Reorganize source tree to use single autotools instance

Change-Id: I7b51f88292e057c6443b12224486f2d0c9f8ae23
Signed-off-by: Damjan Marion <damarion@cisco.com>

1 files changed, 1108 insertions, 0 deletions

diff --git a/src/vnet/fib/fib_walk.c b/src/vnet/fib/fib_walk.c
new file mode 100644
index 00000000000..938f7b8c1c6
--- /dev/null
+++ b/src/vnet/fib/fib_walk.c
@@ -0,0 +1,1108 @@
+/*
+ * Copyright (c) 2016 Cisco and/or its affiliates.
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at:
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <vnet/fib/fib_walk.h>
+#include <vnet/fib/fib_node_list.h>
+
+/**
+ * The flags on a walk
+ */
+typedef enum fib_walk_flags_t_
+{
+    /**
+     * A synchronous walk.
+     * This walk will run to completion, i.e. visit ALL the children.
+     * It is a depth first traversal of the graph.
+     */
+    FIB_WALK_FLAG_SYNC = (1 << 0),
+    /**
+     * An asynchronous walk.
+     * This walk will be scheduled to run in the background. It will thus visits
+     * the children at a later point in time.
+     * It is a depth first traversal of the graph.
+     */
+    FIB_WALK_FLAG_ASYNC = (1 << 1),
+    /**
+     * An indication that the walk is currently executing.
+     */
+    FIB_WALK_FLAG_EXECUTING = (1 << 2),
+} fib_walk_flags_t;
+
+/**
+ * A representation of a graph walk from a parent object to its children
+ */
+typedef struct fib_walk_t_
+{
+    /**
+     * FIB node linkage. This object is not in the FIB object graph,
+     * but it is present in other node's dependency lists, so it needs to
+     * be pointerable to.
+     */
+    fib_node_t fw_node;
+
+    /**
+     * the walk's flags
+     */
+    fib_walk_flags_t fw_flags;
+
+    /**
+     * Sibling index in the dependency list
+     */
+    u32 fw_dep_sibling;
+
+    /**
+     * Sibling index in the list of all walks
+     */
+    u32 fw_prio_sibling;
+
+    /**
+     * Pointer to the node whose dependants this walk is walking
+     */
+    fib_node_ptr_t fw_parent;
+
+    /**
+     * Number of nodes visited by this walk. saved for debugging purposes.
+     */
+    u32 fw_n_visits;
+
+    /**
+     * Time the walk started
+     */
+    f64 fw_start_time;
+
+    /**
+     * The reasons this walk is occuring.
+     * This is a vector ordered in time. The reasons and the front were started
+     * first, and so should be acted first when a node is visisted.
+     */
+    fib_node_back_walk_ctx_t *fw_ctx;
+} fib_walk_t;
+
+/**
+ * @brief The pool of all walk objects
+ */
+static fib_walk_t *fib_walk_pool;
+
+/**
+ * @brief There's only one event type sent to the walk process
+ */
+#define FIB_WALK_EVENT 0
+
+/**
+ * Statistics maintained per-walk queue
+ */
+typedef enum fib_walk_queue_stats_t_
+{
+    FIB_WALK_SCHEDULED,
+    FIB_WALK_COMPLETED,
+} fib_walk_queue_stats_t;
+#define FIB_WALK_QUEUE_STATS_NUM ((fib_walk_queue_stats_t)(FIB_WALK_COMPLETED+1))
+
+#define FIB_WALK_QUEUE_STATS {           \
+    [FIB_WALK_SCHEDULED] = "scheduled",  \
+    [FIB_WALK_COMPLETED] = "completed",  \
+}
+
+#define FOR_EACH_FIB_WALK_QUEUE_STATS(_wqs)   \
+    for ((_wqs) = FIB_WALK_SCHEDULED;         \
+	 (_wqs) < FIB_WALK_QUEUE_STATS_NUM;   \
+	 (_wqs)++)
+
+/**
+ * The names of the walk stats
+ */
+static const char * const fib_walk_queue_stats_names[] = FIB_WALK_QUEUE_STATS;
+/**
+ * The names of the walk reasons
+ */
+static const char * const fib_node_bw_reason_names[] = FIB_NODE_BW_REASONS;
+
+/**
+ * A represenation of one queue of walk
+ */
+typedef struct fib_walk_queue_t_
+{
+    /**
+     * Qeuee stats
+     */
+    u64 fwq_stats[FIB_WALK_QUEUE_STATS_NUM];
+
+    /**
+     * The node list which acts as the queue
+     */
+    fib_node_list_t fwq_queue;
+} fib_walk_queue_t;
+
+/**
+ * A set of priority queues for outstanding walks
+ */
+typedef struct fib_walk_queues_t_
+{
+    fib_walk_queue_t fwqs_queues[FIB_WALK_PRIORITY_NUM];
+} fib_walk_queues_t;
+
+/**
+ * The global queues of outstanding walks
+ */
+static fib_walk_queues_t fib_walk_queues;
+
+/**
+ * The names of the walk priorities
+ */
+static const char * const fib_walk_priority_names[] = FIB_WALK_PRIORITIES;
+
+/**
+ * @brief Histogram stats on the lenths of each walk in elemenets visisted.
+ * Store upto 1<<23 elements in increments of 1<<10
+ */
+#define HISTOGRAM_VISITS_PER_WALK_MAX (1<<23)
+#define HISTOGRAM_VISITS_PER_WALK_INCR (1<<10)
+#define HISTOGRAM_VISITS_PER_WALK_N_BUCKETS \
+    (HISTOGRAM_VISITS_PER_WALK_MAX/HISTOGRAM_VISITS_PER_WALK_INCR)
+static u64 fib_walk_hist_vists_per_walk[HISTOGRAM_VISITS_PER_WALK_N_BUCKETS];
+
+/**
+ * @brief History of state for the last 128 walks
+ */
+#define HISTORY_N_WALKS 128
+#define MAX_HISTORY_REASONS 16
+static u32 history_last_walk_pos;
+typedef struct fib_walk_history_t_ {
+    u32 fwh_n_visits;
+    f64 fwh_duration;
+    f64 fwh_completed;
+    fib_node_ptr_t fwh_parent;
+    fib_walk_flags_t fwh_flags;
+    fib_node_bw_reason_flag_t fwh_reason[MAX_HISTORY_REASONS];
+} fib_walk_history_t;
+static fib_walk_history_t fib_walk_history[HISTORY_N_WALKS];
+
+u8*
+format_fib_walk_priority (u8 *s, va_list ap)
+{
+    fib_walk_priority_t prio = va_arg(ap, fib_walk_priority_t);
+
+    ASSERT(prio < FIB_WALK_PRIORITY_NUM);
+
+    return (format(s, "%s", fib_walk_priority_names[prio]));
+}
+static u8*
+format_fib_walk_queue_stats (u8 *s, va_list ap)
+{
+    fib_walk_queue_stats_t wqs = va_arg(ap, fib_walk_queue_stats_t);
+
+    ASSERT(wqs < FIB_WALK_QUEUE_STATS_NUM);
+
+    return (format(s, "%s", fib_walk_queue_stats_names[wqs]));
+}
+
+static index_t
+fib_walk_get_index (fib_walk_t *fwalk)
+{
+    return (fwalk - fib_walk_pool);
+}
+
+static fib_walk_t *
+fib_walk_get (index_t fwi)
+{
+    return (pool_elt_at_index(fib_walk_pool, fwi));
+}
+
+/*
+ * not static so it can be used in the unit tests
+ */
+u32
+fib_walk_queue_get_size (fib_walk_priority_t prio)
+{
+    return (fib_node_list_get_size(fib_walk_queues.fwqs_queues[prio].fwq_queue));
+}
+
+static fib_node_index_t
+fib_walk_queue_get_front (fib_walk_priority_t prio)
+{
+    fib_node_ptr_t wp;
+
+    fib_node_list_get_front(fib_walk_queues.fwqs_queues[prio].fwq_queue, &wp);
+
+    return (wp.fnp_index);
+}
+
+static void
+fib_walk_destroy (fib_walk_t *fwalk)
+{
+    u32 bucket, ii;
+
+    if (FIB_NODE_INDEX_INVALID != fwalk->fw_prio_sibling)
+    {
+	fib_node_list_elt_remove(fwalk->fw_prio_sibling);
+    }
+    fib_node_child_remove(fwalk->fw_parent.fnp_type,
+			  fwalk->fw_parent.fnp_index,
+			  fwalk->fw_dep_sibling);
+
+    /*
+     * add the stats to the continuous histogram collection.
+     */
+    bucket = (fwalk->fw_n_visits / HISTOGRAM_VISITS_PER_WALK_INCR);
+    bucket = (bucket >= HISTOGRAM_VISITS_PER_WALK_N_BUCKETS ?
+	      HISTOGRAM_VISITS_PER_WALK_N_BUCKETS - 1 :
+	      bucket);
+    fib_walk_hist_vists_per_walk[bucket]++;
+
+    /*
+     * save stats to the recent history
+     */
+
+    fib_walk_history[history_last_walk_pos].fwh_n_visits =
+	fwalk->fw_n_visits;
+    fib_walk_history[history_last_walk_pos].fwh_completed =
+	vlib_time_now(vlib_get_main());
+    fib_walk_history[history_last_walk_pos].fwh_duration =
+	fib_walk_history[history_last_walk_pos].fwh_completed -
+        fwalk->fw_start_time;
+    fib_walk_history[history_last_walk_pos].fwh_parent =
+	fwalk->fw_parent;
+    fib_walk_history[history_last_walk_pos].fwh_flags =
+	fwalk->fw_flags;
+
+    vec_foreach_index(ii, fwalk->fw_ctx)
+    {
+        if (ii < MAX_HISTORY_REASONS)
+        {
+            fib_walk_history[history_last_walk_pos].fwh_reason[ii] =
+                fwalk->fw_ctx[ii].fnbw_reason;
+        }
+    }
+
+    history_last_walk_pos = (history_last_walk_pos + 1) % HISTORY_N_WALKS;
+
+    fib_node_deinit(&fwalk->fw_node);
+    vec_free(fwalk->fw_ctx);
+    pool_put(fib_walk_pool, fwalk);
+}
+
+/**
+ * return code when advancing a walk
+ */
+typedef enum fib_walk_advance_rc_t_
+{
+    /**
+     * The walk is complete
+     */
+    FIB_WALK_ADVANCE_DONE,
+    /**
+     * the walk has more work
+     */
+    FIB_WALK_ADVANCE_MORE,
+    /**
+     * The walk merged with the one in front
+     */
+    FIB_WALK_ADVANCE_MERGE,
+} fib_walk_advance_rc_t;
+
+/**
+ * @brief Advance the walk one element in its work list
+ */
+static fib_walk_advance_rc_t
+fib_walk_advance (fib_node_index_t fwi)
+{
+    fib_node_back_walk_ctx_t *ctx, *old;
+    fib_node_back_walk_rc_t wrc;
+    fib_node_ptr_t sibling;
+    fib_walk_t *fwalk;
+    int more_elts;
+
+    /*
+     * this walk function is re-entrant - walks acan spawn walks.
+     * fib_walk_t objects come from a pool, so they can realloc. we need
+     * to retch from said pool at the appropriate times.
+     */
+    fwalk = fib_walk_get(fwi);
+
+    more_elts = fib_node_list_elt_get_next(fwalk->fw_dep_sibling, &sibling);
+
+    if (more_elts)
+    {
+        old = fwalk->fw_ctx;
+
+	vec_foreach(ctx, fwalk->fw_ctx)
+	{
+	    wrc = fib_node_back_walk_one(&sibling, ctx);
+
+	    fwalk = fib_walk_get(fwi);
+	    fwalk->fw_n_visits++;
+
+	    if (FIB_NODE_BACK_WALK_MERGE == wrc)
+	    {
+		/*
+		 * this walk has merged with the one further along the node's
+		 * dependecy list.
+		 */
+		return (FIB_WALK_ADVANCE_MERGE);
+	    }
+            if (old != fwalk->fw_ctx)
+            {
+                /*
+                 * nasty re-entrant addition of a walk has realloc'd the vector
+                 * break out
+                 */
+		return (FIB_WALK_ADVANCE_MERGE);
+	    }
+	}
+	/*
+	 * move foward to the next node to visit
+	 */
+	more_elts = fib_node_list_advance(fwalk->fw_dep_sibling);
+    }
+
+    if (more_elts)
+    {
+	return (FIB_WALK_ADVANCE_MORE);
+    }
+
+    return (FIB_WALK_ADVANCE_DONE);
+}
+
+/**
+ * @breif Enurmerate the times of sleep between walks
+ */
+typedef enum fib_walk_sleep_type_t_
+{
+    FIB_WALK_SHORT_SLEEP,
+    FIB_WALK_LONG_SLEEP,
+} fib_walk_sleep_type_t;
+
+#define FIB_WALK_N_SLEEP (FIB_WALK_LONG_SLEEP+1)
+
+/**
+ * @brief Durations for the sleep types
+ */
+static f64 fib_walk_sleep_duration[] = {
+    [FIB_WALK_LONG_SLEEP] = 1e-3,
+    [FIB_WALK_SHORT_SLEEP] = 1e-8,
+};
+
+/**
+ * @brief The time quota for a walk. When more than this amount of time is
+ * spent, the walk process will yield.
+ */
+static f64 quota = 1e-4;
+
+/**
+ * Histogram on the amount of work done (in msecs) in each walk
+ */
+#define N_TIME_BUCKETS 128
+#define TIME_INCREMENTS (N_TIME_BUCKETS/2)
+static u64 fib_walk_work_time_taken[N_TIME_BUCKETS];
+
+/**
+ * Histogram on the number of nodes visted in each quota
+ */
+#define N_ELTS_BUCKETS 128
+static u32 fib_walk_work_nodes_visisted_incr = 2;
+static u64 fib_walk_work_nodes_visited[N_ELTS_BUCKETS];
+
+/**
+ * Histogram of the sleep lengths
+ */
+static u64 fib_walk_sleep_lengths[2];
+
+/**
+ * @brief Service the queues
+ * This is not declared static so that it can be unit tested - i know i know...
+ */
+f64
+fib_walk_process_queues (vlib_main_t * vm,
+			 const f64 quota)
+{
+    f64 start_time, consumed_time;
+    fib_walk_sleep_type_t sleep;
+    fib_walk_priority_t prio;
+    fib_walk_advance_rc_t rc;
+    fib_node_index_t fwi;
+    fib_walk_t *fwalk;
+    u32 n_elts;
+    i32 bucket;
+
+    consumed_time = 0;
+    start_time = vlib_time_now(vm);
+    n_elts = 0;
+
+    FOR_EACH_FIB_WALK_PRIORITY(prio)
+    {
+	while (0 != fib_walk_queue_get_size(prio))
+	{
+	    fwi = fib_walk_queue_get_front(prio);
+
+	    /*
+	     * set this walk as executing
+	     */
+	    fwalk = fib_walk_get(fwi);
+	    fwalk->fw_flags |= FIB_WALK_FLAG_EXECUTING;
+
+	    do
+	    {
+		rc = fib_walk_advance(fwi);
+		n_elts++;
+		consumed_time = (vlib_time_now(vm) - start_time);
+	    } while ((consumed_time < quota) &&
+		     (FIB_WALK_ADVANCE_MORE == rc));
+
+	    /*
+	     * if this walk has no more work then pop it from the queue
+	     * and move on to the next.
+	     */
+	    if (FIB_WALK_ADVANCE_MORE != rc)
+	    {
+		fwalk = fib_walk_get(fwi);
+		fib_walk_destroy(fwalk);
+		fib_walk_queues.fwqs_queues[prio].fwq_stats[FIB_WALK_COMPLETED]++;
+	    }
+	    else
+	    {
+		/*
+		 * passed our work quota. sleep time.
+		 */
+		fwalk = fib_walk_get(fwi);
+		fwalk->fw_flags &= ~FIB_WALK_FLAG_EXECUTING;
+		sleep = FIB_WALK_SHORT_SLEEP;
+		goto that_will_do_for_now;
+	    }
+	}
+    }
+    /*
+     * got to the end of all the work
+     */
+    sleep = FIB_WALK_LONG_SLEEP;
+
+that_will_do_for_now:
+
+    /*
+     * collect the stats:
+     *  - for the number of nodes visisted we store 128 increments
+     *  - for the time consumed we store quota/TIME_INCREMENTS increments.
+     */
+    bucket = ((n_elts/fib_walk_work_nodes_visisted_incr) > N_ELTS_BUCKETS ?
+	      N_ELTS_BUCKETS-1 :
+	      n_elts/fib_walk_work_nodes_visisted_incr);
+    ++fib_walk_work_nodes_visited[bucket];
+
+    bucket = (consumed_time - quota) / (quota / TIME_INCREMENTS);
+    bucket += N_TIME_BUCKETS/2;
+    bucket = (bucket < 0 ? 0 : bucket);
+    bucket = (bucket > N_TIME_BUCKETS-1 ? N_TIME_BUCKETS-1 : bucket);
+    ++fib_walk_work_time_taken[bucket];
+
+    ++fib_walk_sleep_lengths[sleep];
+
+    return (fib_walk_sleep_duration[sleep]);
+}
+
+/**
+ * @brief The 'fib-walk' process's main loop.
+ */
+static uword
+fib_walk_process (vlib_main_t * vm,
+		  vlib_node_runtime_t * node,
+		  vlib_frame_t * f)
+{
+    f64 sleep_time;
+
+    sleep_time = fib_walk_sleep_duration[FIB_WALK_SHORT_SLEEP];
+
+    while (1)
+    {
+	vlib_process_wait_for_event_or_clock(vm, sleep_time);
+
+	/*
+	 * there may be lots of event queued between the processes,
+	 * but the walks we want to schedule are in the priority queues,
+	 * so we ignore the process events.
+	 */
+	vlib_process_get_events(vm, NULL);
+
+	sleep_time = fib_walk_process_queues(vm, quota);
+    }
+
+    /*
+     * Unreached
+     */
+    ASSERT(!"WTF");
+    return 0;
+}
+
+/* *INDENT-OFF* */
+VLIB_REGISTER_NODE (fib_walk_process_node,static) = {
+    .function = fib_walk_process,
+    .type = VLIB_NODE_TYPE_PROCESS,
+    .name = "fib-walk",
+};
+/* *INDENT-ON* */
+
+/**
+ * @brief Allocate a new walk object
+ */
+static fib_walk_t *
+fib_walk_alloc (fib_node_type_t parent_type,
+		fib_node_index_t parent_index,
+		fib_walk_flags_t flags,
+		fib_node_back_walk_ctx_t *ctx)
+{
+    fib_walk_t *fwalk;
+
+    pool_get(fib_walk_pool, fwalk);
+
+    fib_node_init(&fwalk->fw_node, FIB_NODE_TYPE_WALK);
+
+    fwalk->fw_flags = flags;
+    fwalk->fw_dep_sibling  = FIB_NODE_INDEX_INVALID;
+    fwalk->fw_prio_sibling = FIB_NODE_INDEX_INVALID;
+    fwalk->fw_parent.fnp_index = parent_index;
+    fwalk->fw_parent.fnp_type = parent_type;
+    fwalk->fw_ctx = NULL;
+    fwalk->fw_start_time = vlib_time_now(vlib_get_main());
+    fwalk->fw_n_visits = 0;
+
+    /*
+     * make a copy of the backwalk context so the depth count remains
+     * the same for each sibling visitsed. This is important in the case
+     * where a parent has a loop via one child, but all the others are not.
+     * if the looped child were visited first, the depth count would exceed, the
+     * max and the walk would terminate before it reached the other siblings.
+     */
+    vec_add1(fwalk->fw_ctx, *ctx);
+
+    return (fwalk);
+}
+
+/**
+ * @brief Enqueue a walk onto the appropriate priority queue. Then signal
+ * the background process there is work to do.
+ */
+static index_t
+fib_walk_prio_queue_enquue (fib_walk_priority_t prio,
+			    fib_walk_t *fwalk)
+{
+    index_t sibling;
+
+    sibling = fib_node_list_push_front(fib_walk_queues.fwqs_queues[prio].fwq_queue,
+				       0,
+				       FIB_NODE_TYPE_WALK,
+				       fib_walk_get_index(fwalk));
+    fib_walk_queues.fwqs_queues[prio].fwq_stats[FIB_WALK_SCHEDULED]++;
+
+    /*
+     * poke the fib-walk process to perform the async walk.
+     * we are not passing it specific data, hence the last two args,
+     * the process will drain the queues
+     */
+    vlib_process_signal_event(vlib_get_main(),
+			      fib_walk_process_node.index,
+			      FIB_WALK_EVENT,
+			      FIB_WALK_EVENT);
+
+    return (sibling);
+}
+
+void
+fib_walk_async (fib_node_type_t parent_type,
+		fib_node_index_t parent_index,
+		fib_walk_priority_t prio,
+		fib_node_back_walk_ctx_t *ctx)
+{
+    fib_walk_t *fwalk;
+
+    if (FIB_NODE_GRAPH_MAX_DEPTH < ++ctx->fnbw_depth)
+    {
+	/*
+	 * The walk has reached the maximum depth. there is a loop in the graph.
+	 * bail.
+	 */
+	return;
+    }
+    if (0 == fib_node_get_n_children(parent_type,
+                                     parent_index))
+    {
+        /*
+         * no children to walk - quit now
+         */
+        return;
+    }
+    if (ctx->fnbw_flags & FIB_NODE_BW_FLAG_FORCE_SYNC)
+    {
+        /*
+         * the originator of the walk wanted it to be synchronous, but the
+         * parent object chose async - denied.
+         */
+        return (fib_walk_sync(parent_type, parent_index, ctx));
+    }
+
+
+    fwalk = fib_walk_alloc(parent_type,
+			   parent_index,
+			   FIB_WALK_FLAG_ASYNC,
+			   ctx);
+
+    fwalk->fw_dep_sibling = fib_node_child_add(parent_type,
+					       parent_index,
+					       FIB_NODE_TYPE_WALK,
+					       fib_walk_get_index(fwalk));
+
+    fwalk->fw_prio_sibling = fib_walk_prio_queue_enquue(prio, fwalk);
+}
+
+/**
+ * @brief Back walk all the children of a FIB node.
+ *
+ * note this is a synchronous depth first walk. Children visited may propagate
+ * the walk to thier children. Other children node types may not propagate,
+ * synchronously but instead queue the walk for later async completion.
+ */
+void
+fib_walk_sync (fib_node_type_t parent_type,
+	       fib_node_index_t parent_index,
+	       fib_node_back_walk_ctx_t *ctx)
+{
+    fib_walk_advance_rc_t rc;
+    fib_node_index_t fwi;
+    fib_walk_t *fwalk;
+
+    if (FIB_NODE_GRAPH_MAX_DEPTH < ++ctx->fnbw_depth)
+    {
+	/*
+	 * The walk has reached the maximum depth. there is a loop in the graph.
+	 * bail.
+	 */
+	return;
+    }
+    if (0 == fib_node_get_n_children(parent_type,
+                                     parent_index))
+    {
+        /*
+         * no children to walk - quit now
+         */
+        return;
+    }
+
+    fwalk = fib_walk_alloc(parent_type,
+			   parent_index,
+			   FIB_WALK_FLAG_SYNC,
+			   ctx);
+
+    fwalk->fw_dep_sibling = fib_node_child_add(parent_type,
+					       parent_index,
+					       FIB_NODE_TYPE_WALK,
+					       fib_walk_get_index(fwalk));
+    fwi = fib_walk_get_index(fwalk);
+
+    while (1)
+    {
+	/*
+	 * set this walk as executing
+	 */
+	fwalk->fw_flags |= FIB_WALK_FLAG_EXECUTING;
+
+	do
+	{
+	    rc = fib_walk_advance(fwi);
+	} while (FIB_WALK_ADVANCE_MORE == rc);
+
+
+	/*
+	 * this walk function is re-entrant - walks can spawn walks.
+	 * fib_walk_t objects come from a pool, so they can realloc. we need
+	 * to re-fetch from said pool at the appropriate times.
+	 */
+	fwalk = fib_walk_get(fwi);
+
+	if (FIB_WALK_ADVANCE_MERGE == rc)
+	{
+	    /*
+	     * this sync walk merged with an walk in front.
+	     * by reqeusting a sync walk the client wanted all children walked,
+	     * so we ditch the walk object in hand and continue with the one
+	     * we merged into
+	     */
+	    fib_node_ptr_t merged_walk;
+
+	    fib_node_list_elt_get_next(fwalk->fw_dep_sibling, &merged_walk);
+
+	    ASSERT(FIB_NODE_INDEX_INVALID != merged_walk.fnp_index);
+	    ASSERT(FIB_NODE_TYPE_WALK == merged_walk.fnp_type);
+
+	    fib_walk_destroy(fwalk);
+
+	    fwi = merged_walk.fnp_index;
+	    fwalk = fib_walk_get(fwi);
+
+	    if (FIB_WALK_FLAG_EXECUTING & fwalk->fw_flags)
+	    {
+		/*
+		 * we are executing a sync walk, and we have met with another
+		 * walk that is also executing. since only one walk executs at once
+		 * (there is no multi-threading) this implies we have met ourselves
+		 * and hence the is a loop in the graph.
+		 * This function is re-entrant, so the walk object we met is being
+		 * acted on in a stack frame below this one. We must therefore not
+		 * continue with it now, but let the stack unwind and along the
+		 * appropriate frame to read the depth count and bail.
+		 */
+		fwalk = NULL;
+		break;
+	    }
+	}
+	else
+	{
+	    /*
+	     * the walk reached the end of the depdency list.
+	     */
+	    break;
+	}
+    }
+
+    if (NULL != fwalk)
+    {
+	fib_walk_destroy(fwalk);
+    }
+}
+
+static fib_node_t *
+fib_walk_get_node (fib_node_index_t index)
+{
+    fib_walk_t *fwalk;
+
+    fwalk = fib_walk_get(index);
+
+    return (&(fwalk->fw_node));
+}
+
+/**
+ * Walk objects are not parents, nor are they locked.
+ * are no-ops
+ */
+static void
+fib_walk_last_lock_gone (fib_node_t *node)
+{
+    ASSERT(0);
+}
+
+static fib_walk_t*
+fib_walk_get_from_node (fib_node_t *node)
+{
+    return ((fib_walk_t*)(((char*)node) -
+			  STRUCT_OFFSET_OF(fib_walk_t, fw_node)));
+}
+
+/**
+ * @brief Another back walk has reach this walk.
+ * Megre them so there is only one left. It is this node being
+ * visited that will remain, so copy or merge the context onto it.
+ */
+static fib_node_back_walk_rc_t
+fib_walk_back_walk_notify (fib_node_t *node,
+			   fib_node_back_walk_ctx_t *ctx)
+{
+    fib_node_back_walk_ctx_t *last;
+    fib_walk_t *fwalk;
+
+    fwalk = fib_walk_get_from_node(node);
+
+    /*
+     * check whether the walk context can be merged with the most recent.
+     * the most recent was the one last added and is thus at the back of the vector.
+     * we can merge walks if the reason for the walk is the same.
+     */
+    last = vec_end(fwalk->fw_ctx) - 1;
+
+    if (last->fnbw_reason == ctx->fnbw_reason)
+    {
+        /*
+         * copy the largest of the depth values. in the presence of a loop,
+         * the same walk will merge with itself. if we take the smaller depth
+         * then it will never end.
+         */
+        last->fnbw_depth = ((last->fnbw_depth >= ctx->fnbw_depth) ?
+                            last->fnbw_depth :
+                            ctx->fnbw_depth);
+    }
+    else
+    {
+        /*
+         * walks could not be merged, this means that the walk infront needs to
+         * perform different action to this one that has caught up. the one in
+         * front was scheduled first so append the new walk context to the back
+         * of the list.
+         */
+        vec_add1(fwalk->fw_ctx, *ctx);
+    }
+
+    return (FIB_NODE_BACK_WALK_MERGE);
+}
+
+/**
+ * The FIB walk's graph node virtual function table
+ */
+static const fib_node_vft_t fib_walk_vft = {
+    .fnv_get = fib_walk_get_node,
+    .fnv_last_lock = fib_walk_last_lock_gone,
+    .fnv_back_walk = fib_walk_back_walk_notify,
+};
+
+void
+fib_walk_module_init (void)
+{
+    fib_walk_priority_t prio;
+
+    FOR_EACH_FIB_WALK_PRIORITY(prio)
+    {
+	fib_walk_queues.fwqs_queues[prio].fwq_queue = fib_node_list_create();
+    }
+
+    fib_node_register_type(FIB_NODE_TYPE_WALK, &fib_walk_vft);
+}
+
+static u8*
+format_fib_walk (u8* s, va_list ap)
+{
+    fib_node_index_t fwi = va_arg(ap, fib_node_index_t);
+    fib_walk_t *fwalk;
+
+    fwalk = fib_walk_get(fwi);
+
+    return (format(s, "  parent:{%s:%d} visits:%d flags:%d",
+		   fib_node_type_get_name(fwalk->fw_parent.fnp_type),
+		   fwalk->fw_parent.fnp_index,
+		   fwalk->fw_n_visits,
+		   fwalk->fw_flags));
+}
+
+static clib_error_t *
+fib_walk_show (vlib_main_t * vm,
+	       unformat_input_t * input,
+	       vlib_cli_command_t * cmd)
+{
+    fib_walk_queue_stats_t wqs;
+    fib_walk_priority_t prio;
+    fib_node_ptr_t sibling;
+    fib_node_index_t fwi;
+    fib_walk_t *fwalk;
+    int more_elts, ii;
+    u8 *s = NULL;
+
+#define USEC 1000000
+    vlib_cli_output(vm, "FIB Walk Quota = %.2fusec:", quota * USEC);
+    vlib_cli_output(vm, "FIB Walk queues:");
+
+    FOR_EACH_FIB_WALK_PRIORITY(prio)
+    {
+	vlib_cli_output(vm, " %U priority queue:",
+			format_fib_walk_priority, prio);
+	vlib_cli_output(vm, "  Stats: ");
+
+	FOR_EACH_FIB_WALK_QUEUE_STATS(wqs)
+	{
+	    vlib_cli_output(vm, "    %U:%d",
+			    format_fib_walk_queue_stats, wqs,
+			    fib_walk_queues.fwqs_queues[prio].fwq_stats[wqs]);
+	}
+	vlib_cli_output(vm, "  Occupancy:%d",
+			fib_node_list_get_size(
+			    fib_walk_queues.fwqs_queues[prio].fwq_queue));
+
+	more_elts = fib_node_list_get_front(
+			fib_walk_queues.fwqs_queues[prio].fwq_queue,
+			&sibling);
+
+	while (more_elts)
+	{
+	    ASSERT(FIB_NODE_INDEX_INVALID != sibling.fnp_index);
+	    ASSERT(FIB_NODE_TYPE_WALK == sibling.fnp_type);
+
+	    fwi = sibling.fnp_index;
+	    fwalk = fib_walk_get(fwi);
+
+	    vlib_cli_output(vm, "  %U", format_fib_walk, fwi);
+
+	    more_elts = fib_node_list_elt_get_next(fwalk->fw_prio_sibling,
+						   &sibling);
+	}
+    }
+
+    vlib_cli_output(vm, "Histogram Statistics:");
+    vlib_cli_output(vm, " Number of Elements visit per-quota:");
+    for (ii = 0; ii < N_ELTS_BUCKETS; ii++)
+    {
+	if (0 != fib_walk_work_nodes_visited[ii])
+	    s = format(s, "%d:%d ",
+		       (ii * fib_walk_work_nodes_visisted_incr),
+		       fib_walk_work_nodes_visited[ii]);
+    }
+    vlib_cli_output(vm, "  %v", s);
+    vec_free(s);
+
+    vlib_cli_output(vm, " Time consumed per-quota (Quota=%f usec):", quota*USEC);
+    s = format(s, "0:%d ", fib_walk_work_time_taken[0]);
+    for (ii = 1; ii < N_TIME_BUCKETS; ii++)
+    {
+	if (0 != fib_walk_work_time_taken[ii])
+	    s = format(s, "%d:%d ", (u32)((((ii - N_TIME_BUCKETS/2) *
+					   (quota / TIME_INCREMENTS)) + quota) *
+					 USEC),
+		       fib_walk_work_time_taken[ii]);
+    }
+    vlib_cli_output(vm, "  %v", s);
+    vec_free(s);
+
+    vlib_cli_output(vm, " Sleep Types:");
+    vlib_cli_output(vm, "  Short  Long:");
+    vlib_cli_output(vm, "  %d %d:",
+		    fib_walk_sleep_lengths[FIB_WALK_SHORT_SLEEP],
+		    fib_walk_sleep_lengths[FIB_WALK_LONG_SLEEP]);
+
+    vlib_cli_output(vm, " Number of Elements visited per-walk:");
+    for (ii = 0; ii < HISTOGRAM_VISITS_PER_WALK_N_BUCKETS; ii++)
+    {
+	if (0 != fib_walk_hist_vists_per_walk[ii])
+	    s = format(s, "%d:%d ",
+		       ii*HISTOGRAM_VISITS_PER_WALK_INCR,
+		       fib_walk_hist_vists_per_walk[ii]);
+    }
+    vlib_cli_output(vm, "  %v", s);
+    vec_free(s);
+
+
+    vlib_cli_output(vm, "Brief History (last %d walks):", HISTORY_N_WALKS);
+    ii = history_last_walk_pos - 1;
+    if (ii < 0)
+        ii = HISTORY_N_WALKS - 1;
+
+    while (ii != history_last_walk_pos)
+    {
+	if (0 != fib_walk_history[ii].fwh_reason[0])
+	{
+            fib_node_back_walk_reason_t reason;
+            u8 *s = NULL;
+            u32 jj;
+
+	    s = format(s, "[@%d]: %s:%d visits:%d duration:%.2f completed:%.2f ",
+                       ii, fib_node_type_get_name(fib_walk_history[ii].fwh_parent.fnp_type),
+                       fib_walk_history[ii].fwh_parent.fnp_index,
+                       fib_walk_history[ii].fwh_n_visits,
+                       fib_walk_history[ii].fwh_duration,
+                       fib_walk_history[ii].fwh_completed);
+            if (FIB_WALK_FLAG_SYNC & fib_walk_history[ii].fwh_flags)
+                s = format(s, "sync, ");
+            if (FIB_WALK_FLAG_ASYNC & fib_walk_history[ii].fwh_flags)
+                s = format(s, "async, ");
+
+            s = format(s, "reason:");
+            jj = 0;
+            while (0 != fib_walk_history[ii].fwh_reason[jj])
+            {
+                FOR_EACH_FIB_NODE_BW_REASON(reason) {
+                    if ((1<<reason) & fib_walk_history[ii].fwh_reason[jj]) {
+                        s = format (s, "%s,", fib_node_bw_reason_names[reason]);
+                    }
+                }
+                jj++;
+            }
+            vlib_cli_output(vm, "%v", s);
+	}
+
+        ii--;
+        if (ii < 0)
+            ii = HISTORY_N_WALKS - 1;
+    }
+
+    return (NULL);
+}
+
+VLIB_CLI_COMMAND (fib_walk_show_command, static) = {
+    .path = "show fib walk",
+    .short_help = "show fib walk",
+    .function = fib_walk_show,
+};
+
+static clib_error_t *
+fib_walk_set_quota (vlib_main_t * vm,
+		    unformat_input_t * input,
+		    vlib_cli_command_t * cmd)
+{
+    clib_error_t * error = NULL;
+    f64 new_quota;
+
+    if (unformat (input, "%f", &new_quota))
+    {
+	quota = new_quota;
+    }
+    else
+    {
+	error = clib_error_return(0 , "Pass a float value");
+    }
+
+    return (error);
+}
+
+VLIB_CLI_COMMAND (fib_walk_set_quota_command, static) = {
+    .path = "set fib walk quota",
+    .short_help = "set fib walk quota",
+    .function = fib_walk_set_quota,
+};
+
+static clib_error_t *
+fib_walk_set_histogram_elements_size (vlib_main_t * vm,
+				      unformat_input_t * input,
+				      vlib_cli_command_t * cmd)
+{
+    clib_error_t * error = NULL;
+    u32 new;
+
+    if (unformat (input, "%d", &new))
+    {
+	fib_walk_work_nodes_visisted_incr = new;
+    }
+    else
+    {
+	error = clib_error_return(0 , "Pass an int value");
+    }
+
+    return (error);
+}
+
+VLIB_CLI_COMMAND (fib_walk_set_histogram_elements_size_command, static) = {
+    .path = "set fib walk histogram elements size",
+    .short_help = "set fib walk histogram elements size",
+    .function = fib_walk_set_histogram_elements_size,
+};
+
+static clib_error_t *
+fib_walk_clear (vlib_main_t * vm,
+		unformat_input_t * input,
+		vlib_cli_command_t * cmd)
+{
+    memset(fib_walk_hist_vists_per_walk, 0, sizeof(fib_walk_hist_vists_per_walk));
+    memset(fib_walk_history, 0, sizeof(fib_walk_history));
+    memset(fib_walk_work_time_taken, 0, sizeof(fib_walk_work_time_taken));
+    memset(fib_walk_work_nodes_visited, 0, sizeof(fib_walk_work_nodes_visited));
+    memset(fib_walk_sleep_lengths, 0, sizeof(fib_walk_sleep_lengths));
+
+    return (NULL);
+}
+
+VLIB_CLI_COMMAND (fib_walk_clear_command, static) = {
+    .path = "clear fib walk",
+    .short_help = "clear fib walk",
+    .function = fib_walk_clear,
+};