Patent Publication Number: US-11646971-B2

Title: Limiting backpressure with bad actors

Description:
RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 16/029,031, filed Jul. 6, 2018, now U.S. Pat. No. 10,721,172. The entire teachings of the above application are incorporated herein by reference. 
    
    
     BACKGROUND 
     Processors, and specifically network processors, route packets to and from destinations on a network. In doing so, the processors can perform direct memory access of packets. Certain processors can route the packets to various internal, and in some cases external, functions. 
     SUMMARY 
     In an embodiment, a method includes, in response to detecting available memory of a destination node of a packet flow of one or more nodes to the destination node being below a particular threshold, marking the destination node as being in a backpressure state. The destination node in the backpressure state, sends a signal indicating a condition of packet backpressure to the one or more nodes of the packet flow, and initiates a timer for a particular time period. The method further marks, at the end of the particular time period, the destination node as being in a bad actor state if the available memory is below the particular threshold, and as being in a good actor state if the memory is above the particular threshold. The method further, in response to marking the destination node as being in a bad actor state, sends a signal to the one or more nodes of the packet flow causing the one or more nodes to drop packets directed to the destination node. 
     In an embodiment, packet flow is the distribution of packets from a first node to a destination node, optionally via intermediary nodes. Nodes along the packet flow from the first node to a node before the destination node can be considered upstream from the destination node in the packet flow. Backpressure is applied from the destination node upstream in the packet flow. 
     In an embodiment, in response to marking the destination node as being in a good actor state, the method sends a signal to the one or more nodes of the packet flow causing the one or more nodes to continue sending packets to the destination node. 
     In an embodiment, a method marking the destination node as being in the bad actor state further includes throwing an interrupt to other entities related to the packet flow. 
     In an embodiment, the destination node is a virtual function (VF) ring. 
     In an embodiment, the threshold is a watermark. 
     In an embodiment, the method further comprises, at startup, initiating the destination node as being in the bad actor state. 
     In an embodiment, sending the signal (e.g., applying backpressure) includes sending a signal to the one or more nodes of the packet flow indicating the destination node is not accepting new packets. 
     In an embodiment, a system includes a processor configured to, in response to detecting available memory of a destination node of a packet flow of one or more nodes to the destination node being below a particular threshold, mark the destination node as being in a backpressure state. The destination node, in the backpressure state sends a signal indicating a condition of packet backpressure to the one or more nodes of the packet flow, and initiating a timer for a particular time period. The processor is further configured to mark, at the end of the particular time period, the destination node as being in a bad actor state if the available memory is below the particular threshold, and as being in a good actor state if the memory is above the particular threshold. In response to marking the destination node as being in a bad actor state, the processor sends a signal to the one or more nodes of the packet flow causing the one or more nodes to drop packets directed to the destination node. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing will be apparent from the following more particular description of example embodiments, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments. 
         FIG.  1 A  is a block diagram illustrating a network processor employing a networking units block. 
         FIG.  1 B  is a block diagram illustrating an example embodiment of elements within the networking units block and their connections to the NCB. 
         FIG.  1 C  is a block diagram illustrating an example embodiment of a System DMA Packet Interface (DPI) Packet (SDP) Interface Unit, DMA Packet Interface Unit, and PCI-Express Interface Unit (PEM). 
         FIGS.  2 A-B  are diagrams illustrating an example embodiment of a virtual function having a plurality of buffers. 
         FIG.  3    is a state diagram illustrating an example embodiment of current methods of applying backpressure. 
         FIG.  4    is a state diagram illustrating an example embodiment of a method of the present disclosure. 
         FIG.  5    is a state diagram illustrating an example embodiment of a method employed by the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     A description of example embodiments follows. 
       FIG.  1 A  is a block diagram illustrating a network processor employing a networking units block  122 . The networking units block  122  is connected to a Near-Coprocessor Bus (NCB), which facilitates communication with other portions of the chip, including memory and memory controllers, and other modules. 
       FIG.  1 B  is a block diagram  120  illustrating an example embodiment of elements within the networking units block  122  and their connections to the NCB. The System DMA (Direct Memory Access) Packet Interface (DPI) Packet (SDP) Interface Unit  102  is coupled with a DMA Packet Interface (DPI) unit  104 . The DPI unit is coupled with the PCI-Express Interface Units (PEM)  114  to receive information from PCI-Express. The SDP  102  can further communicate with a Network Interface Controller (NIC) that exchanges packets with PCIe/SATA/Ethernet. 
       FIG.  1 C  is a block diagram  130  illustrating an example embodiment of a System DMA Packet Interface (DPI) Packet (SDP) Interface Unit, DMA Packet Interface Unit, and PEM unit. The SDP  102  interface unit provides PCIe Endpoint support for a remote host to DMA packets into and out of a hardware processor. The SDP  102  includes a packet input unit (PKI)  108  and a packet output unit (PKO)  110 . The SDP  102  communicates with a DMA Packet Interface (DPI) unit  104  having a direct memory access (DMA) unit  106 . The DPI unit  104  further communicates with PEM units, for example, having virtual functions (VFs) and physical functions (PFs). 
     The SDP  102  has 512 physical SDP input rings that are paired with SDP output rings. A person having ordinary skill in the art can recognize that the exact number of rings can vary, however. A PCIe host or other external device may submit instructions/packets to the SDP  102 , which is a way to feed packets or commands to the chip on which the SDP resides. 
     The SDP  102  can further stop instruction fetches for a physical ring when buffer counts are low. The SDP  102  does not fetch new instructions for a physical ring when the network interface controller (NIC) is exerting backpressure for its respective pair. 
     In processors that handle packets and packet flows, packet flow management to a particular destination node can be a problem. For example, the destination node can run out of buffer space or memory, which can cause packets to be dropped. To prevent packet dropping, backpressure can be applied to upstream flows when a given function (e.g., virtual function (VF) or physical function (PF)) cannot process packets to prevent overflowing a destination node function with packets. Applying backpressure sends a signal to all upstream devices to stop sending packets to the destination node, however, already sent packets to continue along the packet flow to the destination node. However, if the given function cannot process packets for long periods of time, applying back pressure can congest upstream systems and slow other flows unrelated to the given function. In other words, applying backpressure to systems when resources are limited allows more control in software but can cause congestion to upstream systems. 
     In an embodiment of the present disclosure, a multi-state framework can solve this congestion problem. In an embodiment, all destination node functions (e.g., VFs and PFs) are assigned a good actor state, bad actor state, and a backpressure state. Other network elements change their behavior with respect to the destination node based on the assigned state. 
     Limiting backpressure to only destinations node that are behaving in a desirable manner prevents such congestion. Embodiments of the present disclosure identify functions that are not processing packets in a timely manner and prevents them from applying backpressure. Packets sent to bad actor state destinations are dropped, which prevents such packets from filling up local memory. 
     For example, for destinations in the bad actor state, instead of backpressure, hardware drops all packets destined for the destination node. For destinations in the good actor state, network elements send packets to the destination node as in normal operation. For destinations in the backpressure state, network elements act as if backpressure is applied by not sending any new packets to the destination node. 
     At startup, all destinations initialize in bad actor states. After sufficient memory buffers are allocated to send packets, the destination node changes to the good actor state. If a destination node does not have sufficient buffers to send packets, it is placed in the backpressure state and a timer is started for that function. When a destination node starts to allow packet traffic to flow again, the timer stops and resets to zero. If a destination node continues not accepting packets and its timer has reached a programmable threshold, the destination node is moved to the bad actor state. Destination nodes can also move from good actor states to bad actor states if a function is disabled or reset, indicating it cannot receive packets. A destination node can also move to bad actor state if a packet is received and there are no buffers to send it to prevent head of line blocking and allow the packet to be dropped. 
       FIGS.  2 A-B  are diagrams  200  and  250  illustrating an example embodiment of a virtual function  202  having a plurality of buffers  204   a - n . The plurality of buffers  204   a - n  can store data or pointers to data in a separate memory. 
     A configurable watermark level  206  indicates number of buffers that should be available for ideal performance. 
     In one embodiment, the configurable watermark level  206  can indicate a number of buffers to remain empty. A doorbell or other process can determine the number of filled buffers and compare the number of filled buffers to the watermark. In another embodiment, the doorbell or other process can determine the number of empty buffers and compare the number of empty buffers to the watermark. Such a comparison can be performed either in hardware or by a processor. 
     In  FIG.  2 A , the filled buffers  204   a - c  and beyond, which are represented by the buffers having diagonal stripes, are below the configurable watermark level  206 . In  FIG.  2 B , the filled buffers  204   a - c  and beyond are beyond the configurable watermark level  256 . As described above, however, the watermark level can instead represent the number of empty buffers instead of the number of filled buffers. A person having ordinary skill in the art can recognize that such a modification can be made, and that the other principles described in this application apply to either embodiment. 
       FIG.  3    is a state diagram  300  illustrating an example embodiment of current methods of applying backpressure. A node in normal operation  302 , upon having low memory or another trigger from a host, etc., begins applying backpressure  304  by sending a signal to other devices that send it packets. Upon the node regaining adequate free memory, the node signals to the other devices that it has adequate free memory, and returns to normal operation  302 . 
       FIG.  4    is a state diagram  400  illustrating an example embodiment of a method of the present disclosure. Upon startup  402 , a node is initialized into a bad actor state  404  as an assumption. While in the bad actor state  404 , the node does not accept any packets being sent to it. Any packets that have already been sent are dropped by hardware. Further, the node can send a signal to other nodes along the packet flow that the node is in a bad actor state, so that the other nodes do not send new packets to the node in the bad actor state  404 . 
     The node, periodically (e.g., after a set number of clock cycles), checks its buffer levels. If the buffer levels are below the watermark level, indicating there is enough memory, the node transitions to a good actor state  406 . In the good actor state, the node can receive packets normally with no backpressure or dropped packets. 
     From the good actor state  406 , the used memory of the buffer can rise above the watermark as packets are received. In response, the node can transition to a backpressure state  408 . In the backpressure state  408 , a signal is sent to all upstream nodes to stop sending new packets. In addition, upon entering the backpressure state, a timer begins to count time or cycles for a configurable amount of time. If, at the end of the period, the used memory remains above the watermark, the node transitions to the bad actor state  404 , where backpressure is no longer applied, hardware can drop packets, and no more packets are sent to the destination node. However, if, after the period of time has elapsed, the used memory amount falls below the watermark, the node returns to the good actor state  406 . 
     A person having ordinary skill in the art can recognize that, in other embodiments, events can trigger the good actor state  406  transitioning to the bad actor state  404 , such as disabling the node/ring/function, user shutdown of the node/ring/function, an error condition, or no buffers being available. 
       FIG.  5    is a state diagram  500  illustrating an example embodiment of a method employed by the present disclosure. At startup  502 , the node is initialized in a bad actor state  504 . While in the bad actor state  504 , the node does not accept any packets being sent to it. Any packets that have already been sent are dropped by hardware. Further, the node can send a signal to other nodes along the packet flow that the node is in a bad actor state, so that the other nodes do not send new packets to the node in the bad actor state  504 . 
     Upon detecting that the used memory falls below the watermark, the node transitions to a good actor state  506 . When used memory rises above the watermark, the node remains in the good actor state  506  but applies backpressure, as described above, and starts a timer. If, after the timer expires, the used memory remains above the watermark, the node enters the bad actor state  504 . 
       FIGS.  4  and  5    both illustrate respective state diagrams  400  and  500  that solve the same problem with similar solutions. However, the state diagram  400  of  FIG.  4    is represented with a two-state solution after the startup state, where the state diagram  500  of  FIG.  5    is represented with a tri-state solution after the startup state. Effectively, in  FIG.  4   , the backpressure state is a separate, but in  FIG.  5   , the backpressure state becomes part of the good actor state. 
     The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety. 
     While example embodiments have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the embodiments encompassed by the appended claims.