Subject: Re: [PATCH v3 10/14] sched: Introduce arch_cpu_allowed_mask() to limit fallback rq selection

From: Quentin Perret <[email-masked]>
Date: Thu, 19 Nov 2020 09:38:50 +0000

On Friday 13 Nov 2020 at 09:37:15 (+0000), Will Deacon wrote:
> Asymmetric systems may not offer the same level of userspace ISA support
> across all CPUs, meaning that some applications cannot be executed by
> some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
> not feature support for 32-bit applications on both clusters.
> 
> On such a system, we must take care not to migrate a task to an
> unsupported CPU when forcefully moving tasks in select_fallback_rq()
> in response to a CPU hot-unplug operation.
> 
> Introduce an arch_cpu_allowed_mask() hook which, given a task argument,
> allows an architecture to return a cpumask of CPUs that are capable of
> executing that task. The default implementation returns the
> cpu_possible_mask, since sane machines do not suffer from per-cpu ISA
> limitations that affect scheduling. The new mask is used when selecting
> the fallback runqueue as a last resort before forcing a migration to the
> first active CPU.
> 
> Signed-off-by: Will Deacon <[email-masked]>
> ---
>  kernel/sched/core.c | 13 ++++++++++---
>  1 file changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 818c8f7bdf2a..8df38ebfe769 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1696,6 +1696,11 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
>  
>  #ifdef CONFIG_SMP
>  
> +/* Must contain at least one active CPU */
> +#ifndef arch_cpu_allowed_mask
> +#define  arch_cpu_allowed_mask(p)	cpu_possible_mask
> +#endif
> +
>  /*
>   * Per-CPU kthreads are allowed to run on !active && online CPUs, see
>   * __set_cpus_allowed_ptr() and select_fallback_rq().
> @@ -1708,7 +1713,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>  	if (is_per_cpu_kthread(p))
>  		return cpu_online(cpu);
>  
> -	return cpu_active(cpu);
> +	if (!cpu_active(cpu))
> +		return false;
> +
> +	return cpumask_test_cpu(cpu, arch_cpu_allowed_mask(p));
>  }
>  
>  /*
> @@ -2361,10 +2369,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
>  			}
>  			fallthrough;
>  		case possible:
> -			do_set_cpus_allowed(p, cpu_possible_mask);
> +			do_set_cpus_allowed(p, arch_cpu_allowed_mask(p));

Nit: I'm wondering if this should be called arch_cpu_possible_mask()
instead?

In any case:

Reviewed-by: Quentin Perret <[email-masked]?

Thanks,
Quentin

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