netdev - Re: [RFC PATCH net-next v3 0/2] net: veth: Optimizing page pool usage

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Date: Tue, 22 Aug 2023 20:24:04 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Jesper Dangaard Brouer <jbrouer@...hat.com>, Liang Chen
	<liangchen.linux@...il.com>, <hawk@...nel.org>, <horms@...nel.org>,
	<davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<pabeni@...hat.com>
CC: <brouer@...hat.com>, <ilias.apalodimas@...aro.org>,
	<daniel@...earbox.net>, <ast@...nel.org>, <netdev@...r.kernel.org>, Lorenzo
 Bianconi <lorenzo.bianconi@...hat.com>, Stanislav Fomichev <sdf@...gle.com>,
	Maryam Tahhan <mtahhan@...hat.com>
Subject: Re: [RFC PATCH net-next v3 0/2] net: veth: Optimizing page pool usage

On 2023/8/22 5:54, Jesper Dangaard Brouer wrote:
> On 21/08/2023 16.21, Jesper Dangaard Brouer wrote:
>>
>> On 16/08/2023 14.30, Liang Chen wrote:
>>> Page pool is supported for veth, but at the moment pages are not properly
>>> recyled for XDP_TX and XDP_REDIRECT. That prevents veth xdp from fully
>>> leveraging the advantages of the page pool. So this RFC patchset is mainly
>>> to make recycling work for those cases. With that in place, it can be
>>> further optimized by utilizing the napi skb cache. Detailed figures are
>>> presented in each commit message, and together they demonstrate a quite
>>> noticeable improvement.
>>>
>>
>> I'm digging into this code path today.
>>
>> I'm trying to extend this and find a way to support SKBs that used
>> kmalloc (skb->head_frag=0), such that we can remove the
>> skb_head_is_locked() check in veth_convert_skb_to_xdp_buff(), which will
>> allow more SKBs to avoid realloc.  As long as they have enough headroom,
>> which we can dynamically control for netdev TX-packets by adjusting
>> netdev->needed_headroom, e.g. when loading an XDP prog.
>>
>> I noticed netif_receive_generic_xdp() and bpf_prog_run_generic_xdp() can
>> handle SKB kmalloc (skb->head_frag=0).  Going though the code, I don't
>> think it is a bug that generic-XDP allows this.

Is it possible to relaxe other checking too, and implement something like
pskb_expand_head() in xdp core if xdp core need to modify the data?


>>
>> Deep into this rabbit hole, I start to question our approach.
>>   - Perhaps the veth XDP approach for SKBs is wrong?
>>
>> The root-cause of this issue is that veth_xdp_rcv_skb() code path (that
>> handle SKBs) is calling XDP-native function "xdp_do_redirect()". I
>> question, why isn't it using "xdp_do_generic_redirect()"?
>> (I will jump into this rabbit hole now...)

Is there any reason why xdp_do_redirect() can not handle the slab-allocated
data? Can we change the xdp_do_redirect() to handle slab-allocated
data, so that it can benefit other case beside veth too?