思科VPP源码分析（Bihash分析）

版权声明：本文为博主原创文章，未经博主允许不得转载。 https://blog.csdn.net/jqh9804/article/details/55100994

基本概念

VPP里的Bihash全名为Bounded-index extensible hash。它的最大特点是，在查找时是无锁并且线程安全的。修改操作之间会有互斥，但是修改操作时仍然可以进行查找操作。
vpp里的Bihash优化成了两种，bihash_kv_8_8和bihash_kv_24_8,区别在于hash key是8字节还是24字  节。最大限度的利用SSE4.2指令集中的_mm_crc32_u64来进行hash计算。
核心函数在bihash_template.c中。根据包含的头文件是bihash_8_8.h还是bihash_24_8.h，BV宏和BTV  宏将把名字做出对应扩展。例如:BV (clib_bihash_init)扩展为clib_bihash_init_8_8()或者  clib_bihash_init_24_8()。BVT (clib_bihash)扩展为clib_bihash_8_8_t或者为  clib_bihash_24_8_t。

clib_bihash_bucket_t
hash桶

typedef struct
{
  union
  {
    struct
    {
      //这个桶中记录得kv对，在heap中的起始位置
      u32 offset;
      u8 pad[3];
      //这个桶中记录的kv对，一共占用了1 << log2_pages个page
      u8 log2_pages;
    };
    u64 as_u64;
  };
} clib_bihash_bucket_t;

page数据结构，中间包含了kv对。
typedef struct BV (clib_bihash_value)
{
  union
  {
    BVT (clib_bihash_kv) kvp[BIHASH_KVP_PER_PAGE];
    struct BV (clib_bihash_value) * next_free;
  };
} BVT (clib_bihash_value);

核心函数

clib_bihash_init_8_8()和clib_bihash_init_24_8()
初始化bihash，并分配一个独占的内存heap给它。

void BV (clib_bihash_init)
  (BVT (clib_bihash) * h, char *name, u32 nbuckets, uword memory_size)
{
  void *oldheap;

  //方便之后hash值映射到bucket，用&替代昂贵的%操作。
  nbuckets = 1 << (max_log2 (nbuckets));

  h->name = (u8 *) name;
  h->nbuckets = nbuckets;
  h->log2_nbuckets = max_log2 (nbuckets);

  h->mheap = mheap_alloc (0 /* use VM */ , memory_size);

  //常用操作，这样才能使用内存操作函数，注意这里单核心上只能有一个线程独占此操作，不同核心可以并发
  oldheap = clib_mem_set_heap (h->mheap);
  vec_validate_aligned (h->buckets, nbuckets - 1, CLIB_CACHE_LINE_BYTES);
  h->writer_lock = clib_mem_alloc_aligned (CLIB_CACHE_LINE_BYTES,
                       CLIB_CACHE_LINE_BYTES);

  clib_mem_set_heap (oldheap);
}

clib_bihash_search_8_8()和clib_bihash_search_24_8()
给定key，查找value

int BV (clib_bihash_search)
  (const BVT (clib_bihash) * h,
   BVT (clib_bihash_kv) * search_key, BVT (clib_bihash_kv) * valuep)
{
  u64 hash;
  u32 bucket_index;
  uword value_index;
  BVT (clib_bihash_value) * v;
  clib_bihash_bucket_t *b;
  int i;

  ASSERT (valuep);

  //hash计算，者利用了SSE4.2指令集特性。值得记住
  hash = BV (clib_bihash_hash) (search_key);

  //hash值的低log2_nbuckets bit用来索引桶号
  bucket_index = hash & (h->nbuckets - 1);
  b = &h->buckets[bucket_index];

  if (b->offset == 0)
    return -1;

  hash >>= h->log2_nbuckets;

  //在heap中的offset字节开始，属于桶。
  v = BV (clib_bihash_get_value) (h, b->offset);
  //hash值的中间log2_pages bit用来索引桶中的page号
  value_index = hash & ((1 << b->log2_pages) - 1);
  v += value_index;

  //遍历page，查找key值。
  for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
    {
      if (BV (clib_bihash_key_compare) (v->kvp[i].key, search_key->key))
    {
      *valuep = v->kvp[i];
      return 0;
    }
    }
  return -1;
}

clib_bihash_add_del_8_8()和clib_bihash_add_del_24_8()
添加删除kv对

int BV (clib_bihash_add_del)
  (BVT (clib_bihash) * h, BVT (clib_bihash_kv) * add_v, int is_add)
{
  u32 bucket_index;
  clib_bihash_bucket_t *b, tmp_b;
  BVT (clib_bihash_value) * v, *new_v, *save_new_v, *working_copy;
  u32 value_index;
  int rv = 0;
  int i;
  u64 hash, new_hash;
  u32 new_log2_pages;
  u32 cpu_number = os_get_cpu_number ();

  hash = BV (clib_bihash_hash) (add_v);

  //hash值的低log2_nbuckets bit用来索引桶号
  bucket_index = hash & (h->nbuckets - 1);
  b = &h->buckets[bucket_index];

  hash >>= h->log2_nbuckets;

  while (__sync_lock_test_and_set (h->writer_lock, 1))
    ;

  /* First elt in the bucket? */
  if (b->offset == 0)
    {    
      if (is_add == 0)
    {
      rv = -1;
      goto unlock;
    }
      //桶中最初没有kv对，现在分配一个page
      v = BV (value_alloc) (h, 0);
      *v->kvp = *add_v;
      tmp_b.as_u64 = 0;
      tmp_b.offset = BV (clib_bihash_get_offset) (h, v);

      b->as_u64 = tmp_b.as_u64;
      goto unlock;
    }
  //把桶b中的kv对拷贝到缓存中，缓存加入到b中，b中原来的kv对留给下文修改。
  BV (make_working_copy) (h, b);

  //b中原来的kv对page
  v = BV (clib_bihash_get_value) (h, h->saved_bucket.offset);
  //hash值的中间log2_pages bit用来索引桶中的page号
  value_index = hash & ((1 << h->saved_bucket.log2_pages) - 1);
  v += value_index;

  if (is_add)
    {
      /*
       * For obvious (in hindsight) reasons, see if we're supposed to
       * replace an existing key, then look for an empty slot.
       */
      for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
    {
      if (!memcmp (&(v->kvp[i]), &add_v->key, sizeof (add_v->key)))
        {
          //有重复的key值，把value覆盖旧的
          clib_memcpy (&(v->kvp[i]), add_v, sizeof (*add_v));
          CLIB_MEMORY_BARRIER ();
          /* Restore the previous (k,v) pairs */
          //修改完的page重新保存回b中
          b->as_u64 = h->saved_bucket.as_u64;
          goto unlock;
        }
    }
      for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
    {
      if (BV (clib_bihash_is_free) (&(v->kvp[i])))
        {
          //要保存的kv内容拷贝到第一个空闲的空间中
          clib_memcpy (&(v->kvp[i]), add_v, sizeof (*add_v));
          CLIB_MEMORY_BARRIER ();
          b->as_u64 = h->saved_bucket.as_u64;
          goto unlock;
        }
    }
      /* no room at the inn... split case... */
    }
  else
    {
      for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
    {
      if (!memcmp (&(v->kvp[i]), &add_v->key, sizeof (add_v->key)))
        {
          memset (&(v->kvp[i]), 0xff, sizeof (*(add_v)));
          CLIB_MEMORY_BARRIER ();
          b->as_u64 = h->saved_bucket.as_u64;
          goto unlock;
        }
    }
      rv = -3;
      b->as_u64 = h->saved_bucket.as_u64;
      goto unlock;
    }

  //添加kv发现空间不够了，该桶的page数量增加一倍
  new_log2_pages = h->saved_bucket.log2_pages + 1;

expand_again:
  working_copy = h->working_copies[cpu_number];
  //扩充page，其中的kv对需要重新排列下，因为hash值中需要用new_log2_pages个bit来确定page位置
  new_v = BV (split_and_rehash) (h, working_copy, new_log2_pages);
  if (new_v == 0)
    {
      new_log2_pages++;
      goto expand_again;
    }

  /* Try to add the new entry */
  save_new_v = new_v;
  new_hash = BV (clib_bihash_hash) (add_v);
  new_hash >>= h->log2_nbuckets;
  new_hash &= (1 << min_log2 (vec_len (new_v))) - 1;
  new_v += new_hash;

  for (i = 0; i < BIHASH_KVP_PER_PAGE; i++)
    {
      if (BV (clib_bihash_is_free) (&(new_v->kvp[i])))
    {
      clib_memcpy (&(new_v->kvp[i]), add_v, sizeof (*add_v));
      goto expand_ok;
    }
    }
  /* Crap. Try again */
  new_log2_pages++;
  BV (value_free) (h, save_new_v);
  goto expand_again;

expand_ok:
  tmp_b.log2_pages = min_log2 (vec_len (save_new_v));
  tmp_b.offset = BV (clib_bihash_get_offset) (h, save_new_v);
  CLIB_MEMORY_BARRIER ();
  b->as_u64 = tmp_b.as_u64;
  v = BV (clib_bihash_get_value) (h, h->saved_bucket.offset);
  BV (value_free) (h, v);

unlock:
  CLIB_MEMORY_BARRIER ();
  h->writer_lock[0] = 0;
  return rv;
}

make_working_copy_8_8()和make_working_copy_24_8()
用来生成桶内pages的副本，供添加删除修改使用

static inline void
BV (make_working_copy) (BVT (clib_bihash) * h, clib_bihash_bucket_t * b)
{
  BVT (clib_bihash_value) * v;
  clib_bihash_bucket_t working_bucket __attribute__ ((aligned (8)));
  void *oldheap;
  BVT (clib_bihash_value) * working_copy;
  u32 cpu_number = os_get_cpu_number ();

  //working_copies是per-cpu的
  if (cpu_number >= vec_len (h->working_copies))
    {
      oldheap = clib_mem_set_heap (h->mheap);
      vec_validate (h->working_copies, cpu_number);
      clib_mem_set_heap (oldheap);
    }

  /*
   * working_copies are per-cpu so that near-simultaneous
   * updates from multiple threads will not result in sporadic, spurious
   * lookup failures.
   */
  working_copy = h->working_copies[cpu_number];

  //博主觉得saved_bucket又不是per-cpu的，那么working_copies就没必要做成per-cpu了
  h->saved_bucket.as_u64 = b->as_u64;
  oldheap = clib_mem_set_heap (h->mheap);

  if ((1 << b->log2_pages) > vec_len (working_copy))
    {
      vec_validate_aligned (working_copy, (1 << b->log2_pages) - 1,
                sizeof (u64));
      h->working_copies[cpu_number] = working_copy;
    }

  _vec_len (working_copy) = 1 << b->log2_pages;
  clib_mem_set_heap (oldheap);

  v = BV (clib_bihash_get_value) (h, b->offset);

  //b中原有的kv内容拷贝到working_copy中，然后把b的page指向working_copy中的。这样b中的kv其实是副本。
  clib_memcpy (working_copy, v, sizeof (*v) * (1 << b->log2_pages));
  working_bucket.as_u64 = b->as_u64;
  working_bucket.offset = BV (clib_bihash_get_offset) (h, working_copy);
  CLIB_MEMORY_BARRIER ();
  b->as_u64 = working_bucket.as_u64;
  h->working_copies[cpu_number] = working_copy;
}

split_and_rehash_8_8()和split_and_rehash_24_8()
桶中的page数量扩张后，原有的kv需要重新插入一边。

static
BVT (clib_bihash_value) *
BV (split_and_rehash)
  (BVT (clib_bihash) * h,
   BVT (clib_bihash_value) * old_values, u32 new_log2_pages)
{
  BVT (clib_bihash_value) * new_values, *v, *new_v;
  int i, j, k;

  new_values = BV (value_alloc) (h, new_log2_pages);

  //v会遍历原有的每个page
  v = old_values;
  for (i = 0; i < vec_len (old_values); i++)
    {
      u64 new_hash;

      //遍历原有桶中特定page中的kv
      for (j = 0; j < BIHASH_KVP_PER_PAGE; j++)
    {
      if (BV (clib_bihash_is_free) (&(v->kvp[j])) == 0)
        {
          new_hash = BV (clib_bihash_hash) (&(v->kvp[j]));
          new_hash >>= h->log2_nbuckets;
          new_hash &= (1 << new_log2_pages) - 1;

          new_v = &new_values[new_hash];

          for (k = 0; k < BIHASH_KVP_PER_PAGE; k++)
        {
          if (BV (clib_bihash_is_free) (&(new_v->kvp[k])))
            {
              clib_memcpy (&(new_v->kvp[k]), &(v->kvp[j]),
                   sizeof (new_v->kvp[k]));
              goto doublebreak;
            }
        }
          /* Crap. Tell caller to try again */
          BV (value_free) (h, new_values);
          return 0;
        }
    doublebreak:
      ;
    }
      v++;
    }
  return new_values;
}

clib_bihash_value_8_8()和clib_bihash_value_24_8()
分配page用，page用来保存kv对。内存分配以page为单位，分配1 << log2_pages个page，并且对回收的
page做了缓存。但是没有用伙伴算法进行碎片内存合并。

static
BVT (clib_bihash_value) *
BV (value_alloc) (BVT (clib_bihash) * h, u32 log2_pages)
{
  BVT (clib_bihash_value) * rv = 0;
  void *oldheap;

  ASSERT (h->writer_lock[0]);
  //h->freelists用log2_pages值来索引空闲page
  if (log2_pages >= vec_len (h->freelists) || h->freelists[log2_pages] == 0)
    {
          oldheap = clib_mem_set_heap (h->mheap);
          vec_validate (h->freelists, log2_pages);
          //分配1 << log2_pages 个page，方便从hash值中计算出page编号。
          vec_validate_aligned (rv, (1 << log2_pages) - 1, CLIB_CACHE_LINE_BYTES);
          clib_mem_set_heap (oldheap);
          goto initialize;
    }
  rv = h->freelists[log2_pages];
  h->freelists[log2_pages] = rv->next_free;

initialize:
  ASSERT (rv);
  ASSERT (vec_len (rv) == (1 << log2_pages));
  /*
   * Latest gcc complains that the length arg is zero
   * if we replace (1<<log2_pages) with vec_len(rv).
   * No clue.
   */
  memset (rv, 0xff, sizeof (*rv) * (1 << log2_pages));
  return rv;
}