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

[KY Srinivasan <kys@microsoft.com>]

> -----Original Message-----
> From: Alexander Duyck [mailto:alexander.duyck@gmail.com]
> Sent: Wednesday, September 16, 2015 2:39 PM
> To: KY Srinivasan <kys@microsoft.com>; David Laight
> <David.Laight@ACULAB.COM>; 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;
> Jason Wang <jasowang@redhat.com>
> Subject: Re: [PATCH net-next RFC] net: increase LL_MAX_HEADER for Hyper-V
> 
> 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.

You are right; the rndis header can be built as a separate fragment and sent.
Indeed this is what we were doing earlier - on the outgoing path we would allocate
memory for the rndis header. My goal was to avoid this allocation on every packet being
sent and I decided to use the headroom instead. If we can completely avoid all memory
allocation for rndis header, it makes a significant perf difference:

Throughput as measured by iperf on a 40G interface (VM to VM on two nodes) in Gbps.
Scenario #A: LL_MAX_HEADER =128 [no change], needed_headroom = 220 [no change]
Scenario #B: LL_MAX_HEADER =224, needed_headroom = 220 [no change]

Conn#   #A                     #B          
1              6.9                  8.2  
2              13.2                14.9        
4              17.6                16.6        
8              24.1                26.9        
16           24.0                 31.5        
32           24.5                 33.6        
64           31.6                 31.5        
128         29.6                 30.3        

Column A is the existing code where we end up having to allocate more headroom and column B is with
Vitaly's patch. I will experiment with a light-weight allocator for the rndis header.

Regards,

K. Y





> 
> - Alex