VPP-223 Document vlib cooperative multi-tasking threads

Change-Id: I283963b004ed6d91133e7e36811f75834280bbe7
Signed-off-by: Dave Barach <dave@barachs.net>

vlib/vlib/node_funcs.h

@@ -410,13 +410,22 @@ vlib_current_process (vlib_main_t * vm)
   return vlib_get_current_process (vm)->node_runtime.node_index;
 }
 
-/* Anything less than 1e-6 is considered zero. */
+/** Returns TRUE if a process suspend time is less than 1us
+    @param dt - remaining poll time in seconds
+    @returns 1 if dt < 1e-6, 0 otherwise
+*/
 always_inline uword
 vlib_process_suspend_time_is_zero (f64 dt)
 {
   return dt < 1e-6;
 }
 
+/** Suspend a vlib cooperative multi-tasking thread for a period of time
+    @param vm - vlib_main_t *
+    @param dt - suspend interval in seconds
+    @returns VLIB_PROCESS_RESUME_LONGJMP_RESUME, routinely ignored
+*/
+
 always_inline uword
 vlib_process_suspend (vlib_main_t * vm, f64 dt)
 {
@@ -503,7 +512,15 @@ vlib_process_put_event_data (vlib_main_t * vm, void *event_data)
   vec_add1 (nm->recycled_event_data_vectors, event_data);
 }
 
-/* Return type & add any events to data vector. */
+/** Return the first event type which has occurred and a vector of per-event
+    data of that type, or a timeout indication
+
+    @param vm - vlib_main_t pointer
+    @param data_vector - pointer to a (uword *) vector to receive event data
+    @returns either an event type and a vector of per-event instance data,
+    or ~0 to indicate a timeout.
+*/
+
 always_inline uword
 vlib_process_get_events (vlib_main_t * vm, uword ** data_vector)
 {
@@ -654,6 +671,13 @@ vlib_process_wait_for_event_with_type (vlib_main_t * vm,
   return vlib_process_get_events_helper (p, h[0], data_vector);
 }
 
+/** Suspend a cooperative multi-tasking thread
+    Waits for an event, or for the indicated number of seconds to elapse
+    @param vm - vlib_main_t pointer
+    @param dt - timeout, in seconds. 
+    @returns the remaining time interval
+*/
+
 always_inline f64
 vlib_process_wait_for_event_or_clock (vlib_main_t * vm, f64 dt)
 {

vlib/vlib/vlib_process_doc.h

@@ -0,0 +1,147 @@
+/*
+ * 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.
+*/
+
+#error do not #include this file!
+
+/** \file
+
+    Cooperative multi-tasking thread support.
+
+    Vlib provides a lightweight cooperative multi-tasking thread
+    model. Context switching costs a setjmp/longjump pair.  It's not
+    unreasonable to put vlib threads to sleep for 10us.
+
+    The graph node scheduler invokes these processes in much the same
+    way as traditional vector-processing run-to-completion graph
+    nodes; plus-or-minus a setjmp/longjmp pair required to switch
+    stacks. Simply set the vlib_node_registration_t type field to
+    VLIB_NODE_TYPE_PROCESS. Process is a misnomer; these are threads.
+
+    As of this writing, the default stack size is 2<<15;
+    32kb. Initialize the node registration's
+    process_log2_n_stack_bytes member as needed. The graph node
+    dispatcher makes some effort to detect stack overrun. We map a
+    no-access page below each thread stack.
+
+    Process node dispatch functions are expected to be while(1) { }
+    loops which suspend when not otherwise occupied, and which must
+    not run for unreasonably long periods of time.  Unreasonably long
+    is an application-dependent concept. Over the years, we have
+    constructed frame-size sensitive control-plane nodes which will
+    use a much higher fraction of the available CPU bandwidth when the
+    frame size is low. Classic example: modifying forwarding
+    tables. So long as the table-builder leaves the forwarding tables
+    in a valid state, one can suspend the table builder to avoid
+    dropping packets as a result of control-plane activity.
+
+    Process nodes can suspend for fixed amounts of time, or until another
+    entity signals an event, or both. See the example below.
+
+    When running in VLIB process context, one must pay strict attention to
+    loop invariant issues. If one walks a data structure and calls a
+    function which may suspend, one had best know by construction that it
+    cannot change. Often, it s best to simply make a snapshot copy of a
+    data structure, walk the copy at leisure, then free the copy.
+
+    Here's an example:
+
+    <pre>
+    #define EXAMPLE_POLL_PERIOD 10.0
+
+    static uword
+    example_process (vlib_main_t * vm, vlib_node_runtime_t * rt, 
+                     vlib_frame_t * f)
+    {
+      f64 poll_time_remaining;
+      uword event_type, *event_data = 0;
+
+      poll_time_remaining = EXAMPLE_POLL_PERIOD;
+      while (1)
+        {
+          int i;
+
+           // Sleep until next periodic call due, 
+           // or until we receive event(s) 
+           //
+          poll_time_remaining =
+    	    vlib_process_wait_for_event_or_clock (vm, poll_time_remaining);
+
+          event_type = vlib_process_get_events (vm, &event_data);
+          switch (event_type)
+     	    {
+       	    case ~0:		// no events => timeout
+      	      break;
+
+            case EVENT1:
+    	      for (i = 0; i < vec_len (event_data); i++)
+    	        handle_event1 (mm, event_data[i]);
+    	      break;
+
+    	    case EVENT2:
+    	      for (i = 0; i < vec_len (event_data); i++)
+    	        handle_event2 (vm, event_data[i]);
+    	      break;
+
+              // ... and so forth for each event type 
+
+            default:
+              // This should never happen... 
+    	      clib_warning ("BUG: unhandled event type %d", 
+                            event_type);
+    	      break;
+      	    }
+          vec_reset_length (event_data);
+
+          // Timer expired, call periodic function 
+          if (vlib_process_suspend_time_is_zero (poll_time_remaining))
+    	    {
+    	      example_periodic (vm);
+    	      poll_time_remaining = EXAMPLE_POLL_PERIOD;
+    	    }
+        }
+      // NOTREACHED 
+      return 0;
+    }     
+
+    static VLIB_REGISTER_NODE (example_node) = {
+      .function = example_process,
+      .type = VLIB_NODE_TYPE_PROCESS,
+      .name = "example-process",
+    };
+    </pre>
+
+    In this example, the VLIB process node waits for an event to
+    occur, or for 10 seconds to elapse. The code demuxes on the event
+    type, calling the appropriate handler function.
+
+    Each call to vlib_process_get_events returns a vector of
+    per-event-type data passed to successive vlib_process_signal_event
+    calls; vec_len (event_data) >= 1.  It is an error to process only
+    event_data[0].
+
+    Resetting the event_data vector-length to 0 by calling
+    vec_reset_length (event_data) - instead of calling vec_free (...)
+    - means that the event scheme doesn t burn cycles continuously
+    allocating and freeing the event data vector. This is a common
+    coding pattern, well worth using when appropriate.
+*/
+
+/*
+ * fd.io coding-style-patch-verification: ON
+ *
+ * Local Variables:
+ * eval: (c-set-style "gnu")
+ * End:
+ */