netdev - Re: [PATCH net-next] virtio-net: make napi

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Date:   Sun, 20 Aug 2017 16:30:38 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Koichiro Den <den@...ipeden.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH net-next] virtio-net: make napi_tx param easier to grasp

On Sat, Aug 19, 2017 at 2:37 AM, Koichiro Den <den@...ipeden.com> wrote:
> The module param napi_tx needs not to be writable for now since we do
> not have any means of activating/deactivating it online,

A virtio_net device inherits its napi tx mode from the global napi_tx flag
on device up. It is possible to change the parameter and bring a device
down/up to change the device mode.

> @@ -1179,13 +1172,19 @@ static int virtnet_open(struct net_device *dev)
>         struct virtnet_info *vi = netdev_priv(dev);
>         int i;
>
> +       /* Tx napi touches cachelines on the cpu handling tx interrupts. Only
> +        * enable the feature if this is likely affine with the transmit path.
> +        */
> +       if (!vi->affinity_hint_set)
> +               napi_tx = false;
> +

This disables napi globally if a specific device lacks affinity.