpuset.
+ */
+static void __cpuset_cpus_allowed_locked(struct task_struct *tsk, struct cpumask *pmask)
 {
-	unsigned long flags;
 	struct cpuset *cs;
 
-	spin_lock_irqsave(&callback_lock, flags);
-
 	cs = task_cs(tsk);
 	if (cs != &top_cpuset)
 		guarantee_active_cpus(tsk, pmask);
@@ -4153,7 +4142,39 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
 		if (!cpumask_intersects(pmask, cpu_active_mask))
 			cpumask_copy(pmask, possible_mask);
 	}
+}
 
+/**
+ * cpuset_cpus_allowed_locked - return cpus_allowed mask from a task's cpuset.
+ * @tsk: pointer to task_struct from which to obtain cpuset->cpus_allowed.
+ * @pmask: pointer to struct cpumask variable to receive cpus_allowed set.
+ *
+ * Similir to cpuset_cpus_allowed() except that the caller must have acquired
+ * cpuset_mutex.
+ */
+void cpuset_cpus_allowed_locked(struct task_struct *tsk, struct cpumask *pmask)
+{
+	lockdep_assert_held(&cpuset_mutex);
+	__cpuset_cpus_allowed_locked(tsk, pmask);
+}
+
+/**
+ * cpuset_cpus_allowed - return cpus_allowed mask from a task's cpuset.
+ * @tsk: pointer to task_struct from which to obtain cpuset->cpus_allowed.
+ * @pmask: pointer to struct cpumask variable to receive cpus_allowed set.
+ *
+ * Description: Returns the cpumask_var_t cpus_allowed of the cpuset
+ * attached to the specified @tsk.  Guaranteed to return some non-empty
+ * subset of cpu_active_mask, even if this means going outside the
+ * tasks cpuset, except when the task is in the top cpuset.
+ **/
+
+void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&callback_lock, flags);
+	__cpuset_cpus_allowed_locked(tsk, pmask);
 	spin_unlock_irqrestore(&callback_lock, flags);
 }
 
-- 
2.49.0

Re: [PATCHv6 1/2] cgroup/cpuset: Introduce cpuset_cpus_allowed_locked()

[Waiman Long]

On 11/17/25 4:27 AM, Pingfan Liu wrote:
> cpuset_cpus_allowed() uses a reader lock that is sleepable under RT,
> which means it cannot be called inside raw_spin_lock_t context.
>
> Introduce a new cpuset_cpus_allowed_locked() helper that performs the
> same function as cpuset_cpus_allowed() except that the caller must have
> acquired the cpuset_mutex so that no further locking will be needed.
>
> Suggested-by: Waiman Long <longman@redhat.com>
> Signed-off-by: Pingfan Liu <piliu@redhat.com>
> Cc: Waiman Long <longman@redhat.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Johannes Weiner <hannes@cmpxchg.org>
> Cc: "Michal Koutný" <mkoutny@suse.com>
> Cc: linux-kernel@vger.kernel.org
> To: cgroups@vger.kernel.org
> ---
>   include/linux/cpuset.h |  1 +
>   kernel/cgroup/cpuset.c | 51 +++++++++++++++++++++++++++++-------------
>   2 files changed, 37 insertions(+), 15 deletions(-)
>
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index 2ddb256187b51..e057a3123791e 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -75,6 +75,7 @@ extern void dec_dl_tasks_cs(struct task_struct *task);
>   extern void cpuset_lock(void);
>   extern void cpuset_unlock(void);
>   extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
> +extern void cpuset_cpus_allowed_locked(struct task_struct *p, struct cpumask *mask);
>   extern bool cpuset_cpus_allowed_fallback(struct task_struct *p);
>   extern bool cpuset_cpu_is_isolated(int cpu);
>   extern nodemask_t cpuset_mems_allowed(struct task_struct *p);

Ah, the following code should be added to to !CONFIG_CPUSETS section 
after cpuset_cpus_allowed().

#define cpuset_cpus_allowed_locked(p, m)  cpuset_cpus_allowed(p, m)

Or you can add another inline function that just calls 
cpuset_cpus_allowed().

Cheers,
Longman

Re: [PATCHv6 1/2] cgroup/cpuset: Introduce cpuset_cpus_allowed_locked()

[Pingfan Liu]

On Tue, Nov 18, 2025 at 4:37 AM Waiman Long <llong@redhat.com> wrote:
>
> On 11/17/25 4:27 AM, Pingfan Liu wrote:
> > cpuset_cpus_allowed() uses a reader lock that is sleepable under RT,
> > which means it cannot be called inside raw_spin_lock_t context.
> >
> > Introduce a new cpuset_cpus_allowed_locked() helper that performs the
> > same function as cpuset_cpus_allowed() except that the caller must have
> > acquired the cpuset_mutex so that no further locking will be needed.
> >
> > Suggested-by: Waiman Long <longman@redhat.com>
> > Signed-off-by: Pingfan Liu <piliu@redhat.com>
> > Cc: Waiman Long <longman@redhat.com>
> > Cc: Tejun Heo <tj@kernel.org>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: "Michal Koutný" <mkoutny@suse.com>
> > Cc: linux-kernel@vger.kernel.org
> > To: cgroups@vger.kernel.org
> > ---
> >   include/linux/cpuset.h |  1 +
> >   kernel/cgroup/cpuset.c | 51 +++++++++++++++++++++++++++++-------------
> >   2 files changed, 37 insertions(+), 15 deletions(-)
> >
> > diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> > index 2ddb256187b51..e057a3123791e 100644
> > --- a/include/linux/cpuset.h
> > +++ b/include/linux/cpuset.h
> > @@ -75,6 +75,7 @@ extern void dec_dl_tasks_cs(struct task_struct *task);
> >   extern void cpuset_lock(void);
> >   extern void cpuset_unlock(void);
> >   extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
> > +extern void cpuset_cpus_allowed_locked(struct task_struct *p, struct cpumask *mask);
> >   extern bool cpuset_cpus_allowed_fallback(struct task_struct *p);
> >   extern bool cpuset_cpu_is_isolated(int cpu);
> >   extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
>
> Ah, the following code should be added to to !CONFIG_CPUSETS section
> after cpuset_cpus_allowed().
>
> #define cpuset_cpus_allowed_locked(p, m)  cpuset_cpus_allowed(p, m)
>
> Or you can add another inline function that just calls
> cpuset_cpus_allowed().
>

It may be better to make cpuset_cpus_allowed() call
cpuset_cpus_allowed_locked(), following the call chain used under
CONFIG_CPUSETS case.

Thanks,

Pingfan

Re: [PATCHv6 1/2] cgroup/cpuset: Introduce cpuset_cpus_allowed_locked()

[Waiman Long]

On 11/18/25 1:30 AM, Pingfan Liu wrote:
> On Tue, Nov 18, 2025 at 4:37 AM Waiman Long <llong@redhat.com> wrote:
>> On 11/17/25 4:27 AM, Pingfan Liu wrote:
>>> cpuset_cpus_allowed() uses a reader lock that is sleepable under RT,
>>> which means it cannot be called inside raw_spin_lock_t context.
>>>
>>> Introduce a new cpuset_cpus_allowed_locked() helper that performs the
>>> same function as cpuset_cpus_allowed() except that the caller must have
>>> acquired the cpuset_mutex so that no further locking will be needed.
>>>
>>> Suggested-by: Waiman Long <longman@redhat.com>
>>> Signed-off-by: Pingfan Liu <piliu@redhat.com>
>>> Cc: Waiman Long <longman@redhat.com>
>>> Cc: Tejun Heo <tj@kernel.org>
>>> Cc: Johannes Weiner <hannes@cmpxchg.org>
>>> Cc: "Michal Koutný" <mkoutny@suse.com>
>>> Cc: linux-kernel@vger.kernel.org
>>> To: cgroups@vger.kernel.org
>>> ---
>>>    include/linux/cpuset.h |  1 +
>>>    kernel/cgroup/cpuset.c | 51 +++++++++++++++++++++++++++++-------------
>>>    2 files changed, 37 insertions(+), 15 deletions(-)
>>>
>>> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
>>> index 2ddb256187b51..e057a3123791e 100644
>>> --- a/include/linux/cpuset.h
>>> +++ b/include/linux/cpuset.h
>>> @@ -75,6 +75,7 @@ extern void dec_dl_tasks_cs(struct task_struct *task);
>>>    extern void cpuset_lock(void);
>>>    extern void cpuset_unlock(void);
>>>    extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
>>> +extern void cpuset_cpus_allowed_locked(struct task_struct *p, struct cpumask *mask);
>>>    extern bool cpuset_cpus_allowed_fallback(struct task_struct *p);
>>>    extern bool cpuset_cpu_is_isolated(int cpu);
>>>    extern nodemask_t cpuset_mems_allowed(struct task_struct *p);
>> Ah, the following code should be added to to !CONFIG_CPUSETS section
>> after cpuset_cpus_allowed().
>>
>> #define cpuset_cpus_allowed_locked(p, m)  cpuset_cpus_allowed(p, m)
>>
>> Or you can add another inline function that just calls
>> cpuset_cpus_allowed().
>>
> It may be better to make cpuset_cpus_allowed() call
> cpuset_cpus_allowed_locked(), following the call chain used under
> CONFIG_CPUSETS case.

That is fine too.

Cheers,
Longman

>
> Thanks,
>
> Pingfan
>