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diff --git a/Documentation/networking/kcm.rst b/Documentation/networking/kcm.rst new file mode 100644 index 000000000..db0f5560a --- /dev/null +++ b/Documentation/networking/kcm.rst @@ -0,0 +1,290 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================= +Kernel Connection Multiplexor +============================= + +Kernel Connection Multiplexor (KCM) is a mechanism that provides a message based +interface over TCP for generic application protocols. With KCM an application +can efficiently send and receive application protocol messages over TCP using +datagram sockets. + +KCM implements an NxM multiplexor in the kernel as diagrammed below:: + + +------------+ +------------+ +------------+ +------------+ + | KCM socket | | KCM socket | | KCM socket | | KCM socket | + +------------+ +------------+ +------------+ +------------+ + | | | | + +-----------+ | | +----------+ + | | | | + +----------------------------------+ + | Multiplexor | + +----------------------------------+ + | | | | | + +---------+ | | | ------------+ + | | | | | + +----------+ +----------+ +----------+ +----------+ +----------+ + | Psock | | Psock | | Psock | | Psock | | Psock | + +----------+ +----------+ +----------+ +----------+ +----------+ + | | | | | + +----------+ +----------+ +----------+ +----------+ +----------+ + | TCP sock | | TCP sock | | TCP sock | | TCP sock | | TCP sock | + +----------+ +----------+ +----------+ +----------+ +----------+ + +KCM sockets +=========== + +The KCM sockets provide the user interface to the multiplexor. All the KCM sockets +bound to a multiplexor are considered to have equivalent function, and I/O +operations in different sockets may be done in parallel without the need for +synchronization between threads in userspace. + +Multiplexor +=========== + +The multiplexor provides the message steering. In the transmit path, messages +written on a KCM socket are sent atomically on an appropriate TCP socket. +Similarly, in the receive path, messages are constructed on each TCP socket +(Psock) and complete messages are steered to a KCM socket. + +TCP sockets & Psocks +==================== + +TCP sockets may be bound to a KCM multiplexor. A Psock structure is allocated +for each bound TCP socket, this structure holds the state for constructing +messages on receive as well as other connection specific information for KCM. + +Connected mode semantics +======================== + +Each multiplexor assumes that all attached TCP connections are to the same +destination and can use the different connections for load balancing when +transmitting. The normal send and recv calls (include sendmmsg and recvmmsg) +can be used to send and receive messages from the KCM socket. + +Socket types +============ + +KCM supports SOCK_DGRAM and SOCK_SEQPACKET socket types. + +Message delineation +------------------- + +Messages are sent over a TCP stream with some application protocol message +format that typically includes a header which frames the messages. The length +of a received message can be deduced from the application protocol header +(often just a simple length field). + +A TCP stream must be parsed to determine message boundaries. Berkeley Packet +Filter (BPF) is used for this. When attaching a TCP socket to a multiplexor a +BPF program must be specified. The program is called at the start of receiving +a new message and is given an skbuff that contains the bytes received so far. +It parses the message header and returns the length of the message. Given this +information, KCM will construct the message of the stated length and deliver it +to a KCM socket. + +TCP socket management +--------------------- + +When a TCP socket is attached to a KCM multiplexor data ready (POLLIN) and +write space available (POLLOUT) events are handled by the multiplexor. If there +is a state change (disconnection) or other error on a TCP socket, an error is +posted on the TCP socket so that a POLLERR event happens and KCM discontinues +using the socket. When the application gets the error notification for a +TCP socket, it should unattach the socket from KCM and then handle the error +condition (the typical response is to close the socket and create a new +connection if necessary). + +KCM limits the maximum receive message size to be the size of the receive +socket buffer on the attached TCP socket (the socket buffer size can be set by +SO_RCVBUF). If the length of a new message reported by the BPF program is +greater than this limit a corresponding error (EMSGSIZE) is posted on the TCP +socket. The BPF program may also enforce a maximum messages size and report an +error when it is exceeded. + +A timeout may be set for assembling messages on a receive socket. The timeout +value is taken from the receive timeout of the attached TCP socket (this is set +by SO_RCVTIMEO). If the timer expires before assembly is complete an error +(ETIMEDOUT) is posted on the socket. + +User interface +============== + +Creating a multiplexor +---------------------- + +A new multiplexor and initial KCM socket is created by a socket call:: + + socket(AF_KCM, type, protocol) + +- type is either SOCK_DGRAM or SOCK_SEQPACKET +- protocol is KCMPROTO_CONNECTED + +Cloning KCM sockets +------------------- + +After the first KCM socket is created using the socket call as described +above, additional sockets for the multiplexor can be created by cloning +a KCM socket. This is accomplished by an ioctl on a KCM socket:: + + /* From linux/kcm.h */ + struct kcm_clone { + int fd; + }; + + struct kcm_clone info; + + memset(&info, 0, sizeof(info)); + + err = ioctl(kcmfd, SIOCKCMCLONE, &info); + + if (!err) + newkcmfd = info.fd; + +Attach transport sockets +------------------------ + +Attaching of transport sockets to a multiplexor is performed by calling an +ioctl on a KCM socket for the multiplexor. e.g.:: + + /* From linux/kcm.h */ + struct kcm_attach { + int fd; + int bpf_fd; + }; + + struct kcm_attach info; + + memset(&info, 0, sizeof(info)); + + info.fd = tcpfd; + info.bpf_fd = bpf_prog_fd; + + ioctl(kcmfd, SIOCKCMATTACH, &info); + +The kcm_attach structure contains: + + - fd: file descriptor for TCP socket being attached + - bpf_prog_fd: file descriptor for compiled BPF program downloaded + +Unattach transport sockets +-------------------------- + +Unattaching a transport socket from a multiplexor is straightforward. An +"unattach" ioctl is done with the kcm_unattach structure as the argument:: + + /* From linux/kcm.h */ + struct kcm_unattach { + int fd; + }; + + struct kcm_unattach info; + + memset(&info, 0, sizeof(info)); + + info.fd = cfd; + + ioctl(fd, SIOCKCMUNATTACH, &info); + +Disabling receive on KCM socket +------------------------------- + +A setsockopt is used to disable or enable receiving on a KCM socket. +When receive is disabled, any pending messages in the socket's +receive buffer are moved to other sockets. This feature is useful +if an application thread knows that it will be doing a lot of +work on a request and won't be able to service new messages for a +while. Example use:: + + int val = 1; + + setsockopt(kcmfd, SOL_KCM, KCM_RECV_DISABLE, &val, sizeof(val)) + +BFP programs for message delineation +------------------------------------ + +BPF programs can be compiled using the BPF LLVM backend. For example, +the BPF program for parsing Thrift is:: + + #include "bpf.h" /* for __sk_buff */ + #include "bpf_helpers.h" /* for load_word intrinsic */ + + SEC("socket_kcm") + int bpf_prog1(struct __sk_buff *skb) + { + return load_word(skb, 0) + 4; + } + + char _license[] SEC("license") = "GPL"; + +Use in applications +=================== + +KCM accelerates application layer protocols. Specifically, it allows +applications to use a message based interface for sending and receiving +messages. The kernel provides necessary assurances that messages are sent +and received atomically. This relieves much of the burden applications have +in mapping a message based protocol onto the TCP stream. KCM also make +application layer messages a unit of work in the kernel for the purposes of +steering and scheduling, which in turn allows a simpler networking model in +multithreaded applications. + +Configurations +-------------- + +In an Nx1 configuration, KCM logically provides multiple socket handles +to the same TCP connection. This allows parallelism between in I/O +operations on the TCP socket (for instance copyin and copyout of data is +parallelized). In an application, a KCM socket can be opened for each +processing thread and inserted into the epoll (similar to how SO_REUSEPORT +is used to allow multiple listener sockets on the same port). + +In a MxN configuration, multiple connections are established to the +same destination. These are used for simple load balancing. + +Message batching +---------------- + +The primary purpose of KCM is load balancing between KCM sockets and hence +threads in a nominal use case. Perfect load balancing, that is steering +each received message to a different KCM socket or steering each sent +message to a different TCP socket, can negatively impact performance +since this doesn't allow for affinities to be established. Balancing +based on groups, or batches of messages, can be beneficial for performance. + +On transmit, there are three ways an application can batch (pipeline) +messages on a KCM socket. + + 1) Send multiple messages in a single sendmmsg. + 2) Send a group of messages each with a sendmsg call, where all messages + except the last have MSG_BATCH in the flags of sendmsg call. + 3) Create "super message" composed of multiple messages and send this + with a single sendmsg. + +On receive, the KCM module attempts to queue messages received on the +same KCM socket during each TCP ready callback. The targeted KCM socket +changes at each receive ready callback on the KCM socket. The application +does not need to configure this. + +Error handling +-------------- + +An application should include a thread to monitor errors raised on +the TCP connection. Normally, this will be done by placing each +TCP socket attached to a KCM multiplexor in epoll set for POLLERR +event. If an error occurs on an attached TCP socket, KCM sets an EPIPE +on the socket thus waking up the application thread. When the application +sees the error (which may just be a disconnect) it should unattach the +socket from KCM and then close it. It is assumed that once an error is +posted on the TCP socket the data stream is unrecoverable (i.e. an error +may have occurred in the middle of receiving a message). + +TCP connection monitoring +------------------------- + +In KCM there is no means to correlate a message to the TCP socket that +was used to send or receive the message (except in the case there is +only one attached TCP socket). However, the application does retain +an open file descriptor to the socket so it will be able to get statistics +from the socket which can be used in detecting issues (such as high +retransmissions on the socket). |