On 2/8/2020 4:58 AM, Alex Rosenbaum wrote:
On Thu, Feb 6, 2020 at 5:19 PM Tom Talpey <tom@xxxxxxxxxx> wrote:
On 2/6/2020 9:39 AM, Alex Rosenbaum wrote:
On Thu, Feb 6, 2020 at 4:18 PM Tom Talpey <tom@xxxxxxxxxx> wrote:
On 1/8/2020 9:26 AM, Alex Rosenbaum wrote:
A combination of the flow_label field in the IPv6 header and UDP source port
field in RoCE v2.0 are used to identify a group of packets that must be
delivered in order by the network, end-to-end.
These fields are used to create entropy for network routers (ECMP), load
balancers and 802.3ad link aggregation switching that are not aware of RoCE IB
headers.
The flow_label field is defined by a 20 bit hash value. CM based connections
will use a hash function definition based on the service type (QP Type) and
Service ID (SID). Where CM services are not used, the 20 bit hash will be
according to the source and destination QPN values.
Drivers will derive the RoCE v2.0 UDP src_port from the flow_label result.
UDP source port selection must adhere IANA port allocation ranges. Thus we will
be using IANA recommendation for Ephemeral port range of: 49152-65535, or in
hex: 0xC000-0xFFFF.
The below calculations take into account the importance of producing a symmetric
hash result so we can support symmetric hash calculation of network elements.
Hash Calculation for RDMA IP CM Service
=======================================
For RDMA IP CM Services, based on QP1 iMAD usage and connected RC QPs using the
RDMA IP CM Service ID, the flow label will be calculated according to IBTA CM
REQ private data info and Service ID.
Flow label hash function calculations definition will be defined as:
Extract the following fields from the CM IP REQ:
CM_REQ.ServiceID.DstPort [2 Bytes]
CM_REQ.PrivateData.SrcPort [2 Bytes]
u32 hash = DstPort * SrcPort;
hash ^= (hash >> 16);
hash ^= (hash >> 8);
AH_ATTR.GRH.flow_label = hash AND IB_GRH_FLOWLABEL_MASK;
#define IB_GRH_FLOWLABEL_MASK 0x000FFFFF
Sorry it took me a while to respond to this, and thanks for looking
into it since my comments on the previous proposal. I have a concern
with an aspect of this one.
The RoCEv2 destination port is a fixed value, 4791. Therefore the
term
u32 hash = DstPort * SrcPort;
adds no entropy beyond the value of SrcPort.
we're talking about the CM service ports, taken from the
rdma_resolve_route(mca_id, <ip:SrcPort>, <ip:DstPort>, to_msec);
these are the CM level port-space and not the RoCE UDP L4 ports.
we want to use both as these will allow different client instance and
server instance on same nodes will use differen CM ports and hopefully
generate different hash results for multi-flows between these two
servers.
Aha, ok I guess I missed that, and ok.
In turn, the subsequent
hash ^= (hash >> 16);
hash ^= (hash >> 8);
are re-mashing the bits with one another, again, adding no entropy.
I still wonder about this one. It's attempting to reduce the 32-bit
product to 20 bits, but a second xor with the "middle" 16 bits seems
really strange. Mathematically, wouldn't it be better to just take
the modulus of 2^20? If not, are you expecting some behavior in the
hash values that makes the double-xor approach better (in which case
it should be called out)?
Tom.
The function takes into account creating a symmetric hash, so both
active and passive can reconstruct the same flow label results. That's
why we multiply the two CM Port values (16 bit * 16 bit). The results
is a 32 bit value, and we don't want to lose any of of the MSB bit's
by modulus or masking. So we need some folding function from 32 bit to
the 20 bit flow label.
The specific bit shift is something I took from the bond driver:
https://elixir.bootlin.com/linux/latest/source/drivers/net/bonding/bond_main.c#L3407
This proved very good in spreading the flow label in our internal
testing. Other alternative can be suggested, as long as it considers
all bits in the conversion 32->20 bits.
I'm ok with it, but I still don't fully understand why the folding
is necessary. The multiplication is the important part, and it is
the operation that combines the two entropic inputs. The folding just
flips bits from what's basically the same entropy source.
IOW, I think that
u32 hash = (DstPort * SrcPort) & IB_GRH_FLOWLABEL_MASK;
would produce a completely equal benefit, mathematically.
Tom.
Alex
Can you describe how, mathematically, this is not different from simply
using the SrcPort field, and if so, how it contributes to the entropy
differentiation of the incoming streams?
Tom.
Result of the above hash will be kept in the CM's route path record connection
context and will be used all across its vitality for all preceding CM messages
on both ends of the connection (including REP, REJ, DREQ, DREP, ..).
Once connection is established, the corresponding Connected RC QPs, on both
ends of the connection, will update their context with the calculated RDMA IP
CM Service based flow_label and UDP src_port values at the Connect phase of
the active side and Accept phase of the passive side of the connection.
CM will provide to the calculated value of the flow_label hash (20 bit) result
in the 'uint32_t flow_label' field of 'struct ibv_global_route' in 'struct
ibv_ah_attr'.
The 'struct ibv_ah_attr' is passed by the CM to the provider library when
modifying a connected QP's (e.g.: RC) state by calling 'ibv_modify_qp(qp,
ah_attr, attr_mask |= IBV_QP_AV)' or when create a AH for working with
datagram QP's (e.g.: UD) by calling ibv_create_ah(ah_attr).
Hash Calculation for non-RDMA CM Service ID
===========================================
For non CM QP's, the application can define the flow_label value in the
'struct ibv_ah_attr' when modifying the connected QP's (e.g.: RC) or creating
a AH for the datagram QP's (e.g.: UD).
If the provided flow_label value is zero, not set by the application (e.g.:
legacy cases), then verbs providers should use the src.QP[24bit] and
dst.QP[24bit] as input arguments for flow_label calculation.
As QPN's are an array of 3 bytes, the multiplication will result in 6 bytes
value. We'll define a flow_label value as:
DstQPn [3 Bytes]
SrcQPn [3 Bytes]
u64 hash = DstQPn * SrcQPn;
hash ^= (hash >> 20);
hash ^= (hash >> 40);
AH_ATTR.GRH.flow_label = hash AND IB_GRH_FLOWLABEL_MASK;
Hash Calculation for UDP src_port
=================================
Providers supporting RoCEv2 will use the 'flow_label' value as input to
calculate the RoCEv2 UDP src_port, which will be used in the QP context or the
AH context.
UDP src_port calculations from flow label:
[while considering the 14 bits UDP port range according to IANA recommendation]
AH_ATTR.GRH.flow_label [20 bits]
u32 fl_low = fl & 0x03FFF;
u32 fl_high = fl & 0xFC000;
u16 udp_sport = fl_low XOR (fl_high >> 14);
RoCE.UDP.src_port = udp_sport OR IB_ROCE_UDP_ENCAP_VALID_PORT
#define IB_ROCE_UDP_ENCAP_VALID_PORT 0xC000
This is a v2 follow-up on "[RFC] RoCE v2.0 UDP Source Port Entropy" [1]
[1] https://www.spinics.net/lists/linux-rdma/msg73735.html
Signed-off-by: Alex Rosenbaum <alexr@xxxxxxxxxxxx>
