RE: [PATCH net-next RFC] net: increase LL_MAX_HEADER for Hyper-V

[Haiyang Zhang]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> Sent: Wednesday, September 16, 2015 11:50 AM
> To: netdev@xxxxxxxxxxxxxxx
> Cc: David S. Miller <davem@xxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx;
> KY Srinivasan <kys@xxxxxxxxxxxxx>; Haiyang Zhang
> <haiyangz@xxxxxxxxxxxxx>; Jason Wang <jasowang@xxxxxxxxxx>
> 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@xxxxxxxxxx>
> ---
>  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




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