Subject: Re: [PATCH net-next 3/3] virtio_net: Map NAPIs to queues

From: Joe Damato <[email-masked]>
Date: Mon, 13 Jan 2025 09:30:20 -0800

On Mon, Jan 13, 2025 at 12:05:51PM +0800, Jason Wang wrote:
> On Sat, Jan 11, 2025 at 4:26 AM Joe Damato <[email-masked]> wrote:
> >
> > Use netif_queue_set_napi to map NAPIs to queue IDs so that the mapping
> > can be accessed by user apps.
> >
> > $ ethtool -i ens4 | grep driver
> > driver: virtio_net
> >
> > $ sudo ethtool -L ens4 combined 4
> >
> > $ ./tools/net/ynl/pyynl/cli.py \
> >        --spec Documentation/netlink/specs/netdev.yaml \
> >        --dump queue-get --json='{"ifindex": 2}'
> > [{'id': 0, 'ifindex': 2, 'napi-id': 8289, 'type': 'rx'},
> >  {'id': 1, 'ifindex': 2, 'napi-id': 8290, 'type': 'rx'},
> >  {'id': 2, 'ifindex': 2, 'napi-id': 8291, 'type': 'rx'},
> >  {'id': 3, 'ifindex': 2, 'napi-id': 8292, 'type': 'rx'},
> >  {'id': 0, 'ifindex': 2, 'type': 'tx'},
> >  {'id': 1, 'ifindex': 2, 'type': 'tx'},
> >  {'id': 2, 'ifindex': 2, 'type': 'tx'},
> >  {'id': 3, 'ifindex': 2, 'type': 'tx'}]
> >
> > Note that virtio_net has TX-only NAPIs which do not have NAPI IDs, so
> > the lack of 'napi-id' in the above output is expected.
> >
> > Signed-off-by: Joe Damato <[email-masked]>
> > ---
> >  drivers/net/virtio_net.c | 29 ++++++++++++++++++++++++++---
> >  1 file changed, 26 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 4e88d352d3eb..8f0f26cc5a94 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -2804,14 +2804,28 @@ static void virtnet_napi_do_enable(struct virtqueue *vq,
> >  }
> >
> >  static void virtnet_napi_enable_lock(struct virtqueue *vq,
> > -                                    struct napi_struct *napi)
> > +                                    struct napi_struct *napi,
> > +                                    bool need_rtnl)
> >  {
> > +       struct virtnet_info *vi = vq->vdev->priv;
> > +       int q = vq2rxq(vq);
> > +
> >         virtnet_napi_do_enable(vq, napi);
> > +
> > +       if (q < vi->curr_queue_pairs) {
> > +               if (need_rtnl)
> > +                       rtnl_lock();
> 
> Can we tweak the caller to call rtnl_lock() instead to avoid this trick?

The major problem is that if the caller calls rtnl_lock() before
calling virtnet_napi_enable_lock, then virtnet_napi_do_enable (and
thus napi_enable) happen under the lock.

Jakub mentioned in a recent change [1] that napi_enable may soon
need to sleep.

Given the above constraints, the only way to avoid the "need_rtnl"
would be to refactor the code much more, placing calls (or wrappers)
to netif_queue_set_napi in many locations.

IMHO: This implementation seemed cleaner than putting calls to
netif_queue_set_napi throughout the driver.

Please let me know how you'd like to proceed on this.

[1]: https://lore.kernel.org/netdev/[email-masked]/

> > +
> > +               netif_queue_set_napi(vi->dev, q, NETDEV_QUEUE_TYPE_RX, napi);
> > +
> > +               if (need_rtnl)
> > +                       rtnl_unlock();
> > +       }
> >  }
> >
> >  static void virtnet_napi_enable(struct virtqueue *vq, struct napi_struct *napi)
> >  {
> > -       virtnet_napi_enable_lock(vq, napi);
> > +       virtnet_napi_enable_lock(vq, napi, false);
> >  }
> >
> >  static void virtnet_napi_tx_enable(struct virtnet_info *vi,
> > @@ -2848,9 +2862,13 @@ static void refill_work(struct work_struct *work)
> >         for (i = 0; i < vi->curr_queue_pairs; i++) {
> >                 struct receive_queue *rq = &vi->rq[i];
> >
> > +               rtnl_lock();
> > +               netif_queue_set_napi(vi->dev, i, NETDEV_QUEUE_TYPE_RX, NULL);
> > +               rtnl_unlock();
> >                 napi_disable(&rq->napi);
> 
> I wonder if it's better to have a helper to do set napi to NULL as
> well as napi_disable().

There are a couple places where this code is repeated, so I could do
that, but I'd probably employ the same "trick" as above with a flag
for "need_rtnl" in the helper.

I can send a v2 which adds a virtnet_napi_disable_lock and call it
from the 4 sites I see that can use it (virtnet_xdp_set,
virtnet_rx_pause, virtnet_disable_queue_pair, refill_work).

But first.... we need to agree on the flag being passed in to hold
rtnl :)

Please let me know.

Thanks for the review.