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

[Alexander Duyck <alexander.duyck@gmail.com>]

On 09/16/2015 10:55 AM, KY Srinivasan wrote:
>
>> -----Original Message-----
>> From: David Laight [mailto:David.Laight@ACULAB.COM]
>> Sent: Wednesday, September 16, 2015 9:25 AM
>> To: Haiyang Zhang <haiyangz@microsoft.com>; Vitaly Kuznetsov
>> <vkuznets@redhat.com>; netdev@vger.kernel.org
>> Cc: David S. Miller <davem@davemloft.net>; linux-kernel@vger.kernel.org;
>> KY Srinivasan <kys@microsoft.com>; Jason Wang <jasowang@redhat.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@redhat.com]
>>>> Sent: Wednesday, September 16, 2015 11:50 AM
>>>> To: netdev@vger.kernel.org
>>>> Cc: David S. Miller <davem@davemloft.net>; linux-
>> kernel@vger.kernel.org;
>>>> KY Srinivasan <kys@microsoft.com>; Haiyang Zhang
>>>> <haiyangz@microsoft.com>; Jason Wang <jasowang@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 <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
>>> 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.
> Remote NDIS is the protocol used to send packets from the guest to the host. Every packet
> needs to be decorated with the RNDIS header and the maximum room needed for the RNDIS
> header is the hreadroom we want.

I think we get that.  The question is does the Remote NDIS header and 
packet info actually need to be a part of the header data?  I would 
argue that it probably doesn't.

So for example in netvsc_start_xmit it looks like you are calling 
init_page_array in order to populate a set of page buffers, but the 
first buffer for the Remote NDIS protocol is populated as a separate 
page and offset.  As such it doesn't seem like it necessarily needs to 
be a part of the header data but could be maintained perhaps in a 
separate ring buffer, or perhaps just be a separate page that you break 
up to use for each header.

- Alex