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Date:   Fri, 24 May 2019 11:54:00 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        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 2019/5/24 上午11:28, Toshiaki Makita wrote:
> 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.
>

Well, technically the bulk queue passed to TUN could be reused. I admit 
it may save cacheline in ideal case but I wonder how much we could gain 
on real workload. (Note TUN doesn't use napi handler to do XDP, it has a 
NAPI mode but it was mainly used for hardening and XDP was not 
implemented there, maybe we should fix this).

Thanks


>> 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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