Try to monitor planner buffer underruns in the auto_report_buffer_statistics loop as modifying the stepper ISR loop seems like a bad idea

Marlin/src/gcode/queue.cpp

@@ -91,10 +91,14 @@ bool send_ok[BUFSIZE];
 #if ENABLED(BUFFER_MONITORING)
   uint32_t GCodeQueue::command_buffer_underruns = 0;
   bool GCodeQueue::command_buffer_empty = false;
-
   millis_t GCodeQueue::max_command_buffer_empty_duration = 0;
   millis_t GCodeQueue::command_buffer_empty_at = 0;
 
+  uint32_t GCodeQueue::planner_buffer_underruns = 0;
+  bool GCodeQueue::planner_buffer_empty = false;
+  millis_t GCodeQueue::max_planner_buffer_empty_duration = 0;
+  millis_t GCodeQueue::planner_buffer_empty_at = 0;
+
   uint8_t GCodeQueue::auto_buffer_report_interval;
   millis_t GCodeQueue::next_buffer_report_ms;
 #endif
@@ -639,12 +643,14 @@ void GCodeQueue::advance() {
   }
 
   #if ENABLED(BUFFER_MONITORING)
-    static millis_t command_buffer_empty_duration;
+    if (command_buffer_empty) {
-    command_buffer_empty_duration = millis() - command_buffer_empty_at;
+      static millis_t command_buffer_empty_duration;
-    if (command_buffer_empty_duration > max_command_buffer_empty_duration) {
+      command_buffer_empty_duration = millis() - command_buffer_empty_at;
-      max_command_buffer_empty_duration = command_buffer_empty_duration;
+      if (command_buffer_empty_duration > max_command_buffer_empty_duration) {
+        max_command_buffer_empty_duration = command_buffer_empty_duration;
+      }
+      command_buffer_empty = false;
     }
-    command_buffer_empty = false;
   #endif
 
 
@@ -697,15 +703,40 @@ void GCodeQueue::report_buffer_statistics() {
   SERIAL_ECHO("M576");
   SERIAL_ECHOLNPAIR(SP_P_STR, int(planner.moves_free()),
                     SP_B_STR, int(BUFSIZE - length),
+                    " PU", queue.planner_buffer_underruns,
+                    " PD", queue.max_planner_buffer_empty_duration,
                     " BU", queue.command_buffer_underruns,
                     " BD", queue.max_command_buffer_empty_duration,
                     );
 
   command_buffer_underruns = 0;
   max_command_buffer_empty_duration = 0;
+
+  planner_buffer_underruns = 0;
+  max_planner_buffer_empty_duration = 0;
 }
 
 void GCodeQueue::auto_report_buffer_statistics() {
+  // Bit of a hack to try to catch planner buffer underruns without having logic
+  // running inside Stepper::block_phase_isr
+  if (planner.movesplanned() == 0) {
+    if (!planner_buffer_empty) { // if the planner buffer wasn't empty, but now it is
+      planner_buffer_empty = true;
+      planner_buffer_underruns++;
+      planner_buffer_empty_at = millis();
+    }
+  } else if (planner_buffer_empty) { // if the planner buffer was empty, but now it ain't
+    static millis_t planner_buffer_empty_duration;
+    planner_buffer_empty_duration = millis() - planner_buffer_empty_at;
+
+    // if it's longer than the currently tracked max duration, replace it
+    if (planner_buffer_empty_duration > max_planner_buffer_empty_duration) {
+      max_planner_buffer_empty_duration = planner_buffer_empty_duration;
+    }
+
+    planner_buffer_empty = false;
+  }
+
   if (queue.auto_buffer_report_interval && ELAPSED(millis(), queue.next_buffer_report_ms)) {
     queue.next_buffer_report_ms = millis() + 1000UL * queue.auto_buffer_report_interval;
     PORT_REDIRECT(SERIAL_BOTH);

Marlin/src/gcode/queue.h

@@ -156,13 +156,21 @@ public:
     static millis_t max_command_buffer_empty_duration;
     static millis_t command_buffer_empty_at;
 
+    static uint32_t planner_buffer_underruns;
+    static bool planner_buffer_empty;
+    static millis_t max_planner_buffer_empty_duration;
+    static millis_t planner_buffer_empty_at;
+
     /**
      * Report buffer statistics to the host to be able to detect buffer underruns
      * 
      * Returns "M576 " followed by:
-     *  P<uint>  Planner space remaining
+     *  P<uint>   Planner space remaining
-     *  B<uint>  Command buffer space remaining
+     *  B<uint>   Command buffer space remaining
-     *  U<uint>  Number of command buffer underruns since last report
+     *  PU<uint>  Number of planner buffer underruns since last report
+     *  PD<uint>  Max time in ms the planner buffer was empty since last report
+     *  BU<uint>  Number of command buffer underruns since last report
+     *  BD<uint>  Max time in ms the command buffer was empty since last report
      */
     static void report_buffer_statistics();
 

Marlin/src/gcode/stats/M576.cpp

@@ -30,7 +30,18 @@
 #include "../queue.h"
 
 /**
- * M576: Return buffer stats, and optionally set auto-report interval. M576 [S<seconds>]
+ * M576: Return buffer stats, and optionally set auto-report interval.
+ * Usage: M576 [S<seconds>]
+ * 
+ * When called, printer emits the following output:
+ * "M576 P<nn> B<nn> PU<nn> PD<nn> BU<nn> BD<nn>"
+ * Where:
+ *   P: Planner buffers available 
+ *   B: Command buffers available
+ *   PU: Planner buffer underruns since last report
+ *   PD: Maximum time in ms planner buffer was empty since last report
+ *   BU: Command buffer underruns since last report
+ *   BD: Maximum time in ms command buffer was empty since last report
  */
 void GcodeSuite::M576() {
   if (parser.seenval('S')) {