[PATCH v7 13/22] sched: Allow task CPU affinity to be restricted on asymmetric systems

[Peter Zijlstra]

On Tue, May 25, 2021 at 04:14:23PM +0100, Will Deacon wrote:
> @@ -2426,20 +2421,166 @@ 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;
>  }

So sys_sched_setaffinity() releases the user_cpus_ptr thingy ?! How does
that work?

I thought the intended semantics were somethings like:

	A - 0xff			B

	restrict(0xf) // user: 0xff eff: 0xf

					sched_setaffinity(A, 0x3c) // user: 0x3c eff: 0xc

	relax() // user: NULL, eff: 0x3c