> -----Original Message----- > From: Vitaly Kuznetsov [mailto:vkuzn...@redhat.com] > Sent: Wednesday, September 16, 2015 11:50 AM > To: netdev@vger.kernel.org > Cc: David S. Miller <da...@davemloft.net>; linux-ker...@vger.kernel.org; > KY Srinivasan <k...@microsoft.com>; Haiyang Zhang > <haiya...@microsoft.com>; Jason Wang <jasow...@redhat.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 <vkuzn...@redhat.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. Thanks, - Haiyang -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html