Subject: Re: [PATCH net-next v6 1/5] net: move ARFS rmap management to core

From: Jakub Kicinski <[email-masked]>
Date: Mon, 20 Jan 2025 16:59:27 -0800

On Fri, 17 Jan 2025 17:33:31 -0700 Ahmed Zaki wrote:
> +#ifdef CONFIG_RFS_ACCEL
> +static void
> +netif_irq_cpu_rmap_notify(struct irq_affinity_notify *notify,
> +			  const cpumask_t *mask)
> +{
> +	struct napi_struct *napi =
> +		container_of(notify, struct napi_struct, notify);
> +	struct cpu_rmap *rmap = napi->dev->rx_cpu_rmap;
> +	int err;
> +
> +	if (napi->dev->rx_cpu_rmap_auto) {

Can this ever not be true?

> +		err = cpu_rmap_update(rmap, napi->napi_rmap_idx, mask);
> +		if (err)
> +			pr_warn("%s: RMAP update failed (%d)\n",
> +				__func__, err);

netdev_warn(napi->dev, "...) ?

> +	}
> +}
> +
> +static void netif_napi_affinity_release(struct kref *ref)
> +{
> +	struct napi_struct *napi =
> +		container_of(ref, struct napi_struct, notify.kref);
> +	struct cpu_rmap *rmap = napi->dev->rx_cpu_rmap;
> +
> +	if (!napi->dev->rx_cpu_rmap_auto)

Similar question, can it possibly be false without driver bugs?
I think you disable rmap completely if we can't add a single IRQ,
that may be too drastic. Better miss one IRQ than the whole rmap, no?

> +		return;
> +	rmap->obj[napi->napi_rmap_idx] = NULL;
> +	napi->napi_rmap_idx = -1;

Why do we modify NAPI here? Shouldn't caller be responsible for this?

> +	cpu_rmap_put(rmap);
> +}
> +
> +static int napi_irq_cpu_rmap_add(struct napi_struct *napi, int irq)
> +{
> +	struct cpu_rmap *rmap = napi->dev->rx_cpu_rmap;
> +	int rc;
> +
> +	if (!rmap)

Should never happen, I'd ignore this and let the kernel crash below.

> +		return -EINVAL;
> +
> +	napi->notify.notify = netif_irq_cpu_rmap_notify;
> +	napi->notify.release = netif_napi_affinity_release;
> +	cpu_rmap_get(rmap);
> +	rc = cpu_rmap_add(rmap, napi);
> +	if (rc < 0)
> +		goto err_add;
> +
> +	napi->napi_rmap_idx = rc;
> +	rc = irq_set_affinity_notifier(irq, &napi->notify);
> +	if (rc)
> +		goto err_set;
> +
> +	return 0;
> +
> +err_set:
> +	rmap->obj[napi->napi_rmap_idx] = NULL;
> +	napi->napi_rmap_idx = -1;
> +err_add:
> +	cpu_rmap_put(rmap);
> +	return rc;
> +}