netdev - Re: [PATCH net 1/6] virtio-net: initially change the value of tx-frames

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Date: Thu, 21 Sep 2023 15:03:58 +0800
From: Jason Wang <jasowang@...hat.com>
To: Heng Qi <hengqi@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, virtualization@...ts.linux-foundation.org, 
	"Michael S . Tsirkin" <mst@...hat.com>, "David S . Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Alexei Starovoitov <ast@...nel.org>, 
	Daniel Borkmann <daniel@...earbox.net>, Jesper Dangaard Brouer <hawk@...nel.org>, 
	John Fastabend <john.fastabend@...il.com>, Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH net 1/6] virtio-net: initially change the value of tx-frames

On Tue, Sep 19, 2023 at 3:49 PM Heng Qi <hengqi@...ux.alibaba.com> wrote:
>
> Background:
> 1. Commit 0c465be183c7 ("virtio_net: ethtool tx napi configuration") uses
>    tx-frames to toggle napi_tx (0 off and 1 on) if notification coalescing
>    is not supported.
> 2. Commit 31c03aef9bc2 ("virtio_net: enable napi_tx by default") enables
>    napi_tx for all txqs by default.
>
> Status:
> When virtio-net supports notification coalescing, after initialization,
> tx-frames is 0 and napi_tx is true.
>
> Problem:
> When the user only wants to set rx coalescing params using
>            ethtool -C eth0 rx-usecs 10, or
>            ethtool -Q eth0 queue_mask 0x1 -C rx-usecs 10,
> these cmds will carry tx-frames as 0, causing the napi_tx switching condition
> is satisfied. Then the user gets:
>            netlink error: Device or resource busy.
>
> The same happens when trying to set rx-frames, adaptive_rx, adaptive_tx...
>
> Result:

It's probably not the result but how to fix it?

> When notification coalescing feature is negotiated, initially make the
> value of tx-frames to be consistent with napi_tx.
>
> For compatibility with the past, it is still supported to use tx-frames
> to toggle napi_tx.
>
> Reported-by: Xiaoming Zhao <zxm377917@...baba-inc.com>
> Signed-off-by: Heng Qi <hengqi@...ux.alibaba.com>
> ---
>  drivers/net/virtio_net.c | 42 +++++++++++++++++++++++++++++++++-------
>  1 file changed, 35 insertions(+), 7 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index fe7f314d65c9..fd5bc8d59eda 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -4442,13 +4442,6 @@ static int virtnet_probe(struct virtio_device *vdev)
>                 dev->xdp_features |= NETDEV_XDP_ACT_RX_SG;
>         }
>
> -       if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_NOTF_COAL)) {
> -               vi->intr_coal_rx.max_usecs = 0;
> -               vi->intr_coal_tx.max_usecs = 0;
> -               vi->intr_coal_tx.max_packets = 0;
> -               vi->intr_coal_rx.max_packets = 0;
> -       }
> -
>         if (virtio_has_feature(vdev, VIRTIO_NET_F_HASH_REPORT))
>                 vi->has_rss_hash_report = true;
>
> @@ -4523,6 +4516,41 @@ static int virtnet_probe(struct virtio_device *vdev)
>         if (err)
>                 goto free;
>
> +       if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_NOTF_COAL)) {
> +               vi->intr_coal_rx.max_usecs = 0;
> +               vi->intr_coal_tx.max_usecs = 0;
> +               vi->intr_coal_rx.max_packets = 0;
> +
> +               /* Why is this needed?
> +                * If without this setting, consider that when VIRTIO_NET_F_NOTF_COAL is
> +                * negotiated and napi_tx is initially true: when the user sets non tx-frames
> +                * parameters, such as the following cmd or others,
> +                *              ethtool -C eth0 rx-usecs 10.
> +                * Then
> +                * 1. ethtool_set_coalesce() first calls virtnet_get_coalesce() to get
> +                *    the last parameters except rx-usecs. If tx-frames has never been set before,
> +                *    virtnet_get_coalesce() returns with tx-frames=0 in the parameters.
> +                * 2. virtnet_set_coalesce() is then called, according to 1:
> +                *    ec->tx_max_coalesced_frames=0. Now napi_tx switching condition is met.
> +                * 3. If the device is up, the user setting fails:
> +                *             "netlink error: Device or resource busy"
> +                * This is not intuitive. Therefore, we keep napi_tx state consistent with
> +                * tx-frames when VIRTIO_NET_F_NOTF_COAL is negotiated. This behavior is
> +                * compatible with before.
> +                */

Maybe it's sufficient to say it tries to align the behaviour with that
tx NAPI is enabled by default?

Other than these minor issues:

Acked-by: Jason Wang <jasowang@...hat.com>

Thanks

> +               if (vi->sq[0].napi.weight)
> +                       vi->intr_coal_tx.max_packets = 1;
> +               else
> +                       vi->intr_coal_tx.max_packets = 0;
> +       }
> +
> +       if (virtio_has_feature(vi->vdev, VIRTIO_NET_F_VQ_NOTF_COAL)) {
> +               /* The reason is the same as VIRTIO_NET_F_NOTF_COAL. */
> +               for (i = 0; i < vi->max_queue_pairs; i++)
> +                       if (vi->sq[i].napi.weight)
> +                               vi->sq[i].intr_coal.max_packets = 1;
> +       }
> +
>  #ifdef CONFIG_SYSFS
>         if (vi->mergeable_rx_bufs)
>                 dev->sysfs_rx_queue_group = &virtio_net_mrg_rx_group;
> --
> 2.19.1.6.gb485710b
>