by Madalin-Cristian Bucur

[permalink] [raw]

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

> -----Original Message-----
> From: Eric Dumazet [mailto:[email protected]]
>
> 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).

Madalin
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2015-06-15 15:08:28

by Eric Dumazet

[permalink] [raw]

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

On Mon, 2015-06-15 at 13:40 +0000, Madalin-Cristian Bucur wrote:
> > -----Original Message-----
> > From: Eric Dumazet [mailto:[email protected]]
> >
> > 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.