netdev - Re: [PATCH RFC net-next] virtio_net: Relax queue requirement for using XDP

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Date:   Wed, 26 Feb 2020 11:29:44 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     David Ahern <dsahern@...il.com>, David Ahern <dsahern@...nel.org>,
        netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org,
        David Ahern <dahern@...italocean.com>,
        "Michael S . Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH RFC net-next] virtio_net: Relax queue requirement for
 using XDP


On 2020/2/26 上午11:24, David Ahern wrote:
> On 2/25/20 8:00 PM, Jason Wang wrote:
>> On 2020/2/26 上午8:57, David Ahern wrote:
>>> From: David Ahern<dahern@...italocean.com>
>>>
>>> virtio_net currently requires extra queues to install an XDP program,
>>> with the rule being twice as many queues as vcpus. From a host
>>> perspective this means the VM needs to have 2*vcpus vhost threads
>>> for each guest NIC for which XDP is to be allowed. For example, a
>>> 16 vcpu VM with 2 tap devices needs 64 vhost threads.
>>>
>>> The extra queues are only needed in case an XDP program wants to
>>> return XDP_TX. XDP_PASS, XDP_DROP and XDP_REDIRECT do not need
>>> additional queues. Relax the queue requirement and allow XDP
>>> functionality based on resources. If an XDP program is loaded and
>>> there are insufficient queues, then return a warning to the user
>>> and if a program returns XDP_TX just drop the packet. This allows
>>> the use of the rest of the XDP functionality to work without
>>> putting an unreasonable burden on the host.
>>>
>>> Cc: Jason Wang<jasowang@...hat.com>
>>> Cc: Michael S. Tsirkin<mst@...hat.com>
>>> Signed-off-by: David Ahern<dahern@...italocean.com>
>>> ---
>>>    drivers/net/virtio_net.c | 14 ++++++++++----
>>>    1 file changed, 10 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
>>> index 2fe7a3188282..2f4c5b2e674d 100644
>>> --- a/drivers/net/virtio_net.c
>>> +++ b/drivers/net/virtio_net.c
>>> @@ -190,6 +190,8 @@ struct virtnet_info {
>>>        /* # of XDP queue pairs currently used by the driver */
>>>        u16 xdp_queue_pairs;
>>>    +    bool can_do_xdp_tx;
>>> +
>>>        /* I like... big packets and I cannot lie! */
>>>        bool big_packets;
>>>    @@ -697,6 +699,8 @@ static struct sk_buff *receive_small(struct
>>> net_device *dev,
>>>                len = xdp.data_end - xdp.data;
>>>                break;
>>>            case XDP_TX:
>>> +            if (!vi->can_do_xdp_tx)
>>> +                goto err_xdp;
>> I wonder if using spinlock to synchronize XDP_TX is better than dropping
>> here?
> I recall you suggesting that. Sure, it makes for a friendlier user
> experience, but if a spinlock makes this slower then it goes against the
> core idea of XDP.
>
>

Maybe we can do some benchmark. TAP uses spinlock for XDP_TX. If my 
memory is correct, for the best case (no queue contention), it can only 
have ~10% PPS drop on heavy workload.

Thanks