Messages in this thread Patch in this message | | | From | Vitaly Kuznetsov <> | Subject | [PATCH net-next RFC] net: increase LL_MAX_HEADER for Hyper-V | Date | Wed, 16 Sep 2015 17:50:25 +0200 |
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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@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 -- 2.4.3
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