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

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Date:   Thu, 23 May 2019 22:51:34 +0900
From:   Toshiaki Makita <toshiaki.makita1@...il.com>
To:     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 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.
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