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Date:   Fri, 24 May 2019 11:13:03 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
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 2019/5/23 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.

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

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