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

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Date:   Thu, 9 Mar 2023 09:49:30 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
CC:     Xuan Zhuo <xuanzhuo@...ux.alibaba.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>,
        <netdev@...r.kernel.org>
Subject: Re: [PATCH net, stable v1 3/3] virtio_net: add checking sq is full
 inside xdp xmit

On 2023/3/8 20:21, Michael S. Tsirkin wrote:
> 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@...wei.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@...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.
>>>>
>>>
>>>
>>>
>>>  * 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 digged a little deeper. And there was a long discussion about this.

https://patchwork.ozlabs.org/project/netdev/patch/200906031247.05591.rusty@rustcorp.com.au/#70283

> 
>> 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
> 

That's the part I did not understand, the implemention of dev_requeue_skb() does not seem
"non-trivial" to me, maybe the implemention before dev_requeue_skb() was non-trivial.
But now "non-trivial amount of useless work" just seems to add confusion here.

Anyway, according to the discussion between Herbert and Rusty, the problems seems to be:
1. tcpdump may see the packet twice due to requeuing.
2. higher priority skb is not able to preempt the requeued low priority skb.

For 1, it seems fixable.
For 2, I am not a QoS expert, but does the big TCP support added recently and the batching
support in dev_hard_start_xmit() adds more noise to this qos problem?