netdev - Re: [PATCH v19 bpf-next 03/23] net: mvneta: update mb bit before passing the xdp buffer to eBPF layer

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Date:   Mon, 6 Dec 2021 16:11:06 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     John Fastabend <john.fastabend@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com,
        dsahern@...nel.org, brouer@...hat.com, echaudro@...hat.com,
        jasowang@...hat.com, alexander.duyck@...il.com, saeed@...nel.org,
        maciej.fijalkowski@...el.com, magnus.karlsson@...el.com,
        tirthendu.sarkar@...el.com, toke@...hat.com
Subject: Re: [PATCH v19 bpf-next 03/23] net: mvneta: update mb bit before
 passing the xdp buffer to eBPF layer

> Lorenzo Bianconi wrote:
> > Update multi-buffer bit (mb) in xdp_buff to notify XDP/eBPF layer and
> > XDP remote drivers if this is a "non-linear" XDP buffer. Access
> > skb_shared_info only if xdp_buff mb is set in order to avoid possible
> > cache-misses.
> > 
> > Signed-off-by: Lorenzo Bianconi <lorenzo@...nel.org>
> 
> [...]
> 
> > @@ -2320,8 +2325,12 @@ mvneta_swbm_build_skb(struct mvneta_port *pp, struct page_pool *pool,
> >  		      struct xdp_buff *xdp, u32 desc_status)
> >  {
> >  	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
> > -	int i, num_frags = sinfo->nr_frags;
> >  	struct sk_buff *skb;
> > +	u8 num_frags;
> > +	int i;
> > +
> > +	if (unlikely(xdp_buff_is_mb(xdp)))
> > +		num_frags = sinfo->nr_frags;
> 
> Doesn't really need a respin IMO, but rather an observation. Its not
> obvious to me the unlikely/likely pair here is wanted. Seems it could
> be relatively common for some applications sending jumbo frames.
> 
> Maybe worth some experimenting in the future.

Probably for mvneta it will not make any difference but in general I tried to
avoid possible cache-misses here (accessing sinfo pointers). I will carry out
some comparison to see if I can simplify the code.

Regards,
Lorenzo

> 
> >  
> >  	skb = build_skb(xdp->data_hard_start, PAGE_SIZE);
> >  	if (!skb)
> > @@ -2333,6 +2342,9 @@ mvneta_swbm_build_skb(struct mvneta_port *pp, struct page_pool *pool,
> >  	skb_put(skb, xdp->data_end - xdp->data);
> >  	skb->ip_summed = mvneta_rx_csum(pp, desc_status);
> >  
> > +	if (likely(!xdp_buff_is_mb(xdp)))
> > +		goto out;
> > +
> >  	for (i = 0; i < num_frags; i++) {
> >  		skb_frag_t *frag = &sinfo->frags[i];
> 

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