Re: [RFC PATCH 3/5] sched/fair: Update overloaded mask in presence of pushable task

[Shrikanth Hegde <sshegde@linux.ibm.com>]

On 4/9/25 16:45, K Prateek Nayak wrote:

Hi Prateek. Feel free to cc me in the future versions.
This seems interesting way if it can get us rid of newidle balance overheads.

> In presence of pushable tasks on the CPU, set it on the newly introduced
> "overloaded+mask" in sched_domain_shared struct. This will be used by
> the newidle balance to limit the scanning to these overloaded CPUs since
> they contain tasks that could be run on the newly idle target.
> 
> Signed-off-by: K Prateek Nayak <kprateek.nayak@amd.com>
> ---
>   kernel/sched/fair.c | 24 ++++++++++++++++++++++++
>   1 file changed, 24 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 98d3ed2078cd..834fcdd15cac 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8559,6 +8559,24 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
>   	return target;
>   }
>   
> +static inline void update_overloaded_mask(int cpu, bool contains_pushable)
> +{
> +	struct sched_domain_shared *sd_share = rcu_dereference(per_cpu(sd_llc_shared, cpu));
> +	cpumask_var_t overloaded_mask;
> +
> +	if (!sd_share)
> +		return;
> +
> +	overloaded_mask = sd_share->overloaded_mask;
> +	if (!overloaded_mask)
> +		return;
> +
> +	if (contains_pushable)
> +		cpumask_set_cpu(cpu, overloaded_mask);
> +	else
> +		cpumask_clear_cpu(cpu, overloaded_mask);
> +}
> +

I was getting below error when compiling. Noticed that overloaded_mask is a local update and it wouldn't
update the actual overloaded_mask.

Compilation Error:
kernel/sched/fair.c: In function ‘update_overloaded_mask’:
kernel/sched/fair.c:8570:25: error: assignment to expression with array type
  8570 |         overloaded_mask = sd_share->overloaded_mask;
       |                         ^
kernel/sched/fair.c:8571:13: warning: the address of ‘overloaded_mask’ will always evaluate as ‘true’ [-Waddress]
  8571 |         if (!overloaded_mask)



Made the below change. Also you would need rcu protection for sd_share i think.

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index bca3db5d0ac0..818d4769ec55 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8562,19 +8562,18 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
  static inline void update_overloaded_mask(int cpu, bool contains_pushable)
  {
         struct sched_domain_shared *sd_share = rcu_dereference(per_cpu(sd_llc_shared, cpu));
-       cpumask_var_t overloaded_mask;
  
         if (!sd_share)
                 return;
  
-       overloaded_mask = sd_share->overloaded_mask;
-       if (!overloaded_mask)
+       if (!sd_share->overloaded_mask)
                 return;
  
+       guard(rcu)();
         if (contains_pushable)
-               cpumask_set_cpu(cpu, overloaded_mask);
+               cpumask_set_cpu(cpu, sd_share->overloaded_mask);
         else
-               cpumask_clear_cpu(cpu, overloaded_mask);
+               cpumask_clear_cpu(cpu, sd_share->overloaded_mask);
  }

>   static inline bool fair_push_task(struct task_struct *p)
>   {
>   	if (!task_on_rq_queued(p))
> @@ -8606,11 +8624,17 @@ static inline void fair_queue_pushable_tasks(struct rq *rq)
>   static void fair_remove_pushable_task(struct rq *rq, struct task_struct *p)
>   {
>   	plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
> +
> +	if (!has_pushable_tasks(rq))
> +		update_overloaded_mask(rq->cpu, false);
>   }
>   
>   static void fair_add_pushable_task(struct rq *rq, struct task_struct *p)
>   {
>   	if (fair_push_task(p)) {
> +		if (!has_pushable_tasks(rq))
> +			update_overloaded_mask(rq->cpu, true);
> +
>   		plist_del(&p->pushable_tasks, &rq->cfs.pushable_tasks);
>   		plist_node_init(&p->pushable_tasks, p->prio);
>   		plist_add(&p->pushable_tasks, &rq->cfs.pushable_tasks);