• linux 内核中的pid和前缀树

    前言：

    写这个文章的初衷是因为今天手写了一个字典树，然后写字典树以后忽然想到了之前看的技术文章，linux kernel 之前的pid 申请方式已经从 bitmap 变成了 基数树，所以打算写文章再回顾一下这种数据结构算法

    一、内核中pid的申请

    旧的内核中pid的申请就不多说了，是使用的是bitmap去实现的，新版的换为了基数树。申请pid的内核代码在：

    1. struct pid *alloc_pid(struct pid_namespace *ns, pid_t *set_tid,
    2. 		      size_t set_tid_size)
    3. {
    4. .......
    5. }

    继续看代码,申请函数为:

    1. if (tid) {
    2. 			nr = idr_alloc(&tmp->idr, NULL, tid,
    3. 				       tid + 1, GFP_ATOMIC);
    4. 			/*
    5. 			 * If ENOSPC is returned it means that the PID is
    6. 			 * alreay in use. Return EEXIST in that case.
    7. 			 */
    8. 			if (nr == -ENOSPC)
    9. 				nr = -EEXIST;
    10. 		} else {
    11. 			int pid_min = 1;
    12. 			/*
    13. 			 * init really needs pid 1, but after reaching the
    14. 			 * maximum wrap back to RESERVED_PIDS
    15. 			 */
    16. 			if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
    17. 				pid_min = RESERVED_PIDS;
    18.  
    19. 			/*
    20. 			 * Store a null pointer so find_pid_ns does not find
    21. 			 * a partially initialized PID (see below).
    22. 			 */
    23. 			nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
    24. 					      pid_max, GFP_ATOMIC);
    25. 		}

    进程最大的pid号为：

    1.  
    2. int pid_max = PID_MAX_DEFAULT;
    3.  
    4. /*
    5.  * This controls the default maximum pid allocated to a process
    6.  */
    7. #define PID_MAX_DEFAULT (CONFIG_BASE_SMALL ? 0x1000 : 0x8000)

    这里我发现了一个问题，linux 最大的pid 是 0x8000 而不是 0xFFFF,这是为了和老的内核兼容，我们可以通过

    cat /proc/sys/kernel/pid_max

    查询当前内核的最大pid。

    申请pid 函数我们聚焦在

    idr_alloc 和 idr_alloc_cyclic

    再进去看 idr_alloc_cyclic 的实现

    1. int idr_alloc_cyclic(struct idr *idr, void *ptr, int start, int end, gfp_t gfp)
    2. {
    3. 	u32 id = idr->idr_next;
    4. 	int err, max = end > 0 ? end - 1 : INT_MAX;
    5.  
    6. 	if ((int)id < start)
    7. 		id = start;
    8.  
    9. 	err = idr_alloc_u32(idr, ptr, &id, max, gfp);
    10. 	if ((err == -ENOSPC) && (id > start)) {
    11. 		id = start;
    12. 		err = idr_alloc_u32(idr, ptr, &id, max, gfp);
    13. 	}
    14. 	if (err)
    15. 		return err;
    16.  
    17. 	idr->idr_next = id + 1;
    18. 	return id;
    19. }

    具体实现pid 申请的函数在:

    1. int idr_alloc_u32(struct idr *idr, void *ptr, u32 *nextid,
    2. 			unsigned long max, gfp_t gfp)
    3. {
    4. 	struct radix_tree_iter iter;
    5. 	void __rcu **slot;
    6. 	unsigned int base = idr->idr_base;
    7. 	unsigned int id = *nextid;
    8.  
    9. 	if (WARN_ON_ONCE(!(idr->idr_rt.xa_flags & ROOT_IS_IDR)))
    10. 		idr->idr_rt.xa_flags |= IDR_RT_MARKER;
    11.  
    12. 	id = (id < base) ? 0 : id - base;
    13. 	radix_tree_iter_init(&iter, id);
    14. 	slot = idr_get_free(&idr->idr_rt, &iter, gfp, max - base);
    15. 	if (IS_ERR(slot))
    16. 		return PTR_ERR(slot);
    17.  
    18. 	*nextid = iter.index + base;
    19. 	/* there is a memory barrier inside radix_tree_iter_replace() */
    20. 	radix_tree_iter_replace(&idr->idr_rt, &iter, slot, ptr);
    21. 	radix_tree_iter_tag_clear(&idr->idr_rt, &iter, IDR_FREE);
    22.  
    23. 	return 0;
    24. }

    我们需要重点分析这个函数,重要的数据结构radix_tree_iter，我们看一下这个数据结构：

    这是一个基数树的迭代器

    1. /**
    2.  * struct radix_tree_iter - radix tree iterator state
    3.  *
    4.  * @index:	index of current slot
    5.  * @next_index:	one beyond the last index for this chunk
    6.  * @tags:	bit-mask for tag-iterating
    7.  * @node:	node that contains current slot
    8.  *
    9.  * This radix tree iterator works in terms of "chunks" of slots.  A chunk is a
    10.  * subinterval of slots contained within one radix tree leaf node.  It is
    11.  * described by a pointer to its first slot and a struct radix_tree_iter
    12.  * which holds the chunk's position in the tree and its size.  For tagged
    13.  * iteration radix_tree_iter also holds the slots' bit-mask for one chosen
    14.  * radix tree tag.
    15.  */
    16. struct radix_tree_iter {
    17. 	unsigned long	index;
    18. 	unsigned long	next_index;
    19. 	unsigned long	tags;
    20. 	struct radix_tree_node *node;
    21. };

    迭代器中有重要的数据结构

    #define radix_tree_node		xa_node

    也就是替换后 变为了

    struct xa_node*

    我们再继续看 xa_node* 这个数据结构,这是基数树的节点

    1. /*
    2.  * @count is the count of every non-NULL element in the ->slots array
    3.  * whether that is a value entry, a retry entry, a user pointer,
    4.  * a sibling entry or a pointer to the next level of the tree.
    5.  * @nr_values is the count of every element in ->slots which is
    6.  * either a value entry or a sibling of a value entry.
    7.  */
    8. struct xa_node {
    9. 	unsigned char	shift;		/* Bits remaining in each slot */
    10. 	unsigned char	offset;		/* Slot offset in parent */
    11. 	unsigned char	count;		/* Total entry count */
    12. 	unsigned char	nr_values;	/* Value entry count */
    13. 	struct xa_node __rcu *parent;	/* NULL at top of tree */
    14. 	struct xarray	*array;		/* The array we belong to */
    15. 	union {
    16. 		struct list_head private_list;	/* For tree user */
    17. 		struct rcu_head	rcu_head;	/* Used when freeing node */
    18. 	};
    19. 	void __rcu	*slots[XA_CHUNK_SIZE];
    20. 	union {
    21. 		unsigned long	tags[XA_MAX_MARKS][XA_MARK_LONGS];
    22. 		unsigned long	marks[XA_MAX_MARKS][XA_MARK_LONGS];
    23. 	};
    24. };

    这些宏：

    1. #ifndef XA_CHUNK_SHIFT
    2. #define XA_CHUNK_SHIFT		(CONFIG_BASE_SMALL ? 4 : 6)
    3. #endif
    4. #define XA_CHUNK_SIZE		(1UL << XA_CHUNK_SHIFT)

    XA_CHUNK_SIZE 是 64 或者16.

    #define XA_MAX_MARKS		3

    对应的CHUNK是6个位或者 4个位

    然后我们看到pid最大是0x8000, 4 *4 为 16 个 槽，这意味着基数树高最大为3((16 / 6 ) + 1)

    1. #if defined __x86_64__ && !defined __ILP32__
    2. # define __WORDSIZE	64
    3. #else
    4. # define __WORDSIZE	32
    5. #define __WORDSIZE32_SIZE_ULONG		0
    6. #define __WORDSIZE32_PTRDIFF_LONG	0
    7. #endif
    8.  
    9. #ifndef BITS_PER_LONG
    10. # define BITS_PER_LONG __WORDSIZE
    11. #endif

    BITS_PER_LONG 为64 或者32

    1. #define DIV_ROUND_UP(n,d) (((n) + (d) - 1) / (d))
    2.  
    3. /*
    4.  * The IDR API does not expose the tagging functionality of the radix tree
    5.  * to users.  Use tag 0 to track whether a node has free space below it.
    6.  */
    7. #define IDR_FREE	0

    带入计算就是:

    (64 + 64 -1) / 64 = 1

    对应的宏

    1. #define XA_MAX_MARKS		3
    2. #define XA_MARK_LONGS		DIV_ROUND_UP(XA_CHUNK_SIZE, BITS_PER_LONG)

    初始化基数树的迭代器:

    1. /**
    2.  * radix_tree_iter_init - initialize radix tree iterator
    3.  *
    4.  * @iter:	pointer to iterator state
    5.  * @start:	iteration starting index
    6.  * Returns:	NULL
    7.  */
    8. static __always_inline void __rcu **
    9. radix_tree_iter_init(struct radix_tree_iter *iter, unsigned long start)
    10. {
    11. 	/*
    12. 	 * Leave iter->tags uninitialized. radix_tree_next_chunk() will fill it
    13. 	 * in the case of a successful tagged chunk lookup.  If the lookup was
    14. 	 * unsuccessful or non-tagged then nobody cares about ->tags.
    15. 	 *
    16. 	 * Set index to zero to bypass next_index overflow protection.
    17. 	 * See the comment in radix_tree_next_chunk() for details.
    18. 	 */
    19. 	iter->index = 0;
    20. 	iter->next_index = start;
    21. 	return NULL;
    22. }

    主要看idr_get_free，这个函数要看懂

    shift -= RADIX_TREE_MAP_SHIFT;

    shift 最大是16， 每次我们移动是6位，对应的是CHUNKSIZE

    1. if (!tag_get(node, IDR_FREE, offset)) {
    2. 			offset = radix_tree_find_next_bit(node, IDR_FREE,
    3. 							offset + 1);
    4. 			start = next_index(start, node, offset);
    5. 			if (start > max || start == 0)
    6. 				return ERR_PTR(-ENOSPC);
    7. 			while (offset == RADIX_TREE_MAP_SIZE) {
    8. 				offset = node->offset + 1;
    9. 				node = node->parent;
    10. 				if (!node)
    11. 					goto grow;
    12. 				shift = node->shift;
    13. 			}
    14. 			child = rcu_dereference_raw(node->slots[offset]);
    15. 		}

    每次循环如果循环到tag不是IDR_FREE，就会继续找下一个tag，直到找到是IDR_FREE的solt。

    在这里退出循环

    1. else if (!radix_tree_is_internal_node(child))
    2. 			break;

    最后返回空闲槽

    1. 	iter->index = start;
    2. 	if (node)
    3. 		iter->next_index = 1 + min(max, (start | node_maxindex(node)));
    4. 	else
    5. 		iter->next_index = 1;
    6. 	iter->node = node;
    7. 	set_iter_tags(iter, node, offset, IDR_FREE);
    8.  
    9. 	return slot;

    上面的start 就应该是要申请的pid，我们最后看next_index

    1. /*
    2.  * The maximum index which can be stored in a radix tree
    3.  */
    4. static inline unsigned long shift_maxindex(unsigned int shift)
    5. {
    6. 	return (RADIX_TREE_MAP_SIZE << shift) - 1;
    7. }
    8.  
    9. static inline unsigned long node_maxindex(const struct radix_tree_node *node)
    10. {
    11. 	return shift_maxindex(node->shift);
    12. }
    13.  
    14. static unsigned long next_index(unsigned long index,
    15. 				const struct radix_tree_node *node,
    16. 				unsigned long offset)
    17. {
    18. 	return (index & ~node_maxindex(node)) + (offset << node->shift);
    19. }

    所以start 就是要申请的pid。

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  • 原文地址：https://blog.csdn.net/qq_32783703/article/details/133847341