Re: [PATCH v3 net-next 2/2] xdp: add multi-buff support for xdp running in generic mode

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 12/6/23 00:58, Jakub Kicinski wrote:
> On Wed, 6 Dec 2023 00:08:15 +0100 Lorenzo Bianconi wrote:
>> v00 (NS:ns0 - 192.168.0.1/24) <---> (NS:ns1 - 192.168.0.2/24) v01 ==(XDP_REDIRECT)==> v10 (NS:ns1 - 192.168.1.1/24) <---> (NS:ns2 - 192.168.1.2/24) v11
>>
>> - v00: iperf3 client (pinned on core 0)
>> - v11: iperf3 server (pinned on core 7)
>>
>> net-next veth codebase (page_pool APIs):
>> =======================================
>> - MTU  1500: ~ 5.42 Gbps
>> - MTU  8000: ~ 14.1 Gbps
>> - MTU 64000: ~ 18.4 Gbps
>>
>> net-next veth codebase + page_frag_cahe APIs [0]:
>> =================================================
>> - MTU  1500: ~ 6.62 Gbps
>> - MTU  8000: ~ 14.7 Gbps
>> - MTU 64000: ~ 19.7 Gbps
>>
>> xdp_generic codebase + page_frag_cahe APIs (current proposed patch):
>> ====================================================================
>> - MTU  1500: ~ 6.41 Gbps
>> - MTU  8000: ~ 14.2 Gbps
>> - MTU 64000: ~ 19.8 Gbps
>>
>> xdp_generic codebase + page_frag_cahe APIs [1]:
>> ===============================================
> 
> This one should say page pool?
> 
>> - MTU  1500: ~ 5.75 Gbps
>> - MTU  8000: ~ 15.3 Gbps
>> - MTU 64000: ~ 21.2 Gbps
>>
>> It seems page_pool APIs are working better for xdp_generic codebase
>> (except MTU 1500 case) while page_frag_cache APIs are better for
>> veth driver. What do you think? Am I missing something?
> 
> IDK the details of veth XDP very well but IIUC they are pretty much
> the same. Are there any clues in perf -C 0 / 7?
> 
>> [0] Here I have just used napi_alloc_frag() instead of
>> page_pool_dev_alloc_va()/page_pool_dev_alloc() in
>> veth_convert_skb_to_xdp_buff()
>>
>> [1] I developed this PoC to use page_pool APIs for xdp_generic code:
> 
> Why not put the page pool in softnet_data?

First I thought cool that Jakub is suggesting softnet_data, which will
make page_pool (PP) even more central as the netstacks memory layer.

BUT then I realized that PP have a weakness, which is the return/free
path that need to take a normal spin_lock, as that can be called from
any CPU (unlike the RX/alloc case).  Thus, I fear that making multiple
devices share a page_pool via softnet_data, increase the chance of lock
contention when packets are "freed" returned/recycled.

--Jesper

p.s. PP have the page_pool_put_page_bulk() API, but only XDP 
(NIC-drivers) leverage this.