linux-kernel - RE: [PATCH net-next RFC] net: increase LL_MAX

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Date:	Wed, 16 Sep 2015 16:25:03 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Haiyang Zhang' <haiyangz@...rosoft.com>,
	Vitaly Kuznetsov <vkuznets@...hat.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:	"David S. Miller" <davem@...emloft.net>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	KY Srinivasan <kys@...rosoft.com>,
	Jason Wang <jasowang@...hat.com>
Subject: RE: [PATCH net-next RFC] net: increase LL_MAX_HEADER for Hyper-V

From: Haiyang Zhang
> Sent: 16 September 2015 17:09
> > -----Original Message-----
> > From: Vitaly Kuznetsov [mailto:vkuznets@...hat.com]
> > Sent: Wednesday, September 16, 2015 11:50 AM
> > To: netdev@...r.kernel.org
> > Cc: David S. Miller <davem@...emloft.net>; linux-kernel@...r.kernel.org;
> > KY Srinivasan <kys@...rosoft.com>; Haiyang Zhang
> > <haiyangz@...rosoft.com>; Jason Wang <jasowang@...hat.com>
> > Subject: [PATCH net-next RFC] net: increase LL_MAX_HEADER for Hyper-V
> >
> > Commit b08cc79155fc26d0d112b1470d1ece5034651a4b ("hv_netvsc: Eliminate
> >  memory allocation in the packet send path") introduced skb headroom
> > request for Hyper-V netvsc driver:
> >
> >        max_needed_headroom = sizeof(struct hv_netvsc_packet) +
> >                                sizeof(struct rndis_message) +
> >                                NDIS_VLAN_PPI_SIZE + NDIS_CSUM_PPI_SIZE +
> >                                NDIS_LSO_PPI_SIZE + NDIS_HASH_PPI_SIZE;
> >        ...
> >        net->needed_headroom = max_needed_headroom;
> >
> > max_needed_headroom is 220 bytes, it significantly exceeds the
> > LL_MAX_HEADER setting. This causes each skb to be cloned on send path,
> > e.g. for IPv4 case we fall into the following clause
> > (ip_finish_output2()):
> >
> > if (unlikely(skb_headroom(skb) < hh_len && dev->header_ops)) {
> >     ...
> >     skb2 = skb_realloc_headroom(skb, LL_RESERVED_SPACE(dev));
> >     ...
> > }
> >
> > leading to a significant performance regression. Increase LL_MAX_HEADER
> > to make it suitable for netvsc, make it 224 to be 16-aligned.
> > Alternatively we could (partially) revert the commit which introduced
> > skb
> > headroom request restoring manual memory allocation on transmit path.
> >
> > Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> > ---
> >  include/linux/netdevice.h | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> > index 88a0069..7233790 100644
> > --- a/include/linux/netdevice.h
> > +++ b/include/linux/netdevice.h
> > @@ -132,7 +132,9 @@ static inline bool dev_xmit_complete(int rc)
> >   *	used.
> >   */
> >
> > -#if defined(CONFIG_WLAN) || IS_ENABLED(CONFIG_AX25)
> > +#if IS_ENABLED(CONFIG_HYPERV_NET)
> > +# define LL_MAX_HEADER 224
> > +#elif defined(CONFIG_WLAN) || IS_ENABLED(CONFIG_AX25)
> >  # if defined(CONFIG_MAC80211_MESH)
> >  #  define LL_MAX_HEADER 128
> >  # else
> 
> Thanks for the patch.
> To avoid we forget to update that 224 number when we add more things
> into netvsc header, I suggest that we define a macro in netdevice.h such
> as:
> #define HVNETVSC_MAX_HEADER 224
> #define LL_MAX_HEADER HVNETVSC_MAX_HEADER
> 
> And, put a note in netvsc code saying the header reservation shouldn't
> exceed HVNETVSC_MAX_HEADER, or you need to update HVNETVSC_MAX_HEADER.

Am I right in thinking this is adding an extra 96 unused bytes to the front
of almost all skb just so that hyper-v can make its link level header
contiguous with whatever follows (IP header ?).

Doesn't sound ideal.

	David

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