[PATCH v6 06/21] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection

[Peter Zijlstra]

On Tue, May 18, 2021 at 10:47:10AM +0100, Will Deacon wrote:
> diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
> index 03dee12d2b61..bc4ac3c525e6 100644
> --- a/include/linux/mmu_context.h
> +++ b/include/linux/mmu_context.h
> @@ -14,4 +14,12 @@
>  static inline void leave_mm(int cpu) { }
>  #endif
>  
> +/*
> + * CPUs that are capable of running task @p. By default, we assume a sane,
> + * homogeneous system. Must contain at least one active CPU.
> + */
> +#ifndef task_cpu_possible_mask
> +# define task_cpu_possible_mask(p)	cpu_possible_mask
> +#endif

#ifndef task_cpu_possible_mask
# define task_cpu_possible_mask(p)	cpu_possible_mask
# define task_cpu_possible(cpu, p)	true
#else
# define task_cpu_possible(cpu, p)	cpumask_test_cpu((cpu), task_cpu_possible_mask(p))
#endif

> +
>  #endif
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 5226cc26a095..482f7fdca0e8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1813,8 +1813,11 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>  		return cpu_online(cpu);
>  
>  	/* Non kernel threads are not allowed during either online or offline. */
>  	if (!(p->flags & PF_KTHREAD))
> -		return cpu_active(cpu);
+		return cpu_active(cpu) && task_cpu_possible(cpu, p);

>  	/* KTHREAD_IS_PER_CPU is always allowed. */
>  	if (kthread_is_per_cpu(p))

Would something like that make sense?