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 4:49 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 03:57 PM, KY Srinivasan wrote:
> >
> >> -----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
> 
> I get the performance implications, but that is increasing the memory
> footprint for every driver in the system that is holding any outstanding
> transmit buffers.  Also it will likely have a negative impact on
> transmit performance as it increases the truesize of every outgoing buffer.
> 
> The other thing I don't get is why hv_netvsc_packet was being included
> in this allocation as well.  It seems like it is just metadata that is
> used for each outgoing frame.  Odds are you could probably make it a
> separate allocation as well, however if nothing else you should probably
> look at rearranging the structure to fill the holes as it looks like you
> have about 16 bytes of wasted space due to the arrangement of 32b and
> 64b values.  Fixing that would allow you to reduce your needed_headroom
> which may also help to improve things.

Agreed; I will address this as well.

K. Y
> 
> - Alex