Messages in this thread | | | From | KY Srinivasan <> | Subject | RE: [PATCH net-next RFC] net: increase LL_MAX_HEADER for Hyper-V | Date | Wed, 16 Sep 2015 23:58:12 +0000 |
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> -----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
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