Date:   Fri, 13 Jan 2017 10:01:41 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Mack <daniel@...que.org>
Cc:     ast@...com, dh.herrmann@...il.com, daniel@...earbox.net,
        netdev@...r.kernel.org, davem@...emloft.net
Subject: Re: [PATCH v2 1/2] bpf: add a longest prefix match trie map
 implementation

On Thu, Jan 12, 2017 at 06:29:21PM +0100, Daniel Mack wrote:
> This trie implements a longest prefix match algorithm that can be used
> to match IP addresses to a stored set of ranges.
> 
> Internally, data is stored in an unbalanced trie of nodes that has a
> maximum height of n, where n is the prefixlen the trie was created
> with.
> 
> Tries may be created with prefix lengths that are multiples of 8, in
> the range from 8 to 2048. The key used for lookup and update operations
> is a struct bpf_lpm_trie_key, and the value is a uint64_t.
> 
> The code carries more information about the internal implementation.
> 
> Signed-off-by: Daniel Mack <daniel@...que.org>
> Reviewed-by: David Herrmann <dh.herrmann@...il.com>
> ---
>  include/uapi/linux/bpf.h |   7 +
>  kernel/bpf/Makefile      |   2 +-
>  kernel/bpf/lpm_trie.c    | 499 +++++++++++++++++++++++++++++++++++++++++++++++
>  3 files changed, 507 insertions(+), 1 deletion(-)
>  create mode 100644 kernel/bpf/lpm_trie.c
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 0eb0e87..d564277 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -63,6 +63,12 @@ struct bpf_insn {
>  	__s32	imm;		/* signed immediate constant */
>  };
>  
> +/* Key of an a BPF_MAP_TYPE_LPM_TRIE entry */
> +struct bpf_lpm_trie_key {
> +	__u32	prefixlen;	/* up to 32 for AF_INET, 128 for AF_INET6 */
> +	__u8	data[0];	/* Arbitrary size */
> +};
> +
>  /* BPF syscall commands, see bpf(2) man-page for details. */
>  enum bpf_cmd {
>  	BPF_MAP_CREATE,
> @@ -89,6 +95,7 @@ enum bpf_map_type {
>  	BPF_MAP_TYPE_CGROUP_ARRAY,
>  	BPF_MAP_TYPE_LRU_HASH,
>  	BPF_MAP_TYPE_LRU_PERCPU_HASH,
> +	BPF_MAP_TYPE_LPM_TRIE,
>  };
>  
>  enum bpf_prog_type {
> diff --git a/kernel/bpf/Makefile b/kernel/bpf/Makefile
> index 1276474..e1ce4f4 100644
> --- a/kernel/bpf/Makefile
> +++ b/kernel/bpf/Makefile
> @@ -1,7 +1,7 @@
>  obj-y := core.o
>  
>  obj-$(CONFIG_BPF_SYSCALL) += syscall.o verifier.o inode.o helpers.o
> -obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o
> +obj-$(CONFIG_BPF_SYSCALL) += hashtab.o arraymap.o percpu_freelist.o bpf_lru_list.o lpm_trie.o
>  ifeq ($(CONFIG_PERF_EVENTS),y)
>  obj-$(CONFIG_BPF_SYSCALL) += stackmap.o
>  endif
> diff --git a/kernel/bpf/lpm_trie.c b/kernel/bpf/lpm_trie.c
> new file mode 100644
> index 0000000..7f6d47e
> --- /dev/null
> +++ b/kernel/bpf/lpm_trie.c
> @@ -0,0 +1,499 @@
> +/*
> + * Longest prefix match list implementation
> + *
> + * Copyright (c) 2016,2017 Daniel Mack
> + * Copyright (c) 2016 David Herrmann
> + *
> + * This file is subject to the terms and conditions of version 2 of the GNU
> + * General Public License.  See the file COPYING in the main directory of the
> + * Linux distribution for more details.
> + */
> +
> +#include <linux/bpf.h>
> +#include <linux/err.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/vmalloc.h>
> +#include <net/ipv6.h>
> +
> +struct lpm_trie_node;
> +
> +struct lpm_trie_node {
> +	struct rcu_head rcu;
> +	struct lpm_trie_node __rcu	*child[2];
> +	void				*value;
> +	u32				prefixlen;
> +	u8				data[0];
> +};
> +
> +struct lpm_trie {
> +	struct bpf_map			map;
> +	struct lpm_trie_node __rcu	*root;
> +	size_t				n_entries;
> +	size_t				max_prefixlen;
> +	size_t				data_size;
> +	raw_spinlock_t			lock;
> +};
> +
> +/*
> + * This trie implements a longest prefix match algorithm that can be used to
> + * match IP addresses to a stored set of ranges.
> + *
> + * Data stored in @data of struct bpf_lpm_key and struct lpm_trie_node is
> + * interpreted as big endian, so data[0] stores the most significant byte.
> + *
> + * Match ranges are internally stored in instances of struct lpm_trie_node
> + * which each contain their prefix length as well as two pointers that may
> + * lead to more nodes containing more specific matches. Each node also stores
> + * a value that is defined by and returned to userspace via the update_elem
> + * and lookup functions.
> + *
> + * For instance, let's start with a trie that was created with a prefix length
> + * of 32, so it can be used for IPv4 addresses, and one single element that
> + * matches 192.168.0.0/16. The data array would hence contain
> + * [0xc0, 0xa8, 0x00, 0x00] in big-endian notation. This documentation will
> + * stick to IP-address notation for readability though.
> + *
> + * As the trie is empty initially, the new node (1) will be places as root
> + * node, denoted as (R) in the example below. As there are no other node, both
> + * child pointers are %NULL.
> + *
> + *              +----------------+
> + *              |       (1)  (R) |
> + *              | 192.168.0.0/16 |
> + *              |    value: 1    |
> + *              |   [0]    [1]   |
> + *              +----------------+
> + *
> + * Next, let's add a new node (2) matching 192.168.0.0/24. As there is already
> + * a node with the same data and a smaller prefix (ie, a less specific one),
> + * node (2) will become a child of (1). In child index depends on the next bit
> + * that is outside of that (1) matches, and that bit is 0, so (2) will be

s/outside of that/outside of what/ ?
or you meant something else?

> + * child[0] of (1):
> + *
> + *              +----------------+
> + *              |       (1)  (R) |
> + *              | 192.168.0.0/16 |
> + *              |    value: 1    |
> + *              |   [0]    [1]   |
> + *              +----------------+
> + *                   |
> + *    +----------------+
> + *    |       (2)      |
> + *    | 192.168.0.0/24 |
> + *    |    value: 2    |
> + *    |   [0]    [1]   |
> + *    +----------------+
> + *
> + * The child[1] slot of (1) could be filled with another node which has bit #17
> + * (the next bit after the ones that (1) matches on) set to 1. For instance,
> + * 192.168.128.0/24:
> + *
> + *              +----------------+
> + *              |       (1)  (R) |
> + *              | 192.168.0.0/16 |
> + *              |    value: 1    |
> + *              |   [0]    [1]   |
> + *              +----------------+
> + *                   |      |
> + *    +----------------+  +------------------+
> + *    |       (2)      |  |        (3)       |
> + *    | 192.168.0.0/24 |  | 192.168.128.0/24 |
> + *    |    value: 2    |  |     value: 3     |
> + *    |   [0]    [1]   |  |    [0]    [1]    |
> + *    +----------------+  +------------------+
> + *
> + * Let's add another node (4) to the game for 192.168.1.0/24. In order to place
> + * it, node (1) is looked at first, and because (4) of the semantics laid out
> + * above (bit #17 is 0), it would normally be attached to (1) as child[0].
> + * However, that slot is already allocated, so a new node is needed in between.
> + * That node is does not have a value attached to it and it will never be

s/node is does/node does/

> + * returned to users as result of a lookup. It is only there to differenciate

s/differenciate/differentiate/

> + * the traversal further. It will get a prefix as wide as necessary to
> + * distinguish its two children:
> + *
> + *                      +----------------+
> + *                      |       (1)  (R) |
> + *                      | 192.168.0.0/16 |
> + *                      |    value: 1    |
> + *                      |   [0]    [1]   |
> + *                      +----------------+
> + *                           |      |
> + *            +----------------+  +------------------+
> + *            |       (4)  (I) |  |        (3)       |
> + *            | 192.168.0.0/23 |  | 192.168.128.0/24 |
> + *            |    value: ---  |  |     value: 3     |
> + *            |   [0]    [1]   |  |    [0]    [1]    |
> + *            +----------------+  +------------------+
> + *                 |      |
> + *  +----------------+  +----------------+
> + *  |       (2)      |  |       (5)      |
> + *  | 192.168.0.0/24 |  | 192.168.1.0/24 |
> + *  |    value: 2    |  |     value: 5   |
> + *  |   [0]    [1]   |  |   [0]    [1]   |
> + *  +----------------+  +----------------+
> + *
> + * 192.168.1.1/32 would be a child of (5) etc.
> + *
> + * An intermediate node will be turned into a 'real' node on demand. In the
> + * example above, (4) would be re-used if 192.168.0.0/23 is added to the trie.
> + *
> + * A fully populated trie would have a height of 32 nodes, as the trie was
> + * created with a prefix length of 32.
> + *
> + * The lookup starts at the root node. If the current node matches and if there
> + * is a child that can be used to become more specific, the trie is traversed
> + * downwards. The last node in the traversal that is a non-intermediate one is
> + * returned.
> + */
> +
> +static inline int extract_bit(const u8 *data, size_t index)
> +{
> +	return !!(data[index / 8] & (1 << (7 - (index % 8))));
> +}
> +
> +/**
> + * longest_prefix_match() - determine the longest prefix
> + * @trie:	The trie to get internal sizes from
> + * @node:	The node to operate on
> + * @key:	The key to compare to @node
> + *
> + * Determine the longest prefix of @node that matches the bits in @key.
> + */
> +static size_t longest_prefix_match(const struct lpm_trie *trie,
> +				   const struct lpm_trie_node *node,
> +				   const struct bpf_lpm_trie_key *key)
> +{
> +	size_t prefixlen = 0;
> +	int i;
> +
> +	for (i = 0; i < trie->data_size; i++) {
> +		size_t b;
> +
> +		b = 8 - fls(node->data[i] ^ key->data[i]);
> +		prefixlen += b;
> +
> +		if (prefixlen >= node->prefixlen || prefixlen >= key->prefixlen)
> +			return min(node->prefixlen, key->prefixlen);
> +
> +		if (b < 8)
> +			break;
> +	}
> +
> +	return prefixlen;
> +}
> +
> +/* Called from syscall or from eBPF program */
> +static void *trie_lookup_elem(struct bpf_map *map, void *_key)
> +{
> +	struct lpm_trie_node *node, *found = NULL;
> +	struct bpf_lpm_trie_key *key = _key;
> +	struct lpm_trie *trie =
> +		container_of(map, struct lpm_trie, map);
> +
> +	/* Start walking the trie from the root node ... */
> +
> +	for (node = rcu_dereference(trie->root); node;) {
> +		unsigned int next_bit;
> +		size_t matchlen;
> +
> +		/*
> +		 * Determine the longest prefix of @node that matches @key.
> +		 * If it's the maximum possible prefix for this trie, we have
> +		 * an exact match and can return it directly.
> +		 */

useful comments, but not in networking style. bpf is using networking style.
please reformat.

> +		matchlen = longest_prefix_match(trie, node, key);
> +		if (matchlen == trie->max_prefixlen)
> +			return node->value;
> +
> +		/*
> +		 * If the number of bits that match is smaller than the prefix
> +		 * length of @node, bail out and return the node we have seen
> +		 * last in the traversal (ie, the parent).
> +		 */
> +		if (matchlen < node->prefixlen)
> +			break;
> +
> +		/*
> +		 * Consider this node as return candidate unless it is an
> +		 * artificially added intermediate one, in which case ->value
> +		 * is %NULL
> +		 */
> +		if (node->value)
> +			found = node;
> +
> +		/*
> +		 * If the node match is fully satisfied, let's see if we can
> +		 * become more specific. Determine the next bit in the key and
> +		 * traverse down.
> +		 */
> +		next_bit = extract_bit(key->data, node->prefixlen);
> +		node = rcu_dereference(node->child[next_bit]);
> +	}
> +
> +	return found ? found->value : NULL;
> +}
> +
> +static struct lpm_trie_node *lpm_trie_node_alloc(const struct lpm_trie *trie,
> +						 const void *value)
> +{
> +	struct lpm_trie_node *node;
> +	gfp_t gfp = GFP_ATOMIC | __GFP_NOWARN;
> +
> +	node = kmalloc(sizeof(struct lpm_trie_node) + trie->data_size, gfp);
> +	if (!node)
> +		return ERR_PTR(-ENOMEM);
> +
> +	if (value) {
> +		node->value = kmemdup(value, trie->map.value_size, gfp);

can you make value to be part of the node? similar to how hash map is done?
that will help avoid 2nd allocation, will speedup insertion and will
help converting this code to user pre-allocated elements.
I suspect the concern was that for many inner nodes that value is null ?
But in your use case the value_size will be == 0 eventually,
so by embedding it when can save memory too, since 'value' pointer will
be replaced with boolean present flag ?
So potentially less memory and less cache misses?

Overall algorithm is indeed straightforward and simple which is great,
but I would still like to see some performance numbers. Looks like
the best case for single 32-bit element it needs 4 xors and compares
which is fine. For mostly populate trie it's 4xors * 32 depth
which is pretty good too, but cache misses on pointer walks may
kill performance unless we're hitting the same path all the time.
I think it's all acceptable due to simplicity of the implementation
which we may improve later if it turns out to be a bottle neck for
some use cases. We just need a baseline to have realistic expectations.

Thanks!

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