Documentation/networking: Add net DIM documentation

Net DIM is a generic algorithm, purposed for dynamically
optimizing network devices interrupt moderation. This
document describes how it works and how to use it.
Signed-off-by: Tal Gilboa <talgi@mellanox.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/net_dim.txt

+Net DIM - Generic Network Dynamic Interrupt Moderation
+======================================================
+
+Author:
+	Tal Gilboa <talgi@mellanox.com>
+
+
+Contents
+=========
+
+- Assumptions
+- Introduction
+- The Net DIM Algorithm
+- Registering a Network Device to DIM
+- Example
+
+Part 0: Assumptions
+======================
+
+This document assumes the reader has basic knowledge in network drivers
+and in general interrupt moderation.
+
+
+Part I: 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.
+
+
+Part II: The Net DIM Algorithm
+===============================
+
+Each iteration of the Net DIM algorithm follows these steps:
+1. Calculates new data sample.
+2. Compares it to previous sample.
+3. Makes a decision - suggests interrupt moderation configuration fields.
+4. 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.
+
+
+Part III: Registering a Network Device to DIM
+==============================================
+
+Net DIM API exposes the main function net_dim(struct net_dim *dim,
+struct net_dim_sample end_sample). 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: struct net_dim and struct net_dim_sample. Struct net_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.
+Struct net_dim_sample describes a data sample, which will be compared to the
+data sample stored in struct net_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
+struct net_dim to the net_dim() function call. It is advised for each entity
+using Net DIM to hold a struct net_dim as part of its data structure and use it
+as the main Net DIM API object. The struct net_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.
+
+
+Part IV: 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.
+
+my_driver.c:
+
+#include <linux/net_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 net_dim from struct work_struct */
+	struct net_dim *dim = container_of(work, struct net_dim,
+					   work);
+	/* Do interrupt moderation related stuff */
+	...
+
+	/* Signal net DIM work is done and it should move to next iteration */
+	dim->state = NET_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 net_dim_sample dim_sample;
+	...
+	/* Initiate data sample struct with current data */
+	net_dim_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);
+	...
+}