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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Mar 08, 2023 at 04:13:12PM +0800, Yunsheng Lin wrote:
> On 2023/3/8 15:14, Xuan Zhuo wrote:
> > On Wed, 8 Mar 2023 14:59:36 +0800, Yunsheng Lin <linyunsheng@huawei.com> wrote:
> >> 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@openresty.com>
> >>> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> >>> Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
> >>> Acked-by: Michael S. Tsirkin <mst@redhat.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.
> >>
> > 
> > 
> > 
> >  * netdev_tx_t (*ndo_start_xmit)(struct sk_buff *skb,
> >  *                               struct net_device *dev);
> >  *	Called when a packet needs to be transmitted.
> >  *	Returns NETDEV_TX_OK.  Can return NETDEV_TX_BUSY, but you should stop
> >  *	the queue before that can happen; it's for obsolete devices and weird
> >  *	corner cases, but the stack really does a non-trivial amount
> >  *	of useless work if you return NETDEV_TX_BUSY.
> >  *	Required; cannot be NULL.
> 
> Thanks for the pointer. It is intersting, it seems most driver is not flollowing
> the suggestion.

Yes - I don't know why.

> I found out why the above comment was added, but I am not sure I understand
> what does "non-trivial amount of useless work" means yet.
> https://lists.linuxfoundation.org/pipermail/virtualization/2015-April/029718.html

dev_requeue_skb 

-- 
MST