CPUs with a NULL sched domain are removed from the HKR_TYPE_SCHED
isolation mask. The two following checks are equialent:
- !housekeeping_runtime_test_cpu(cpu, HKR_TYPE_SCHED)
- on_null_domain(rq)

Remove on_null_domain() and the redundant checks.

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
---
 kernel/sched/fair.c | 18 ++++++------------
 1 file changed, 6 insertions(+), 12 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 3e0f2a0f153f..9657c8f2176b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -11830,11 +11830,6 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
 
 }
 
-static inline int on_null_domain(struct rq *rq)
-{
-	return unlikely(!rcu_dereference_sched(rq->sd));
-}
-
 #ifdef CONFIG_NO_HZ_COMMON
 /*
  * NOHZ idle load balancing (ILB) details:
@@ -12040,7 +12035,7 @@ void nohz_balance_exit_idle(struct rq *rq)
 	SCHED_WARN_ON(rq != this_rq());
 
 	/* If we're a completely isolated CPU, we don't play: */
-	if (on_null_domain(rq))
+	if (!housekeeping_runtime_test_cpu(cpu_of(rq), HKR_TYPE_SCHED))
 		return;
 
 	if (likely(!rq->nohz_tick_stopped))
@@ -12090,12 +12085,8 @@ void nohz_balance_enter_idle(int cpu)
 	 */
 	rq->has_blocked_load = 1;
 
-	/* Spare idle load balancing on CPUs that don't want to be disturbed: */
-	if (!housekeeping_runtime_test_cpu(cpu, HKR_TYPE_SCHED))
-		return;
-
 	/* If we're a completely isolated CPU, we don't play: */
-	if (on_null_domain(rq))
+	if (!housekeeping_runtime_test_cpu(cpu, HKR_TYPE_SCHED))
 		return;
 
 	/*
@@ -12504,11 +12495,14 @@ static __latent_entropy void sched_balance_softirq(struct softirq_action *h)
  */
 void sched_balance_trigger(struct rq *rq)
 {
+	int cpu = cpu_of(rq);
+
 	/*
 	 * Don't need to rebalance while attached to NULL domain or
 	 * runqueue CPU is not active
 	 */
-	if (unlikely(on_null_domain(rq) || !cpu_active(cpu_of(rq))))
+	if (unlikely(!housekeeping_runtime_test_cpu(cpu, HKR_TYPE_SCHED)) ||
+	    !cpu_active(cpu))
 		return;
 
 	if (time_after_eq(jiffies, rq->next_balance))
-- 
2.25.1

On Wed, Apr 03, 2024 at 05:05:38PM +0200, Pierre Gondois wrote:
> CPUs with a NULL sched domain are removed from the HKR_TYPE_SCHED
> isolation mask. The two following checks are equialent:
> - !housekeeping_runtime_test_cpu(cpu, HKR_TYPE_SCHED)
> - on_null_domain(rq)
> 
> Remove on_null_domain() and the redundant checks.
> 
> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
> ---
>  kernel/sched/fair.c | 18 ++++++------------
>  1 file changed, 6 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 3e0f2a0f153f..9657c8f2176b 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11830,11 +11830,6 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
>  
>  }
>  
> -static inline int on_null_domain(struct rq *rq)
> -{
> -	return unlikely(!rcu_dereference_sched(rq->sd));
> -}
> -
>  #ifdef CONFIG_NO_HZ_COMMON
>  /*
>   * NOHZ idle load balancing (ILB) details:
> @@ -12040,7 +12035,7 @@ void nohz_balance_exit_idle(struct rq *rq)
>  	SCHED_WARN_ON(rq != this_rq());
>  
>  	/* If we're a completely isolated CPU, we don't play: */
> -	if (on_null_domain(rq))
> +	if (!housekeeping_runtime_test_cpu(cpu_of(rq), HKR_TYPE_SCHED))
>  		return;
>  
>  	if (likely(!rq->nohz_tick_stopped))

This seems broken, the whole null domain can happen with cpusets, but
this housekeeping nonsense is predicated on CPU_ISOLATION and none of
that is mandatory for CPUSETS.

Hello Peter,

On 4/4/24 09:27, Peter Zijlstra wrote:
> On Wed, Apr 03, 2024 at 05:05:38PM +0200, Pierre Gondois wrote:
>> CPUs with a NULL sched domain are removed from the HKR_TYPE_SCHED
>> isolation mask. The two following checks are equialent:
>> - !housekeeping_runtime_test_cpu(cpu, HKR_TYPE_SCHED)
>> - on_null_domain(rq)
>>
>> Remove on_null_domain() and the redundant checks.
>>
>> Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
>> ---
>>   kernel/sched/fair.c | 18 ++++++------------
>>   1 file changed, 6 insertions(+), 12 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 3e0f2a0f153f..9657c8f2176b 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -11830,11 +11830,6 @@ static void sched_balance_domains(struct rq *rq, enum cpu_idle_type idle)
>>   
>>   }
>>   
>> -static inline int on_null_domain(struct rq *rq)
>> -{
>> -	return unlikely(!rcu_dereference_sched(rq->sd));
>> -}
>> -
>>   #ifdef CONFIG_NO_HZ_COMMON
>>   /*
>>    * NOHZ idle load balancing (ILB) details:
>> @@ -12040,7 +12035,7 @@ void nohz_balance_exit_idle(struct rq *rq)
>>   	SCHED_WARN_ON(rq != this_rq());
>>   
>>   	/* If we're a completely isolated CPU, we don't play: */
>> -	if (on_null_domain(rq))
>> +	if (!housekeeping_runtime_test_cpu(cpu_of(rq), HKR_TYPE_SCHED))
>>   		return;
>>   
>>   	if (likely(!rq->nohz_tick_stopped))
> 
> This seems broken, the whole null domain can happen with cpusets, but
> this housekeeping nonsense is predicated on CPU_ISOLATION and none of
> that is mandatory for CPUSETS.

ok right,
I will try to remove this implicit dependency,

Regards,
Pierre