[PATCH v2 10/10] Documentation/core-api: Add min heap API introduction

Kuan-Wei Chiu
2024-10-20T04:03:12Z

Introduce an overview of the min heap API, detailing its usage and
functionality. The documentation aims to provide developers with a
clear understanding of how to implement and utilize min heaps within
the Linux kernel, enhancing the overall accessibility of this data
structure.

Signed-off-by: Kuan-Wei Chiu <visitorckw@gmail.com>
---
Changes in v2:
- Wrapped lines at 80 columns.
- Updated Example Usage.
- Added documentation for the ability to pass NULL to the swp in
min_heap_callbacks to use the builtin swap.

Documentation/core-api/index.rst | 1 +
Documentation/core-api/min_heap.rst | 300 ++++++++++++++++++++++++++++
2 files changed, 301 insertions(+)
create mode 100644 Documentation/core-api/min_heap.rst

diff --git a/Documentation/core-api/index.rst b/Documentation/core-api/index.rst
index 6a875743dd4b..563b8fc0002f 100644
--- a/Documentation/core-api/index.rst
+++ b/Documentation/core-api/index.rst
@@ -52,6 +52,7 @@ Library functionality that is used throughout the kernel.
wrappers/atomic_bitops
floating-point
union_find
+ min_heap

Low level entry and exit
========================
diff --git a/Documentation/core-api/min_heap.rst b/Documentation/core-api/min_heap.rst
new file mode 100644
index 000000000000..0c636c8b7aa5
--- /dev/null
+++ b/Documentation/core-api/min_heap.rst
@@ -0,0 +1,300 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+============
+Min Heap API
+============
+
+Introduction
+============
+
+The Min Heap API provides a set of functions and macros for managing min-heaps
+in the Linux kernel. A min-heap is a binary tree structure where the value of
+each node is less than or equal to the values of its children, ensuring that
+the smallest element is always at the root.
+
+This document provides a guide to the Min Heap API, detailing how to define and
+use min-heaps. Users should not directly call functions with **__min_heap_*()**
+prefixes, but should instead use the provided macro wrappers.
+
+In addition to the standard version of the functions, the API also includes a
+set of inline versions for performance-critical scenarios. These inline
+functions have the same names as their non-inline counterparts but include an
+**_inline** suffix. For example, **__min_heap_init_inline** and its
+corresponding macro wrapper **min_heap_init_inline**. The inline versions allow
+custom comparison and swap functions to be called directly, rather than through
+indirect function calls. This can significantly reduce overhead, especially
+when CONFIG_MITIGATION_RETPOLINE is enabled, as indirect function calls become
+more expensive. As with the non-inline versions, it is important to use the
+macro wrappers for inline functions instead of directly calling the functions
+themselves.
+
+Data Structures
+===============
+
+Min-Heap Definition
+-------------------
+
+The core data structure for representing a min-heap is defined using the
+**MIN_HEAP_PREALLOCATED** and **DEFINE_MIN_HEAP** macros. These macros allow
+you to define a min-heap with a preallocated buffer or dynamically allocated
+memory.
+
+Example:
+
+.. code-block:: c
+
+ #define MIN_HEAP_PREALLOCATED(_type, _name, _nr)
+ struct _name {
+ int nr; /* Number of elements in the heap */
+ int size; /* Maximum number of elements that can be held */
+ _type *data; /* Pointer to the heap data */
+ _type preallocated[_nr]; /* Static preallocated array */
+ }
+
+ #define DEFINE_MIN_HEAP(_type, _name) MIN_HEAP_PREALLOCATED(_type, _name, 0)
+
+A typical heap structure will include a counter for the number of elements
+(`nr`), the maximum capacity of the heap (`size`), and a pointer to an array of
+elements (`data`). Optionally, you can specify a static array for preallocated
+heap storage using **MIN_HEAP_PREALLOCATED**.
+
+Min Heap Callbacks
+------------------
+
+The **struct min_heap_callbacks** provides customization options for ordering
+elements in the heap and swapping them. It contains two function pointers:
+
+.. code-block:: c
+
+ struct min_heap_callbacks {
+ bool (*less)(const void *lhs, const void *rhs, void *args);
+ void (*swp)(void *lhs, void *rhs, void *args);
+ };
+
+- **less** is the comparison function used to establish the order of elements.
+- **swp** is a function for swapping elements in the heap. If swp is set to
+ NULL, the default swap function will be used, which swaps the elements based on their size
+
+Macro Wrappers
+==============
+
+The following macro wrappers are provided for interacting with the heap in a
+user-friendly manner. Each macro corresponds to a function that operates on the
+heap, and they abstract away direct calls to internal functions.
+
+Each macro accepts various parameters that are detailed below.
+
+Heap Initialization
+--------------------
+
+.. code-block:: c
+
+ min_heap_init(heap, data, size);
+
+- **heap**: A pointer to the min-heap structure to be initialized.
+- **data**: A pointer to the buffer where the heap elements will be stored. If
+ `NULL`, the preallocated buffer within the heap structure will be used.
+- **size**: The maximum number of elements the heap can hold.
+
+This macro initializes the heap, setting its initial state. If `data` is
+`NULL`, the preallocated memory inside the heap structure will be used for
+storage. Otherwise, the user-provided buffer is used. The operation is **O(1)**.
+
+**Inline Version:** min_heap_init_inline(heap, data, size)
+
+Accessing the Top Element
+-------------------------
+
+.. code-block:: c
+
+ element = min_heap_peek(heap);
+
+- **heap**: A pointer to the min-heap from which to retrieve the smallest
+ element.
+
+This macro returns a pointer to the smallest element (the root) of the heap, or
+`NULL` if the heap is empty. The operation is **O(1)**.
+
+**Inline Version:** min_heap_peek_inline(heap)
+
+Heap Insertion
+--------------
+
+.. code-block:: c
+
+ success = min_heap_push(heap, element, callbacks, args);
+
+- **heap**: A pointer to the min-heap into which the element should be inserted.
+- **element**: A pointer to the element to be inserted into the heap.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the
+ `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro inserts an element into the heap. It returns `true` if the insertion
+was successful and `false` if the heap is full. The operation is **O(log n)**.
+
+**Inline Version:** min_heap_push_inline(heap, element, callbacks, args)
+
+Heap Removal
+------------
+
+.. code-block:: c
+
+ success = min_heap_pop(heap, callbacks, args);
+
+- **heap**: A pointer to the min-heap from which to remove the smallest element.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the
+ `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro removes the smallest element (the root) from the heap. It returns
+`true` if the element was successfully removed, or `false` if the heap is
+empty. The operation is **O(log n)**.
+
+**Inline Version:** min_heap_pop_inline(heap, callbacks, args)
+
+Heap Maintenance
+----------------
+
+You can use the following macros to maintain the heap's structure:
+
+.. code-block:: c
+
+ min_heap_sift_down(heap, pos, callbacks, args);
+
+- **heap**: A pointer to the min-heap.
+- **pos**: The index from which to start sifting down.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the
+ `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro restores the heap property by moving the element at the specified
+index (`pos`) down the heap until it is in the correct position. The operation
+is **O(log n)**.
+
+**Inline Version:** min_heap_sift_down_inline(heap, pos, callbacks, args)
+
+.. code-block:: c
+
+ min_heap_sift_up(heap, idx, callbacks, args);
+
+- **heap**: A pointer to the min-heap.
+- **idx**: The index of the element to sift up.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the
+ `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro restores the heap property by moving the element at the specified
+index (`idx`) up the heap. The operation is **O(log n)**.
+
+**Inline Version:** min_heap_sift_up_inline(heap, idx, callbacks, args)
+
+.. code-block:: c
+
+ min_heapify_all(heap, callbacks, args);
+
+- **heap**: A pointer to the min-heap.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the
+ `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro ensures that the entire heap satisfies the heap property. It is
+called when the heap is built from scratch or after many modifications. The
+operation is **O(n)**.
+
+**Inline Version:** min_heapify_all_inline(heap, callbacks, args)
+
+Removing Specific Elements
+--------------------------
+
+.. code-block:: c
+
+ success = min_heap_del(heap, idx, callbacks, args);
+
+- **heap**: A pointer to the min-heap.
+- **idx**: The index of the element to delete.
+- **callbacks**: A pointer to a `struct min_heap_callbacks` providing the
+ `less` and `swp` functions.
+- **args**: Optional arguments passed to the `less` and `swp` functions.
+
+This macro removes an element at the specified index (`idx`) from the heap and
+restores the heap property. The operation is **O(log n)**.
+
+**Inline Version:** min_heap_del_inline(heap, idx, callbacks, args)
+
+Other Utilities
+===============
+
+- **min_heap_full(heap)**: Checks whether the heap is full.
+ Complexity: **O(1)**.
+
+.. code-block:: c
+
+ bool full = min_heap_full(heap);
+
+- `heap`: A pointer to the min-heap to check.
+
+This macro returns `true` if the heap is full, otherwise `false`.
+
+**Inline Version:** min_heap_full_inline(heap)
+
+- **min_heap_empty(heap)**: Checks whether the heap is empty.
+ Complexity: **O(1)**.
+
+.. code-block:: c
+
+ bool empty = min_heap_empty(heap);
+
+- `heap`: A pointer to the min-heap to check.
+
+This macro returns `true` if the heap is empty, otherwise `false`.
+
+**Inline Version:** min_heap_empty_inline(heap)
+
+Example Usage
+=============
+
+An example usage of the min-heap API would involve defining a heap structure,
+initializing it, and inserting and removing elements as needed.
+
+.. code-block:: c
+
+ #include <linux/min_heap.h>
+
+ int my_less_function(const void *lhs, const void *rhs, void *args) {
+ return (*(int *)lhs < *(int *)rhs);
+ }
+
+ struct min_heap_callbacks heap_cb = {
+ .less = my_less_function, /* Comparison function for heap order */
+ .swp = NULL, /* Use default swap function */
+ };
+
+ void example_usage(void) {
+ /* Pre-populate the buffer with elements */
+ int buffer[5] = {5, 2, 8, 1, 3};
+ /* Declare a min-heap */
+ DEFINE_MIN_HEAP(int, my_heap);
+
+ /* Initialize the heap with preallocated buffer and size */
+ min_heap_init(&my_heap, buffer, 5);
+
+ /* Build the heap using min_heapify_all */
+ my_heap.nr = 5; /* Set the number of elements in the heap */
+ min_heapify_all(&my_heap, &heap_cb, NULL);
+
+ /* Peek at the top element (should be 1 in this case) */
+ int *top = min_heap_peek(&my_heap);
+ pr_info("Top element: %d\n", *top);
+
+ /* Pop the top element (1) and get the new top (2) */
+ min_heap_pop(&my_heap, &heap_cb, NULL);
+ top = min_heap_peek(&my_heap);
+ pr_info("New top element: %d\n", *top);
+
+ /* Insert a new element (0) and recheck the top */
+ int new_element = 0;
+ min_heap_push(&my_heap, &new_element, &heap_cb, NULL);
+ top = min_heap_peek(&my_heap);
+ pr_info("Top element after insertion: %d\n", *top);
+ }
--
2.34.1