Subject: Re: [PATCH v2 net-next 1/2] net: veth: add page_pool for page recycling

From: Lorenzo Bianconi <[email-masked]>
Date: Sun, 23 Apr 2023 16:20:46 +0200

> 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
- max_headsize = SKB_WITH_OVERHEAD(VETH_BUF_SIZE - VETH_XDP_HEADROOM) = 1470

Even in this case we will need the consume a full page. In fact, performances
are a little bit worse:

MTU 1500: tcp throughput ~ 8.3Gbps

Do you agree or am I missing something?

Regards,
Lorenzo

> 
> 
> >  		if (!nskb) {
> > -			put_page(page);
> > +			page_pool_put_full_page(rq->page_pool, page, true);
> >  			goto drop;
> >  		}
> >  
> >  		skb_reserve(nskb, VETH_XDP_HEADROOM);
> > +		skb_copy_header(nskb, skb);
> > +		skb_mark_for_recycle(nskb);
> > +
> >  		size = min_t(u32, skb->len, max_head_size);
> >  		if (skb_copy_bits(skb, 0, nskb->data, size)) {
> >  			consume_skb(nskb);
> > @@ -745,7 +750,6 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
> >  		}
> >  		skb_put(nskb, size);
> >  
> > -		skb_copy_header(nskb, skb);
> >  		head_off = skb_headroom(nskb) - skb_headroom(skb);
> >  		skb_headers_offset_update(nskb, head_off);
> >  
> > @@ -754,7 +758,7 @@ static int veth_convert_skb_to_xdp_buff(struct veth_rq *rq,
> >  		len = skb->len - off;
> >  
> >  		for (i = 0; i < MAX_SKB_FRAGS && off < skb->len; i++) {
> > -			page = alloc_page(GFP_ATOMIC | __GFP_NOWARN);
> > +			page = page_pool_dev_alloc_pages(rq->page_pool);
> >  			if (!page) {
> >  				consume_skb(nskb);
> >  				goto drop;
> > @@ -1002,11 +1006,37 @@ static int veth_poll(struct napi_struct *napi, int budget)
> >  	return done;
> >  }
> >  
> > +static int veth_create_page_pool(struct veth_rq *rq)
> > +{
> > +	struct page_pool_params pp_params = {
> > +		.order = 0,
> > +		.pool_size = VETH_RING_SIZE,
> 
> It seems better to allocate different poo_size according to
> the mtu, so that the best proformance is achiced using the
> least memory?
>