Re: [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:
> +static int restrict_cpus_allowed_ptr(struct task_struct *p,
> +				     struct cpumask *new_mask,
> +				     const struct cpumask *subset_mask)
> +{
> +	struct rq_flags rf;
> +	struct rq *rq;
> +	int err;
> +	struct cpumask *user_mask = NULL;
> +
> +	if (!p->user_cpus_ptr) {
> +		user_mask = kmalloc(cpumask_size(), GFP_KERNEL);

		if (!user_mask)
			return -ENOMEM;
	}

?

> +
> +	rq = task_rq_lock(p, &rf);
> +
> +	/*
> +	 * Forcefully restricting the affinity of a deadline task is
> +	 * likely to cause problems, so fail and noisily override the
> +	 * mask entirely.
> +	 */
> +	if (task_has_dl_policy(p) && dl_bandwidth_enabled()) {
> +		err = -EPERM;
> +		goto err_unlock;
> +	}
> +
> +	if (!cpumask_and(new_mask, &p->cpus_mask, subset_mask)) {
> +		err = -EINVAL;
> +		goto err_unlock;
> +	}
> +
> +	/*
> +	 * We're about to butcher the task affinity, so keep track of what
> +	 * the user asked for in case we're able to restore it later on.
> +	 */
> +	if (user_mask) {
> +		cpumask_copy(user_mask, p->cpus_ptr);
> +		p->user_cpus_ptr = user_mask;
> +	}
> +
> +	return __set_cpus_allowed_ptr_locked(p, new_mask, 0, rq, &rf);
> +
> +err_unlock:
> +	task_rq_unlock(rq, p, &rf);
> +	kfree(user_mask);
> +	return err;
> +}