Commit 16b269436b72 ("sched/deadline: Modify cpudl::free_cpus
to reflect rd->online") introduced the cpudl_set/clear_freecpu
functions to allow the cpu_dl::free_cpus mask to be manipulated
by the deadline scheduler class rq_on/offline callbacks so the
mask would also reflect this state.

Commit 9659e1eeee28 ("sched/deadline: Remove cpu_active_mask
from cpudl_find()") removed the check of the cpu_active_mask to
save some processing on the premise that the cpudl::free_cpus
mask already reflected the runqueue online state.

Unfortunately, there are cases where it is possible for the
cpudl_clear function to set the free_cpus bit for a CPU when the
deadline runqueue is offline. When this occurs while a CPU is
connected to the default root domain the flag may retain the bad
state after the CPU has been unplugged. Later, a different CPU
that is transitioning through the default root domain may push a
deadline task to the powered down CPU when cpudl_find sees its
free_cpus bit is set. If this happens the task will not have the
opportunity to run.

One example is outlined here:
https://lore.kernel.org/lkml/20250110233010.2339521-1-opendmb@gmail.com

Another occurs when the last deadline task is migrated from a
CPU that has an offlined runqueue. The dequeue_task member of
the deadline scheduler class will eventually call cpudl_clear
and set the free_cpus bit for the CPU.

This commit modifies the cpudl_clear function to be aware of the
online state of the deadline runqueue so that the free_cpus mask
can be updated appropriately.

It is no longer necessary to manage the mask outside of the
cpudl_set/clear functions so the cpudl_set/clear_freecpu
functions are removed. In addition, since the free_cpus mask is
now only updated under the cpudl lock the code was changed to
use the non-atomic __cpumask functions.

Signed-off-by: Doug Berger <opendmb@gmail.com>
---
 kernel/sched/cpudeadline.c | 34 +++++++++-------------------------
 kernel/sched/cpudeadline.h |  4 +---
 kernel/sched/deadline.c    |  8 ++++----
 3 files changed, 14 insertions(+), 32 deletions(-)

diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
index cdd740b3f774..d612d5c6c61a 100644
--- a/kernel/sched/cpudeadline.c
+++ b/kernel/sched/cpudeadline.c
@@ -166,12 +166,13 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
  * cpudl_clear - remove a CPU from the cpudl max-heap
  * @cp: the cpudl max-heap context
  * @cpu: the target CPU
+ * @online: the online state of the deadline runqueue
  *
  * Notes: assumes cpu_rq(cpu)->lock is locked
  *
  * Returns: (void)
  */
-void cpudl_clear(struct cpudl *cp, int cpu)
+void cpudl_clear(struct cpudl *cp, int cpu, bool online)
 {
 	int old_idx, new_cpu;
 	unsigned long flags;
@@ -184,7 +185,7 @@ void cpudl_clear(struct cpudl *cp, int cpu)
 	if (old_idx == IDX_INVALID) {
 		/*
 		 * Nothing to remove if old_idx was invalid.
-		 * This could happen if a rq_offline_dl is
+		 * This could happen if rq_online_dl or rq_offline_dl is
 		 * called for a CPU without -dl tasks running.
 		 */
 	} else {
@@ -195,9 +196,12 @@ void cpudl_clear(struct cpudl *cp, int cpu)
 		cp->elements[new_cpu].idx = old_idx;
 		cp->elements[cpu].idx = IDX_INVALID;
 		cpudl_heapify(cp, old_idx);
-
-		cpumask_set_cpu(cpu, cp->free_cpus);
 	}
+	if (unlikely(!online))
+		__cpumask_clear_cpu(cpu, cp->free_cpus);
+	else
+		__cpumask_set_cpu(cpu, cp->free_cpus);
+
 	raw_spin_unlock_irqrestore(&cp->lock, flags);
 }
 
@@ -228,7 +232,7 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl)
 		cp->elements[new_idx].cpu = cpu;
 		cp->elements[cpu].idx = new_idx;
 		cpudl_heapify_up(cp, new_idx);
-		cpumask_clear_cpu(cpu, cp->free_cpus);
+		__cpumask_clear_cpu(cpu, cp->free_cpus);
 	} else {
 		cp->elements[old_idx].dl = dl;
 		cpudl_heapify(cp, old_idx);
@@ -237,26 +241,6 @@ void cpudl_set(struct cpudl *cp, int cpu, u64 dl)
 	raw_spin_unlock_irqrestore(&cp->lock, flags);
 }
 
-/*
- * cpudl_set_freecpu - Set the cpudl.free_cpus
- * @cp: the cpudl max-heap context
- * @cpu: rd attached CPU
- */
-void cpudl_set_freecpu(struct cpudl *cp, int cpu)
-{
-	cpumask_set_cpu(cpu, cp->free_cpus);
-}
-
-/*
- * cpudl_clear_freecpu - Clear the cpudl.free_cpus
- * @cp: the cpudl max-heap context
- * @cpu: rd attached CPU
- */
-void cpudl_clear_freecpu(struct cpudl *cp, int cpu)
-{
-	cpumask_clear_cpu(cpu, cp->free_cpus);
-}
-
 /*
  * cpudl_init - initialize the cpudl structure
  * @cp: the cpudl max-heap context
diff --git a/kernel/sched/cpudeadline.h b/kernel/sched/cpudeadline.h
index 11c0f1faa7e1..d7699468eedd 100644
--- a/kernel/sched/cpudeadline.h
+++ b/kernel/sched/cpudeadline.h
@@ -19,8 +19,6 @@ struct cpudl {
 
 int  cpudl_find(struct cpudl *cp, struct task_struct *p, struct cpumask *later_mask);
 void cpudl_set(struct cpudl *cp, int cpu, u64 dl);
-void cpudl_clear(struct cpudl *cp, int cpu);
+void cpudl_clear(struct cpudl *cp, int cpu, bool online);
 int  cpudl_init(struct cpudl *cp);
-void cpudl_set_freecpu(struct cpudl *cp, int cpu);
-void cpudl_clear_freecpu(struct cpudl *cp, int cpu);
 void cpudl_cleanup(struct cpudl *cp);
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index e2d51f4306b3..ef91871b14d8 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1832,7 +1832,7 @@ static void dec_dl_deadline(struct dl_rq *dl_rq, u64 deadline)
 	if (!dl_rq->dl_nr_running) {
 		dl_rq->earliest_dl.curr = 0;
 		dl_rq->earliest_dl.next = 0;
-		cpudl_clear(&rq->rd->cpudl, rq->cpu);
+		cpudl_clear(&rq->rd->cpudl, rq->cpu, rq->online);
 		cpupri_set(&rq->rd->cpupri, rq->cpu, rq->rt.highest_prio.curr);
 	} else {
 		struct rb_node *leftmost = rb_first_cached(&dl_rq->root);
@@ -2878,9 +2878,10 @@ static void rq_online_dl(struct rq *rq)
 	if (rq->dl.overloaded)
 		dl_set_overload(rq);
 
-	cpudl_set_freecpu(&rq->rd->cpudl, rq->cpu);
 	if (rq->dl.dl_nr_running > 0)
 		cpudl_set(&rq->rd->cpudl, rq->cpu, rq->dl.earliest_dl.curr);
+	else
+		cpudl_clear(&rq->rd->cpudl, rq->cpu, true);
 }
 
 /* Assumes rq->lock is held */
@@ -2889,8 +2890,7 @@ static void rq_offline_dl(struct rq *rq)
 	if (rq->dl.overloaded)
 		dl_clear_overload(rq);
 
-	cpudl_clear(&rq->rd->cpudl, rq->cpu);
-	cpudl_clear_freecpu(&rq->rd->cpudl, rq->cpu);
+	cpudl_clear(&rq->rd->cpudl, rq->cpu, false);
 }
 
 void __init init_sched_dl_class(void)
-- 
2.34.1

Hi!

On 11/08/25 12:05, Doug Berger wrote:
> Commit 16b269436b72 ("sched/deadline: Modify cpudl::free_cpus
> to reflect rd->online") introduced the cpudl_set/clear_freecpu
> functions to allow the cpu_dl::free_cpus mask to be manipulated
> by the deadline scheduler class rq_on/offline callbacks so the
> mask would also reflect this state.
> 
> Commit 9659e1eeee28 ("sched/deadline: Remove cpu_active_mask
> from cpudl_find()") removed the check of the cpu_active_mask to
> save some processing on the premise that the cpudl::free_cpus
> mask already reflected the runqueue online state.
> 
> Unfortunately, there are cases where it is possible for the
> cpudl_clear function to set the free_cpus bit for a CPU when the
> deadline runqueue is offline. When this occurs while a CPU is
> connected to the default root domain the flag may retain the bad
> state after the CPU has been unplugged. Later, a different CPU
> that is transitioning through the default root domain may push a
> deadline task to the powered down CPU when cpudl_find sees its
> free_cpus bit is set. If this happens the task will not have the
> opportunity to run.
> 
> One example is outlined here:
> https://lore.kernel.org/lkml/20250110233010.2339521-1-opendmb@gmail.com
> 
> Another occurs when the last deadline task is migrated from a
> CPU that has an offlined runqueue. The dequeue_task member of
> the deadline scheduler class will eventually call cpudl_clear
> and set the free_cpus bit for the CPU.
> 
> This commit modifies the cpudl_clear function to be aware of the
> online state of the deadline runqueue so that the free_cpus mask
> can be updated appropriately.
> 
> It is no longer necessary to manage the mask outside of the
> cpudl_set/clear functions so the cpudl_set/clear_freecpu
> functions are removed. In addition, since the free_cpus mask is
> now only updated under the cpudl lock the code was changed to
> use the non-atomic __cpumask functions.
> 
> Signed-off-by: Doug Berger <opendmb@gmail.com>
> ---

Thanks for this new approach, it looks good to me.

I would like to stress test it a little more, and have a comment below.

>  kernel/sched/cpudeadline.c | 34 +++++++++-------------------------
>  kernel/sched/cpudeadline.h |  4 +---
>  kernel/sched/deadline.c    |  8 ++++----
>  3 files changed, 14 insertions(+), 32 deletions(-)
> 
> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
> index cdd740b3f774..d612d5c6c61a 100644
> --- a/kernel/sched/cpudeadline.c
> +++ b/kernel/sched/cpudeadline.c
> @@ -166,12 +166,13 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
>   * cpudl_clear - remove a CPU from the cpudl max-heap
>   * @cp: the cpudl max-heap context
>   * @cpu: the target CPU
> + * @online: the online state of the deadline runqueue
>   *
>   * Notes: assumes cpu_rq(cpu)->lock is locked
>   *
>   * Returns: (void)
>   */
> -void cpudl_clear(struct cpudl *cp, int cpu)
> +void cpudl_clear(struct cpudl *cp, int cpu, bool online)
>  {
>  	int old_idx, new_cpu;
>  	unsigned long flags;
> @@ -184,7 +185,7 @@ void cpudl_clear(struct cpudl *cp, int cpu)
>  	if (old_idx == IDX_INVALID) {
>  		/*
>  		 * Nothing to remove if old_idx was invalid.
> -		 * This could happen if a rq_offline_dl is
> +		 * This could happen if rq_online_dl or rq_offline_dl is
>  		 * called for a CPU without -dl tasks running.
>  		 */
>  	} else {
> @@ -195,9 +196,12 @@ void cpudl_clear(struct cpudl *cp, int cpu)
>  		cp->elements[new_cpu].idx = old_idx;
>  		cp->elements[cpu].idx = IDX_INVALID;
>  		cpudl_heapify(cp, old_idx);
> -
> -		cpumask_set_cpu(cpu, cp->free_cpus);
>  	}
> +	if (unlikely(!online))

Isn't using likely(online) more direct and cleaner? :)

> +		__cpumask_clear_cpu(cpu, cp->free_cpus);
> +	else
> +		__cpumask_set_cpu(cpu, cp->free_cpus);
> +
>  	raw_spin_unlock_irqrestore(&cp->lock, flags);
>  }

Best,
Juri

On 8/12/2025 8:14 AM, Juri Lelli wrote:
> Hi!
> 
> On 11/08/25 12:05, Doug Berger wrote:
>> Commit 16b269436b72 ("sched/deadline: Modify cpudl::free_cpus
>> to reflect rd->online") introduced the cpudl_set/clear_freecpu
>> functions to allow the cpu_dl::free_cpus mask to be manipulated
>> by the deadline scheduler class rq_on/offline callbacks so the
>> mask would also reflect this state.
>>
>> Commit 9659e1eeee28 ("sched/deadline: Remove cpu_active_mask
>> from cpudl_find()") removed the check of the cpu_active_mask to
>> save some processing on the premise that the cpudl::free_cpus
>> mask already reflected the runqueue online state.
>>
>> Unfortunately, there are cases where it is possible for the
>> cpudl_clear function to set the free_cpus bit for a CPU when the
>> deadline runqueue is offline. When this occurs while a CPU is
>> connected to the default root domain the flag may retain the bad
>> state after the CPU has been unplugged. Later, a different CPU
>> that is transitioning through the default root domain may push a
>> deadline task to the powered down CPU when cpudl_find sees its
>> free_cpus bit is set. If this happens the task will not have the
>> opportunity to run.
>>
>> One example is outlined here:
>> https://lore.kernel.org/lkml/20250110233010.2339521-1-opendmb@gmail.com
>>
>> Another occurs when the last deadline task is migrated from a
>> CPU that has an offlined runqueue. The dequeue_task member of
>> the deadline scheduler class will eventually call cpudl_clear
>> and set the free_cpus bit for the CPU.
>>
>> This commit modifies the cpudl_clear function to be aware of the
>> online state of the deadline runqueue so that the free_cpus mask
>> can be updated appropriately.
>>
>> It is no longer necessary to manage the mask outside of the
>> cpudl_set/clear functions so the cpudl_set/clear_freecpu
>> functions are removed. In addition, since the free_cpus mask is
>> now only updated under the cpudl lock the code was changed to
>> use the non-atomic __cpumask functions.
>>
>> Signed-off-by: Doug Berger <opendmb@gmail.com>
>> ---
> 
> Thanks for this new approach, it looks good to me.
> 
> I would like to stress test it a little more, and have a comment below.
> 
Thanks for the review. I appreciate any testing you are willing to do.

>>   kernel/sched/cpudeadline.c | 34 +++++++++-------------------------
>>   kernel/sched/cpudeadline.h |  4 +---
>>   kernel/sched/deadline.c    |  8 ++++----
>>   3 files changed, 14 insertions(+), 32 deletions(-)
>>
>> diff --git a/kernel/sched/cpudeadline.c b/kernel/sched/cpudeadline.c
>> index cdd740b3f774..d612d5c6c61a 100644
>> --- a/kernel/sched/cpudeadline.c
>> +++ b/kernel/sched/cpudeadline.c
>> @@ -166,12 +166,13 @@ int cpudl_find(struct cpudl *cp, struct task_struct *p,
>>    * cpudl_clear - remove a CPU from the cpudl max-heap
>>    * @cp: the cpudl max-heap context
>>    * @cpu: the target CPU
>> + * @online: the online state of the deadline runqueue
>>    *
>>    * Notes: assumes cpu_rq(cpu)->lock is locked
>>    *
>>    * Returns: (void)
>>    */
>> -void cpudl_clear(struct cpudl *cp, int cpu)
>> +void cpudl_clear(struct cpudl *cp, int cpu, bool online)
>>   {
>>   	int old_idx, new_cpu;
>>   	unsigned long flags;
>> @@ -184,7 +185,7 @@ void cpudl_clear(struct cpudl *cp, int cpu)
>>   	if (old_idx == IDX_INVALID) {
>>   		/*
>>   		 * Nothing to remove if old_idx was invalid.
>> -		 * This could happen if a rq_offline_dl is
>> +		 * This could happen if rq_online_dl or rq_offline_dl is
>>   		 * called for a CPU without -dl tasks running.
>>   		 */
>>   	} else {
>> @@ -195,9 +196,12 @@ void cpudl_clear(struct cpudl *cp, int cpu)
>>   		cp->elements[new_cpu].idx = old_idx;
>>   		cp->elements[cpu].idx = IDX_INVALID;
>>   		cpudl_heapify(cp, old_idx);
>> -
>> -		cpumask_set_cpu(cpu, cp->free_cpus);
>>   	}
>> +	if (unlikely(!online))
> 
> Isn't using likely(online) more direct and cleaner? :)
> 
Sure! V2 on the way.>> +		__cpumask_clear_cpu(cpu, cp->free_cpus);
>> +	else
>> +		__cpumask_set_cpu(cpu, cp->free_cpus);
>> +
>>   	raw_spin_unlock_irqrestore(&cp->lock, flags);
>>   }
> 
> Best,
> Juri
> 

Thanks again,
     Doug