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

[Joe Damato <jdamato@fastly.com>]

On Thu, Jan 23, 2025 at 01:13:10PM -0700, Ahmed Zaki wrote:
> 
> 
> On 2025-01-23 12:28 p.m., Joe Damato wrote:
> > On Fri, Jan 17, 2025 at 05:33:31PM -0700, Ahmed Zaki wrote:
> > > Add a new netdev flag "rx_cpu_rmap_auto". Drivers supporting ARFS should
> > > set the flag via netif_enable_cpu_rmap() and core will allocate and manage
> > > the ARFS rmap. Freeing the rmap is also done by core when the netdev is
> > > freed.
> > > 
> > > For better IRQ affinity management, move the IRQ rmap notifier inside the
> > > napi_struct. Consequently, add new notify.notify and notify.release
> > > functions: netif_irq_cpu_rmap_notify() and netif_napi_affinity_release().
> > > 
> > > Acked-by: David Arinzon <darinzon@amazon.com>
> > > Signed-off-by: Ahmed Zaki <ahmed.zaki@intel.com>
> > 
> > [...]
> > 
> > > diff --git a/net/core/dev.c b/net/core/dev.c
> > > index fe5f5855593d..dbb63005bc2b 100644
> > > --- a/net/core/dev.c
> > > +++ b/net/core/dev.c
> > > @@ -6862,6 +6862,141 @@ void netif_queue_set_napi(struct net_device *dev, unsigned int queue_index,
> > >   }
> > >   EXPORT_SYMBOL(netif_queue_set_napi);
> > > +#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;
> > 
> > I wonder if this generates a warning with some compilers? err is
> > defined not used if !napi->dev->rx_cpu_rmap_auto ? Not sure.
> > 
> > > +	if (napi->dev->rx_cpu_rmap_auto) {
> > > +		err = cpu_rmap_update(rmap, napi->napi_rmap_idx, mask);
> > > +		if (err)
> > > +			pr_warn("%s: RMAP update failed (%d)\n",
> > > +				__func__, err);
> > > +	}
> > > +}
> > > +
> > > +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)
> > > +		return;
> > > +	rmap->obj[napi->napi_rmap_idx] = NULL;
> > > +	napi->napi_rmap_idx = -1;
> > > +	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)
> > > +		return -EINVAL;
> > > +
> > > +	napi->notify.notify = netif_irq_cpu_rmap_notify;
> > > +	napi->notify.release = netif_napi_affinity_release;
> > 
> > Maybe the callbacks should only be set at the end after everything
> > else is successful, just before the return 0 ?
> > 
> 
> I believe this is needed before irq_set_affinity_notifier(), OW we could
> have some racing. I can move it there if you like.
> 
> > > +	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;
> > > +}
> > 
> > [...]
> > 
> > > +void netif_napi_set_irq_locked(struct napi_struct *napi, int irq)
> > > +{
> > > +	int rc;
> > > +
> > > +	if (!napi->dev->rx_cpu_rmap_auto)
> > > +		goto out;
> > 
> > Maybe the above if statement could be extended to be something like:
> > 
> > if (!napi->dev->rx_cpu_rmap_auto || napi->irq < 0)
> >    goto out;
> > 
> > then you can omit the irq > 0 checks in the code below, potentially?
> 
> I am afraid I don't get this, the checks below one is for the new irq (could
> be valid or -1) and one for the existing (nap->irq).

Ah yes, my mistake; I misread the other half of the if statement
below. My apologies.

> > 
> > > +	/* Remove existing rmap entries */
> > > +	if (napi->irq != irq && napi->irq > 0)
> > > +		irq_set_affinity_notifier(napi->irq, NULL);
> > > +
> > > +	if (irq > 0) {
> > > +		rc = napi_irq_cpu_rmap_add(napi, irq);
> > > +		if (rc) {
> > > +			netdev_warn(napi->dev, "Unable to update ARFS map (%d)\n",
> > > +				    rc);
> > > +			netif_disable_cpu_rmap(napi->dev);
> > > +		}
> > > +	}
> > > +
> > > +out:
> > > +	napi->irq = irq;
> > > +}
> > > +EXPORT_SYMBOL(netif_napi_set_irq_locked);
> > > +
> 
> Thanks.
>