Date:   Wed, 21 Mar 2018 12:37:18 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Tal Gilboa <talgi@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [PATCH net-next V2] Documentation/networking: Add net DIM
 documentation

On 03/21/2018 11:33 AM, Tal Gilboa wrote:
> 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@...lanox.com>
> ---
>  Documentation/networking/net_dim.txt | 174 +++++++++++++++++++++++++++++++++++
>  1 file changed, 174 insertions(+)
>  create mode 100644 Documentation/networking/net_dim.txt
> 
> diff --git a/Documentation/networking/net_dim.txt b/Documentation/networking/net_dim.txt
> new file mode 100644
> index 0000000..9cb31c5
> --- /dev/null
> +++ b/Documentation/networking/net_dim.txt
> @@ -0,0 +1,174 @@
> +Net DIM - Generic Network Dynamic Interrupt Moderation
> +======================================================
> +
> +Author:
> +	Tal Gilboa <talgi@...lanox.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.

I think a short description of the predefined profiles could help.

> +
> +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

Is it completely up to the driver to decide when to call net_dim()?
So it could be based on TX traffic, RX traffic, time, queue depths, etc.?

> +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

(continuing my question from above:)
or on each interrupt.  But the hardware could also be doing interrupt mitigation,
so each interrupt doesn't always correlate to anything specific.

> +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);
> +	...
> +}
> 

Reviewed-by: Randy Dunlap <rdunlap@...radead.org>

thanks,
-- 
~Randy

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