注册如下的命令trace add，开启记录报文信息，参数为节点名称和记录报文数量。

VLIB_CLI_COMMAND (add_trace_cli,static) = {.path = "trace add",.short_help = "trace add   [filter] [verbose]",.function = cli_add_trace_buffer,
};

命令行指定的节点需要支持trace功能，对应标志VLIB_NODE_FLAG_TRACE_SUPPORTED。

static clib_error_t *
cli_add_trace_buffer (vlib_main_t * vm,unformat_input_t * input, vlib_cli_command_t * cmd)
{if (vnet_trace_placeholder == 0)vec_validate_aligned (vnet_trace_placeholder, 2048,CLIB_CACHE_LINE_BYTES);node = vlib_get_node (vm, node_index);if ((node->flags & VLIB_NODE_FLAG_TRACE_SUPPORTED) == 0){ error = clib_error_create ("node '%U' doesn't support per-node ""tracing. There may be another way to ""initiate trace on this node.",format_vlib_node_name, vm, node_index);goto done;}u32 filter_table = classify_get_trace_chain ();if (filter && filter_table == ~0){         error = clib_error_create ("No packet trace filter configured...");goto done;}trace_update_capture_options (add, node_index, filter, verbose);

如果指定的trace报文数量为最大值，修改为50。遍历每个线程，找到线程中的vlib_trace_node_t结构（或者新分配），将指定的报文数量设置到limit成员。

void
trace_update_capture_options (u32 add, u32 node_index, u32 filter, u8 verbose)
{vlib_trace_main_t *tm;vlib_trace_node_t *tn;if (add == ~0)add = 50;foreach_vlib_main (){tm = &this_vlib_main->trace_main;tm->verbose = verbose;vec_validate (tm->nodes, node_index);tn = tm->nodes + node_index;/** Adding 0 makes no real sense, and there wa no other way* to explicilty zero-out the limits and count, so make* an "add 0" request really be "set to 0".*/if (add == 0)tn->limit = tn->count = 0;elsetn->limit += add;}

将每个线程的vlib_trace_main_t结构成员trace_enable设置为1。

  foreach_vlib_main (){tm = &this_vlib_main->trace_main;tm->trace_enable = 1;}vlib_enable_disable_pkt_trace_filter (! !filter);

dpdk-input

节点dpdk-input定义如下，其中指定了VLIB_NODE_FLAG_TRACE_SUPPORTED标志，因此可在trace add命令中使用。

VLIB_REGISTER_NODE (dpdk_input_node) = {.type = VLIB_NODE_TYPE_INPUT,.name = "dpdk-input",.sibling_of = "device-input",.flags = VLIB_NODE_FLAG_TRACE_SUPPORTED,

节点dpdk-input的报文处理函数中与trace相关代码如下，首先获取剩余的可用数量n_trace，如果没有开启trace，limit为零；或者已经trace结束，n_trace都为零。

static_always_inline u32
dpdk_device_input (vlib_main_t * vm, dpdk_main_t * dm, dpdk_device_t * xd,vlib_node_runtime_t * node, u32 thread_index, u16 queue_id)
{u32 *buffers;struct rte_mbuf **mb;/* packet trace if enabled */if (PREDICT_FALSE ((n_trace = vlib_get_trace_count (vm, node)))){n_left = n_rx_packets;buffers = ptd->buffers;mb = ptd->mbufs;next = ptd->next;

vlib_trace_buffer设置报文的trace标志等。之后，函数vlib_add_trace分配节点的vlib_trace_header_t结构，初始化dpdk_rx_trace_t结构成员。最后，记录以上使用的trace数量。

      while (n_trace && n_left){b0 = vlib_get_buffer (vm, buffers[0]);if (single_next == 0)next_index = next[0];if (PREDICT_TRUE(vlib_trace_buffer(vm, node, next_index, b0, /* follow_chain */ 0))){dpdk_rx_trace_t *t0 =vlib_add_trace (vm, node, b0, sizeof t0[0]);t0->queue_index = queue_id;t0->device_index = xd->device_index;t0->buffer_index = vlib_get_buffer_index (vm, b0);clib_memcpy_fast (&t0->mb, mb[0], sizeof t0->mb);clib_memcpy_fast (&t0->buffer, b0,sizeof b0[0] - sizeof b0->pre_data);clib_memcpy_fast (t0->buffer.pre_data, b0->data,sizeof t0->buffer.pre_data);clib_memcpy_fast (&t0->data, mb[0]->buf_addr + mb[0]->data_off,sizeof t0->data);n_trace--;}n_left--;buffers++;mb++;next++;}vlib_set_trace_count (vm, node, n_trace);}

首先判断当前进程是否开启了trace，为报文结构vlib_buffer_t设置标志VLIB_BUFFER_IS_TRACED，函数vlib_buffer_make_trace_handle获取trace_handle。

always_inline __clib_warn_unused_result int
vlib_trace_buffer (vlib_main_t * vm,vlib_node_runtime_t * r,u32 next_index, vlib_buffer_t * b, int follow_chain)
{         vlib_trace_main_t *tm = &vm->trace_main;vlib_trace_header_t **h; if (PREDICT_FALSE (tm->trace_enable == 0))return 0;vlib_trace_next_frame (vm, r, next_index);pool_get (tm->trace_buffer_pool, h);do      {b->flags |= VLIB_BUFFER_IS_TRACED;b->trace_handle = vlib_buffer_make_trace_handle(vm->thread_index, h - tm->trace_buffer_pool);}while (follow_chain && (b = vlib_get_next_buffer (vm, b)));

如下，trace_handle由当前线程索引，和trace_buffer_pool池索引组成。

always_inline u32
vlib_buffer_make_trace_handle (u32 thread, u32 pool_index)
{u32 rv;ASSERT (thread < 0xff);ASSERT (pool_index < 0x00FFFFFF);rv = (thread << 24) | (pool_index & 0x00FFFFFF);return rv;
} 

函数vlib_add_trace_inline分配vlib_trace_header_t结构以及n_data_bytes大小的内存。为了实现向量操作，n_data_bytes按照vlib_trace_header_t大小对其。分配（1+n_data_words）数量的vlib_trace_header_t结构，其中后者n_data_words乘以sizeof（vlib_trace_header_t）表示n_data_bytes分配的实际内存大小。

之后，记录当前时间等信息。返回vlib_trace_header_t的data成员在dpdk_device_input函数中初始化。

always_inline void *
vlib_add_trace_inline (vlib_main_t * vm,vlib_node_runtime_t * r, vlib_buffer_t * b,u32 n_data_bytes)
{vlib_trace_main_t *tm = &vm->trace_main;vlib_trace_header_t *h;u32 n_data_words, trace_index;trace_index = vlib_buffer_get_trace_index (b);if (PREDICT_FALSE (pool_is_free_index (tm->trace_buffer_pool, trace_index)))return vnet_trace_placeholder;n_data_bytes = round_pow2 (n_data_bytes, sizeof (h[0]));n_data_words = n_data_bytes / sizeof (h[0]);vec_add2_aligned (tm->trace_buffer_pool[trace_index], h, 1 + n_data_words,sizeof (h[0]));h->time = vm->cpu_time_last_node_dispatch;h->n_data = n_data_words;h->node_index = r->node_index;return h->data;