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

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Date:   Thu, 23 May 2019 20:35:50 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     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>
Cc:     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 2019/05/23 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.

> So why not just fix that
> instead?

The queues are shared with packets from stack sent from peer. That's
because I needed the lock. I have tried to separate the queues, one for
redirect and one for stack, but receiver side got too complicated and it
ended up with worse performance.

-- 
Toshiaki Makita