Subject: Re: [PATCH v8 11/19] sched: Allow task CPU affinity to be restricted on asymmetric systems

From: Valentin Schneider <[email-masked]>
Date: Fri, 04 Jun 2021 18:12:32 +0100

On 02/06/21 17:47, 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;
> +	}
> +

Shouldn't that be done before any of the bailouts above, so we can
potentially restore the mask even if we end up forcefully expanding the
affinity?

> +	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;
> +}
> +
> +/*
> + * Restrict the CPU affinity of task @p so that it is a subset of
> + * task_cpu_possible_mask() and point @p->user_cpu_ptr to a copy of the
> + * old affinity mask. If the resulting mask is empty, we warn and walk
> + * up the cpuset hierarchy until we find a suitable mask.
> + */
> +void force_compatible_cpus_allowed_ptr(struct task_struct *p)
> +{
> +	cpumask_var_t new_mask;
> +	const struct cpumask *override_mask = task_cpu_possible_mask(p);
> +
> +	alloc_cpumask_var(&new_mask, GFP_KERNEL);
> +
> +	/*
> +	 * __migrate_task() can fail silently in the face of concurrent
> +	 * offlining of the chosen destination CPU, so take the hotplug
> +	 * lock to ensure that the migration succeeds.
> +	 */
> +	cpus_read_lock();

I'm thinking this might not be required with:

  http://lore.kernel.org/r/[email-masked]

but then again this isn't merged yet :-)

> +	if (!cpumask_available(new_mask))
> +		goto out_set_mask;
> +
> +	if (!restrict_cpus_allowed_ptr(p, new_mask, override_mask))
> +		goto out_free_mask;
> +
> +	/*
> +	 * We failed to find a valid subset of the affinity mask for the
> +	 * task, so override it based on its cpuset hierarchy.
> +	 */
> +	cpuset_cpus_allowed(p, new_mask);
> +	override_mask = new_mask;
> +
> +out_set_mask:
> +	if (printk_ratelimit()) {
> +		printk_deferred("Overriding affinity for process %d (%s) to CPUs %*pbl\n",
> +				task_pid_nr(p), p->comm,
> +				cpumask_pr_args(override_mask));
> +	}
> +
> +	WARN_ON(set_cpus_allowed_ptr(p, override_mask));
> +out_free_mask:
> +	cpus_read_unlock();
> +	free_cpumask_var(new_mask);
> +}
> +
> +static int
> +__sched_setaffinity(struct task_struct *p, const struct cpumask *mask);
> +
> +/*
> + * Restore the affinity of a task @p which was previously restricted by a
> + * call to force_compatible_cpus_allowed_ptr(). This will clear (and free)
> + * @p->user_cpus_ptr.
> + */
> +void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
> +{
> +	unsigned long flags;
> +	struct cpumask *mask = p->user_cpus_ptr;
> +
> +	/*
> +	 * Try to restore the old affinity mask. If this fails, then
> +	 * we free the mask explicitly to avoid it being inherited across
> +	 * a subsequent fork().
> +	 */
> +	if (!mask || !__sched_setaffinity(p, mask))
> +		return;
> +
> +	raw_spin_lock_irqsave(&p->pi_lock, flags);
> +	release_user_cpus_ptr(p);
> +	raw_spin_unlock_irqrestore(&p->pi_lock, flags);

AFAICT an affinity change can happen between __sched_setaffinity() and
reacquiring the ->pi_lock. Right now this can't be another
force_compatible_cpus_allowed_ptr() because this is only driven by
arch_setup_new_exec() against current, so we should be fine, but here be
dragons.

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