zebra: backpressure - Zebra push back on Buffer/Stream creation

Currently, the way zebra works is it creates pthread per client (BGP is
of interest in this case) and this thread loops itself in zserv_read()
to check for any incoming data. If there is one, then it reads,
validates and adds it in the ibuf_fifo signalling the main thread to
process the message. The main thread when it gets a change, processes
the message, and invokes the function pointer registered in the header
command. (Ex: zserv_handlers).

Finally, if all of this was successful, this task reschedules itself and
loops in zserv_read() again

However, if there are already items on ibuf FIFO, that means zebra is
slow in processing. And with the current mechanism if Zebra main is
busy, the ibuf FIFO keeps growing holding up the memory.

Show memory zebra:(Example: 15k streams hoarding ~160 MB of data)
--- qmem libfrr ---
Stream             :       44 variable   3432352    15042 161243800

Fix:
Client IO Thread: (zserv_read)
 - Stop doing the read events when we know there are X number of items
   on the FIFO already.(X - zebra zapi-packets <1-10000> (Default-1000)

 - Determine the number of items on the zserv->ibuf_fifo. Subtract this
   from the work items and only pull the number of items off that would
   take us to X items on the ibuf_fifo again.

 - If the number of items in the ibuf_fifo has reached to the maximum
      * Either initially when zserv_read() is called (or)
      * when processing the remainders of the incoming buffer
   the client IO thread is woken by the the zebra main.

Main thread: (zserv_process_message)
If the client ibuf always schedules a wakeup to the client IO to read
more items from the socked buffer. This way we ensure
 - Client IO thread always tries to read the socket buffer and add more
   items to the ibuf_fifo (until max limit)
 - hidden config change (zebra zapi-packets <>) is taken into account

Ticket: #3390099

Signed-off-by: Donald Sharp <[email protected]>
Signed-off-by: Rajasekar Raja <[email protected]>

zebra/zserv.c

@@ -306,22 +306,40 @@ static void zserv_write(struct event *thread)
  * this task reschedules itself.
  *
  * Any failure in any of these actions is handled by terminating the client.
+ *
+ * The client's input buffer ibuf_fifo can have a maximum items as configured
+ * in the packets_to_process. This way we are not filling up the FIFO more
+ * than the maximum when the zebra main is busy. If the fifo has space, we
+ * reschedule ourselves to read more.
+ *
+ * The main thread processes the items in ibuf_fifo and always signals the
+ * client IO thread.
  */
 static void zserv_read(struct event *thread)
 {
 	struct zserv *client = EVENT_ARG(thread);
 	int sock;
 	size_t already;
 	struct stream_fifo *cache;
-	uint32_t p2p_orig;
-
-	uint32_t p2p;
+	uint32_t p2p;	    /* Temp p2p used to process */
+	uint32_t p2p_orig;  /* Configured p2p (Default-1000) */
+	int p2p_avail;	    /* How much space is available for p2p */
 	struct zmsghdr hdr;
+	size_t client_ibuf_fifo_cnt = stream_fifo_count_safe(client->ibuf_fifo);
 
 	p2p_orig = atomic_load_explicit(&zrouter.packets_to_process,
 					memory_order_relaxed);
+	p2p_avail = p2p_orig - client_ibuf_fifo_cnt;
+
+    /*
+     * Do nothing if ibuf_fifo count has reached its max limit. Otherwise
+     * proceed and reschedule ourselves if there is space in the ibuf_fifo.
+     */
+	if (p2p_avail <= 0)
+		return;
+
+	p2p = p2p_avail;
 	cache = stream_fifo_new();
-	p2p = p2p_orig;
 	sock = EVENT_FD(thread);
 
 	while (p2p) {
@@ -421,7 +439,7 @@ static void zserv_read(struct event *thread)
 		p2p--;
 	}
 
-	if (p2p < p2p_orig) {
+	if (p2p < (uint32_t)p2p_avail) {
 		uint64_t time_now = monotime(NULL);
 
 		/* update session statistics */
@@ -435,19 +453,23 @@ static void zserv_read(struct event *thread)
 			while (cache->head)
 				stream_fifo_push(client->ibuf_fifo,
 						 stream_fifo_pop(cache));
+			/* Need to update count as main thread could have processed few */
+			client_ibuf_fifo_cnt =
+				stream_fifo_count_safe(client->ibuf_fifo);
 		}
 
 		/* Schedule job to process those packets */
 		zserv_event(client, ZSERV_PROCESS_MESSAGES);
-
 	}
 
 	if (IS_ZEBRA_DEBUG_PACKET)
-		zlog_debug("Read %d packets from client: %s", p2p_orig - p2p,
-			   zebra_route_string(client->proto));
+		zlog_debug("Read %d packets from client: %s. Current ibuf fifo count: %zu. Conf P2p %d",
+			   p2p_avail - p2p, zebra_route_string(client->proto),
+			   client_ibuf_fifo_cnt, p2p_orig);
 
-	/* Reschedule ourselves */
-	zserv_client_event(client, ZSERV_CLIENT_READ);
+	/* Reschedule ourselves since we have space in ibuf_fifo */
+	if (client_ibuf_fifo_cnt != p2p_orig)
+		zserv_client_event(client, ZSERV_CLIENT_READ);
 
 	stream_fifo_free(cache);
 
@@ -483,14 +505,20 @@ static void zserv_client_event(struct zserv *client,
  * as the task argument.
  *
  * Each message is popped off the client's input queue and the action associated
- * with the message is executed. This proceeds until there are no more messages,
- * an error occurs, or the processing limit is reached.
+ * with the message is executed. This proceeds until an error occurs, or the
+ * processing limit is reached.
  *
  * The client's I/O thread can push at most zrouter.packets_to_process messages
  * onto the input buffer before notifying us there are packets to read. As long
  * as we always process zrouter.packets_to_process messages here, then we can
  * rely on the read thread to handle queuing this task enough times to process
  * everything on the input queue.
+ *
+ * If the client ibuf always schedules a wakeup to the client IO to read more
+ * items from the socked buffer. This way we ensure
+ *  - Client IO thread always tries to read the socket buffer and add more
+ *    items to the ibuf_fifo (until max limit)
+ *  - the hidden config change (zebra zapi-packets <>) is taken into account.
  */
 static void zserv_process_messages(struct event *thread)
 {
@@ -524,6 +552,9 @@ static void zserv_process_messages(struct event *thread)
 	/* Reschedule ourselves if necessary */
 	if (need_resched)
 		zserv_event(client, ZSERV_PROCESS_MESSAGES);
+
+	/* Signal a wake up to client IO thread */
+	zserv_client_event(client, ZSERV_CLIENT_READ);
 }
 
 int zserv_send_message(struct zserv *client, struct stream *msg)