netdev - Re: [PATCH net, stable v1 3/3] virtio_net: add checking sq is full inside xdp xmit

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Message-ID: <7eea924e-5cc3-8584-af95-04587f303f8f@huawei.com>
Date:   Wed, 8 Mar 2023 14:59:36 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, <netdev@...r.kernel.org>
CC:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...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>,
        <virtualization@...ts.linux-foundation.org>, <bpf@...r.kernel.org>,
        Yichun Zhang <yichun@...nresty.com>,
        Alexander Duyck <alexanderduyck@...com>
Subject: Re: [PATCH net, stable v1 3/3] virtio_net: add checking sq is full
 inside xdp xmit

On 2023/3/8 10:49, Xuan Zhuo wrote:
> If the queue of xdp xmit is not an independent queue, then when the xdp
> xmit used all the desc, the xmit from the __dev_queue_xmit() may encounter
> the following error.
> 
> net ens4: Unexpected TXQ (0) queue failure: -28
> 
> This patch adds a check whether sq is full in xdp xmit.
> 
> Fixes: 56434a01b12e ("virtio_net: add XDP_TX support")
> Reported-by: Yichun Zhang <yichun@...nresty.com>
> Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
> Reviewed-by: Alexander Duyck <alexanderduyck@...com>
> Acked-by: Michael S. Tsirkin <mst@...hat.com>
> ---
>  drivers/net/virtio_net.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 46bbddaadb0d..1a309cfb4976 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -767,6 +767,9 @@ static int virtnet_xdp_xmit(struct net_device *dev,
>  	}
>  	ret = nxmit;
>  
> +	if (!is_xdp_raw_buffer_queue(vi, sq - vi->sq))
> +		check_sq_full_and_disable(vi, dev, sq);
> +

Sorry if I missed something obvious here.

As the comment in start_xmit(), the current skb is added to the sq->vq, so
NETDEV_TX_BUSY can not be returned.

	/* If running out of space, stop queue to avoid getting packets that we
	 * are then unable to transmit.
	 * An alternative would be to force queuing layer to requeue the skb by
	 * returning NETDEV_TX_BUSY. However, NETDEV_TX_BUSY should not be
	 * returned in a normal path of operation: it means that driver is not
	 * maintaining the TX queue stop/start state properly, and causes
	 * the stack to do a non-trivial amount of useless work.
	 * Since most packets only take 1 or 2 ring slots, stopping the queue
	 * early means 16 slots are typically wasted.
	 */

It there any reason not to check the sq->vq->num_free at the begin of start_xmit(),
if the space is not enough for the current skb, TX queue is stopped and NETDEV_TX_BUSY
is return to the stack to requeue the current skb.

It seems it is the pattern that most network driver follow, and it seems we can avoid
calling check_sq_full_and_disable() in this patch and not wasting 16 slots as mentioned
in the comment above.


>  	if (flags & XDP_XMIT_FLUSH) {
>  		if (virtqueue_kick_prepare(sq->vq) && virtqueue_notify(sq->vq))
>  			kicks = 1;
>