Re: [PATCH v11 08/16] sched: Allow task CPU affinity to be restricted on asymmetric systems

[Peter Zijlstra]

On Fri, Jul 30, 2021 at 12:24:35PM +0100, Will Deacon wrote:
> @@ -2783,20 +2778,173 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>  
>  	__do_set_cpus_allowed(p, new_mask, flags);
>  
> -	return affine_move_task(rq, p, &rf, dest_cpu, flags);
> +	if (flags & SCA_USER)
> +		release_user_cpus_ptr(p);
> +
> +	return affine_move_task(rq, p, rf, dest_cpu, flags);
>  
>  out:
> -	task_rq_unlock(rq, p, &rf);
> +	task_rq_unlock(rq, p, rf);
>  
>  	return ret;
>  }

> +void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
> +{
> +	unsigned long flags;
> +	struct cpumask *mask = p->user_cpus_ptr;
> +
> +	/*
> +	 * Try to restore the old affinity mask. If this fails, then
> +	 * we free the mask explicitly to avoid it being inherited across
> +	 * a subsequent fork().
> +	 */
> +	if (!mask || !__sched_setaffinity(p, mask))
> +		return;
> +
> +	raw_spin_lock_irqsave(&p->pi_lock, flags);
> +	release_user_cpus_ptr(p);
> +	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
> +}

Both these are a problem on RT.

The easiest recourse is simply never freeing the CPU mask (except on
exit). The alternative is something like the below I suppose..

I'm leaning towards the former option, wdyt?

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -2733,6 +2733,7 @@ static int __set_cpus_allowed_ptr_locked
 	const struct cpumask *cpu_allowed_mask = task_cpu_possible_mask(p);
 	const struct cpumask *cpu_valid_mask = cpu_active_mask;
 	bool kthread = p->flags & PF_KTHREAD;
+	struct cpumask *user_mask = NULL;
 	unsigned int dest_cpu;
 	int ret = 0;
 
@@ -2792,9 +2793,13 @@ static int __set_cpus_allowed_ptr_locked
 	__do_set_cpus_allowed(p, new_mask, flags);
 
 	if (flags & SCA_USER)
-		release_user_cpus_ptr(p);
+		swap(user_mask, p->user_cpus_ptr);
+
+	ret = affine_move_task(rq, p, rf, dest_cpu, flags);
+
+	kfree(user_mask);
 
-	return affine_move_task(rq, p, rf, dest_cpu, flags);
+	return ret;
 
 out:
 	task_rq_unlock(rq, p, rf);
@@ -2954,8 +2959,10 @@ void relax_compatible_cpus_allowed_ptr(s
 		return;
 
 	raw_spin_lock_irqsave(&p->pi_lock, flags);
-	release_user_cpus_ptr(p);
+	p->user_cpus_ptr = NULL;
 	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+
+	kfree(mask);
 }
 
 void set_task_cpu(struct task_struct *p, unsigned int new_cpu)