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

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Message-ID: <5a4564dc-af93-4305-49a4-5ca16d737bc3@huawei.com>
Date:   Wed, 8 Mar 2023 16:13:12 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
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>,
        <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 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.

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

> 
> It does not affect the XDP TX. It is just that there are some waste on the
> path of the protocol stack.
> 
> For example, TCP will do some unnecessary work based on the return value here.

I thought it was handled by the sched layer as below, TCP does not need to be
be aware of that? Am I missed something.

https://elixir.bootlin.com/linux/v6.3-rc1/source/net/sched/sch_generic.c#L362

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