Re: [RFC,v3 02/10] dpaa_eth: add support for DPAA Ethernet

[Eric Dumazet <eric.dumazet@gmail.com>]

On Mon, 2015-06-15 at 13:40 +0000, Madalin-Cristian Bucur wrote:
> > -----Original Message-----
> > From: Eric Dumazet [mailto:eric.dumazet@gmail.com]
> > 
> > On Wed, 2015-04-29 at 17:56 +0300, Madalin Bucur wrote:
> > > This introduces the Freescale Data Path Acceleration Architecture
> > > (DPAA) Ethernet driver (dpaa_eth) that builds upon the DPAA QMan,
> > > BMan, PAMU and FMan drivers to deliver Ethernet connectivity on
> > > the Freescale DPAA QorIQ platforms.
> > ...
> > 
> > > +	/* We're going to store the skb backpointer at the beginning
> > > +	 * of the data buffer, so we need a privately owned skb
> > > +	 */
> > > +
> > > +	/* Code borrowed from skb_unshare(). */
> > > +	if (skb_cloned(skb)) {
> > > +		struct sk_buff *nskb = skb_copy(skb, GFP_ATOMIC);
> > > +
> > > +		/* Finally, create a contig FD from this skb */
> > > +		skb_to_contig_fd(priv, skb, &fd, countptr, &offset);
> > > +
> > > +		kfree_skb(skb);
> > > +		skb = nskb;
> > > +		/* skb_copy() has now linearized the skbuff. */
> > > +	}
> > > +
> > 
> > You know that TCP packets are clones, right ?
> > This code is killing performance.
> > 
> > TCP allows the headers part being modified by a driver if needed.
> > 
> > You should use skb_header_cloned() instead.
> 
> Thank you, I'll address this. I plan to do something like this:
> 
> +       if (!nonlinear) {
> +               /* We're going to store the skb backpointer at the beginning
> +                * of the data buffer, so we need a privately owned skb
> +                */
> +
> +               /* make sure skb is not shared, skb_cow_head() assumes it's not */
> +               skb = skb_share_check(skb, GFP_ATOMIC);
> +               if (!skb)
> +                       goto enomem;
> +
> +               /* verify the skb head is not cloned */
> +               if (skb_cow_head(skb, priv->tx_headroom))
> +                       goto enomem;
> +
> +               nonlinear = skb_is_nonlinear(skb);
> +       }
> 
> but I'm not sure the skb_share_check() is required on tx.
> I'm also a bit puzzled by the aliasing between shared and cloned terms
> (i.e. skb_unshare() could be named something like skb_unclone();
>  the skb_share_check() not only checks but also unshares an skb so
>  the already taken skb_unshare() name would probably fit too).

A driver can ask tx skbs to be not shared, even if pktgen is used with
"clone=X" on it.

dev->priv_flags         &= ~IFF_TX_SKB_SHARING;

This way, you can avoid skb_share_check() completely. Only pktgen will
care.

Most skb_share_check() calls in drivers/net are related to input paths
on virtual drivers.