netdev - Re: [PATCH v2 net-next 1/2] net: veth: add page

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Message-ID: <0c1790dc-dbeb-8664-64ca-1f71e6c4f3a9@huawei.com>
Date:   Mon, 24 Apr 2023 19:58:07 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
CC:     Lorenzo Bianconi <lorenzo@...nel.org>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>, <davem@...emloft.net>,
        <edumazet@...gle.com>, <kuba@...nel.org>, <pabeni@...hat.com>,
        <hawk@...nel.org>, <john.fastabend@...il.com>, <ast@...nel.org>,
        <daniel@...earbox.net>
Subject: Re: [PATCH v2 net-next 1/2] net: veth: add page_pool for page
 recycling

On 2023/4/24 17:17, Lorenzo Bianconi wrote:
>> On 2023/4/23 22:20, Lorenzo Bianconi wrote:
>>>> On 2023/4/23 2:54, Lorenzo Bianconi wrote:
>>>>>  struct veth_priv {
>>>>> @@ -727,17 +729,20 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
>>>>>  			goto drop;
>>>>>  
>>>>>  		/* Allocate skb head */
>>>>> -		page = alloc_page(GFP_ATOMIC | __GFP_NOWARN);
>>>>> +		page = page_pool_dev_alloc_pages(rq->page_pool);
>>>>>  		if (!page)
>>>>>  			goto drop;
>>>>>  
>>>>>  		nskb = build_skb(page_address(page), PAGE_SIZE);
>>>>
>>>> If page pool is used with PP_FLAG_PAGE_FRAG, maybe there is some additional
>>>> improvement for the MTU 1500B case, it seem a 4K page is able to hold two skb.
>>>> And we can reduce the memory usage too, which is a significant saving if page
>>>> size is 64K.
>>>
>>> please correct if I am wrong but I think the 1500B MTU case does not fit in the
>>> half-page buffer size since we need to take into account VETH_XDP_HEADROOM.
>>> In particular:
>>>
>>> - VETH_BUF_SIZE = 2048
>>> - VETH_XDP_HEADROOM = 256 + 2 = 258
>>
>> On some arch the NET_IP_ALIGN is zero.
>>
>> I suppose XDP_PACKET_HEADROOM are for xdp_frame and data_meta, it seems
>> xdp_frame is only 40 bytes for 64 bit arch and max size of metalen is 32
>> as xdp_metalen_invalid() suggest, is there any other reason why we need
>> 256 bytes here?
> 
> XDP_PACKET_HEADROOM must be greater than (40 + 32)B because you may want push
> new data at the beginning of the xdp_buffer/xdp_frame running
> bpf_xdp_adjust_head() helper.
> I think 256B has been selected for XDP_PACKET_HEADROOM since it is 4 cachelines
> (but I can be wrong).
> There was a discussion in the past to reduce XDP_PACKET_HEADROOM to 192B but
> this is not merged yet and it is not related to this series. We can address
> your comments in a follow-up patch when XDP_PACKET_HEADROOM series is merged.

It worth mentioning that the performance gain in this patch is at the cost of
more memory usage, at most of VETH_RING_SIZE(256) + PP_ALLOC_CACHE_SIZE(128)
pages is used.

IMHO, it seems better to limit the memory usage as much as possible, or provide a
way to disable/enable page pool for user.