netdev - Re: [PATCH bpf-next 3/3] veth: Support bulk XDP

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Date:   Fri, 24 May 2019 12:28:51 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jason Wang <jasowang@...hat.com>,
        Toshiaki Makita <toshiaki.makita1@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        netdev@...r.kernel.org, xdp-newbies@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH bpf-next 3/3] veth: Support bulk XDP_TX

On 2019/05/24 12:13, Jason Wang wrote:
> On 2019/5/23 下午9:51, Toshiaki Makita wrote:
>> On 19/05/23 (木) 22:29:27, Jesper Dangaard Brouer wrote:
>>> On Thu, 23 May 2019 20:35:50 +0900
>>> Toshiaki Makita <makita.toshiaki@....ntt.co.jp> wrote:
>>>
>>>> On 2019/05/23 20:25, Toke Høiland-Jørgensen wrote:
>>>>> Toshiaki Makita <makita.toshiaki@....ntt.co.jp> writes:
>>>>>> This improves XDP_TX performance by about 8%.
>>>>>>
>>>>>> Here are single core XDP_TX test results. CPU consumptions are taken
>>>>>> from "perf report --no-child".
>>>>>>
>>>>>> - Before:
>>>>>>
>>>>>>    7.26 Mpps
>>>>>>
>>>>>>    _raw_spin_lock  7.83%
>>>>>>    veth_xdp_xmit  12.23%
>>>>>>
>>>>>> - After:
>>>>>>
>>>>>>    7.84 Mpps
>>>>>>
>>>>>>    _raw_spin_lock  1.17%
>>>>>>    veth_xdp_xmit   6.45%
>>>>>>
>>>>>> Signed-off-by: Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
>>>>>> ---
>>>>>>   drivers/net/veth.c | 26 +++++++++++++++++++++++++-
>>>>>>   1 file changed, 25 insertions(+), 1 deletion(-)
>>>>>>
>>>>>> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
>>>>>> index 52110e5..4edc75f 100644
>>>>>> --- a/drivers/net/veth.c
>>>>>> +++ b/drivers/net/veth.c
>>>>>> @@ -442,6 +442,23 @@ static int veth_xdp_xmit(struct net_device
>>>>>> *dev, int n,
>>>>>>       return ret;
>>>>>>   }
>>>>>>   +static void veth_xdp_flush_bq(struct net_device *dev)
>>>>>> +{
>>>>>> +    struct xdp_tx_bulk_queue *bq = this_cpu_ptr(&xdp_tx_bq);
>>>>>> +    int sent, i, err = 0;
>>>>>> +
>>>>>> +    sent = veth_xdp_xmit(dev, bq->count, bq->q, 0);
>>>>>
>>>>> Wait, veth_xdp_xmit() is just putting frames on a pointer ring. So
>>>>> you're introducing an additional per-cpu bulk queue, only to avoid
>>>>> lock
>>>>> contention around the existing pointer ring. But the pointer ring is
>>>>> per-rq, so if you have lock contention, this means you must have
>>>>> multiple CPUs servicing the same rq, no?
>>>>
>>>> Yes, it's possible. Not recommended though.
>>>>
>>>
>>> I think the general per-cpu TX bulk queue is overkill.  There is a loop
>>> over packets in veth_xdp_rcv(struct veth_rq *rq, budget, *status), and
>>> the caller veth_poll() will call veth_xdp_flush(rq->dev).
>>>
>>> Why can't you store this "temp" bulk array in struct veth_rq ?
>>
>> Of course I can. But I thought tun has the same problem and we can
>> decrease memory footprint by sharing the same storage between devices.
> 
> 
> For TUN and for its fast path where vhost passes a bulk of XDP frames
> (through msg_control) to us, we probably just need a temporary bulk
> array in tun_xdp_one() instead of a global one. I can post patch or
> maybe you if you're interested in this.

Of course you/I can. What I'm concerned is that could be waste of cache
line when softirq runs veth napi handler and then tun napi handler.

> 
> Thanks
> 
> 
>> Or if other devices want to reduce queues so that we can use XDP on
>> many-cpu servers and introduce locks, we can use this storage for that
>> case as well.
>>
>> Still do you prefer veth-specific solution?
>>
>>>
>>> You could even alloc/create it on the stack of veth_poll() and send it
>>> along via a pointer to veth_xdp_rcv).
>>>
>>
>> Toshiaki Makita
> 
> 

-- 
Toshiaki Makita