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diff --git a/Documentation/networking/net_dim.rst b/Documentation/networking/net_dim.rst new file mode 100644 index 0000000000..3bed9fd953 --- /dev/null +++ b/Documentation/networking/net_dim.rst @@ -0,0 +1,176 @@ +====================================================== +Net DIM - Generic Network Dynamic Interrupt Moderation +====================================================== + +:Author: Tal Gilboa <talgi@mellanox.com> + +.. contents:: :depth: 2 + +Assumptions +=========== + +This document assumes the reader has basic knowledge in network drivers +and in general interrupt moderation. + + +Introduction +============ + +Dynamic Interrupt Moderation (DIM) (in networking) refers to changing the +interrupt moderation configuration of a channel in order to optimize packet +processing. The mechanism includes an algorithm which decides if and how to +change moderation parameters for a channel, usually by performing an analysis on +runtime data sampled from the system. Net DIM is such a mechanism. In each +iteration of the algorithm, it analyses a given sample of the data, compares it +to the previous sample and if required, it can decide to change some of the +interrupt moderation configuration fields. The data sample is composed of data +bandwidth, the number of packets and the number of events. The time between +samples is also measured. Net DIM compares the current and the previous data and +returns an adjusted interrupt moderation configuration object. In some cases, +the algorithm might decide not to change anything. The configuration fields are +the minimum duration (microseconds) allowed between events and the maximum +number of wanted packets per event. The Net DIM algorithm ascribes importance to +increase bandwidth over reducing interrupt rate. + + +Net DIM Algorithm +================= + +Each iteration of the Net DIM algorithm follows these steps: + +#. Calculates new data sample. +#. Compares it to previous sample. +#. Makes a decision - suggests interrupt moderation configuration fields. +#. Applies a schedule work function, which applies suggested configuration. + +The first two steps are straightforward, both the new and the previous data are +supplied by the driver registered to Net DIM. The previous data is the new data +supplied to the previous iteration. The comparison step checks the difference +between the new and previous data and decides on the result of the last step. +A step would result as "better" if bandwidth increases and as "worse" if +bandwidth reduces. If there is no change in bandwidth, the packet rate is +compared in a similar fashion - increase == "better" and decrease == "worse". +In case there is no change in the packet rate as well, the interrupt rate is +compared. Here the algorithm tries to optimize for lower interrupt rate so an +increase in the interrupt rate is considered "worse" and a decrease is +considered "better". Step #2 has an optimization for avoiding false results: it +only considers a difference between samples as valid if it is greater than a +certain percentage. Also, since Net DIM does not measure anything by itself, it +assumes the data provided by the driver is valid. + +Step #3 decides on the suggested configuration based on the result from step #2 +and the internal state of the algorithm. The states reflect the "direction" of +the algorithm: is it going left (reducing moderation), right (increasing +moderation) or standing still. Another optimization is that if a decision +to stay still is made multiple times, the interval between iterations of the +algorithm would increase in order to reduce calculation overhead. Also, after +"parking" on one of the most left or most right decisions, the algorithm may +decide to verify this decision by taking a step in the other direction. This is +done in order to avoid getting stuck in a "deep sleep" scenario. Once a +decision is made, an interrupt moderation configuration is selected from +the predefined profiles. + +The last step is to notify the registered driver that it should apply the +suggested configuration. This is done by scheduling a work function, defined by +the Net DIM API and provided by the registered driver. + +As you can see, Net DIM itself does not actively interact with the system. It +would have trouble making the correct decisions if the wrong data is supplied to +it and it would be useless if the work function would not apply the suggested +configuration. This does, however, allow the registered driver some room for +manoeuvre as it may provide partial data or ignore the algorithm suggestion +under some conditions. + + +Registering a Network Device to DIM +=================================== + +Net DIM API exposes the main function net_dim(). +This function is the entry point to the Net +DIM algorithm and has to be called every time the driver would like to check if +it should change interrupt moderation parameters. The driver should provide two +data structures: :c:type:`struct dim <dim>` and +:c:type:`struct dim_sample <dim_sample>`. :c:type:`struct dim <dim>` +describes the state of DIM for a specific object (RX queue, TX queue, +other queues, etc.). This includes the current selected profile, previous data +samples, the callback function provided by the driver and more. +:c:type:`struct dim_sample <dim_sample>` describes a data sample, +which will be compared to the data sample stored in :c:type:`struct dim <dim>` +in order to decide on the algorithm's next +step. The sample should include bytes, packets and interrupts, measured by +the driver. + +In order to use Net DIM from a networking driver, the driver needs to call the +main net_dim() function. The recommended method is to call net_dim() on each +interrupt. Since Net DIM has a built-in moderation and it might decide to skip +iterations under certain conditions, there is no need to moderate the net_dim() +calls as well. As mentioned above, the driver needs to provide an object of type +:c:type:`struct dim <dim>` to the net_dim() function call. It is advised for +each entity using Net DIM to hold a :c:type:`struct dim <dim>` as part of its +data structure and use it as the main Net DIM API object. +The :c:type:`struct dim_sample <dim_sample>` should hold the latest +bytes, packets and interrupts count. No need to perform any calculations, just +include the raw data. + +The net_dim() call itself does not return anything. Instead Net DIM relies on +the driver to provide a callback function, which is called when the algorithm +decides to make a change in the interrupt moderation parameters. This callback +will be scheduled and run in a separate thread in order not to add overhead to +the data flow. After the work is done, Net DIM algorithm needs to be set to +the proper state in order to move to the next iteration. + + +Example +======= + +The following code demonstrates how to register a driver to Net DIM. The actual +usage is not complete but it should make the outline of the usage clear. + +.. code-block:: c + + #include <linux/dim.h> + + /* Callback for net DIM to schedule on a decision to change moderation */ + void my_driver_do_dim_work(struct work_struct *work) + { + /* Get struct dim from struct work_struct */ + struct dim *dim = container_of(work, struct dim, + work); + /* Do interrupt moderation related stuff */ + ... + + /* Signal net DIM work is done and it should move to next iteration */ + dim->state = DIM_START_MEASURE; + } + + /* My driver's interrupt handler */ + int my_driver_handle_interrupt(struct my_driver_entity *my_entity, ...) + { + ... + /* A struct to hold current measured data */ + struct dim_sample dim_sample; + ... + /* Initiate data sample struct with current data */ + dim_update_sample(my_entity->events, + my_entity->packets, + my_entity->bytes, + &dim_sample); + /* Call net DIM */ + net_dim(&my_entity->dim, dim_sample); + ... + } + + /* My entity's initialization function (my_entity was already allocated) */ + int my_driver_init_my_entity(struct my_driver_entity *my_entity, ...) + { + ... + /* Initiate struct work_struct with my driver's callback function */ + INIT_WORK(&my_entity->dim.work, my_driver_do_dim_work); + ... + } + +Dynamic Interrupt Moderation (DIM) library API +============================================== + +.. kernel-doc:: include/linux/dim.h + :internal: |