Subject: Re: [PATCH v3 07/14] sched: Introduce restrict_cpus_allowed_ptr() to limit task CPU affinity

From: Quentin Perret <[email-masked]>
Date: Thu, 19 Nov 2020 09:18:20 +0000

Hey Will,

On Friday 13 Nov 2020 at 09:37:12 (+0000), Will Deacon wrote:
> -static int __set_cpus_allowed_ptr(struct task_struct *p,
> -				  const struct cpumask *new_mask, bool check)
> +static int __set_cpus_allowed_ptr_locked(struct task_struct *p,
> +					 const struct cpumask *new_mask,
> +					 bool check,
> +					 struct rq *rq,
> +					 struct rq_flags *rf)
>  {
>  	const struct cpumask *cpu_valid_mask = cpu_active_mask;
>  	unsigned int dest_cpu;
> -	struct rq_flags rf;
> -	struct rq *rq;
>  	int ret = 0;

Should we have a lockdep assertion here?

> -	rq = task_rq_lock(p, &rf);
>  	update_rq_clock(rq);
>  
>  	if (p->flags & PF_KTHREAD) {
> @@ -1929,7 +1923,7 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>  	if (task_running(rq, p) || p->state == TASK_WAKING) {
>  		struct migration_arg arg = { p, dest_cpu };
>  		/* Need help from migration thread: drop lock and wait. */
> -		task_rq_unlock(rq, p, &rf);
> +		task_rq_unlock(rq, p, rf);
>  		stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
>  		return 0;
>  	} else if (task_on_rq_queued(p)) {
> @@ -1937,20 +1931,69 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
>  		 * OK, since we're going to drop the lock immediately
>  		 * afterwards anyway.
>  		 */
> -		rq = move_queued_task(rq, &rf, p, dest_cpu);
> +		rq = move_queued_task(rq, rf, p, dest_cpu);
>  	}
>  out:
> -	task_rq_unlock(rq, p, &rf);
> +	task_rq_unlock(rq, p, rf);

And that's a little odd to have here no? Can we move it back on the
caller's side?

>  	return ret;
>  }

Thanks,
Quentin

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