sched: Improve timer vs task priority check

Rename sched_tasks_busy() to sched_check_set_tasks_busy() and change
it to only return true if tasks are active (running or requested) for
two consecutive calls.  This makes it less likely that timers will
yield to tasks except when tasks really are notably backlogged.

This also makes it less likely that multiple steppers controlling the
same rail will be interrupted by tasks mid-step.  This should slightly
improve the timing, and make it less likely that a halt during
homing/probing will occur with these steppers taking a different
number of total steps.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

1 files changed, 10 insertions, 6 deletions

diff --git a/src/sched.c b/src/sched.c
index 44cce558..51d159bc 100644
--- a/src/sched.c
+++ b/src/sched.c
@@ -1,6 +1,6 @@
 // Basic scheduling functions and startup/shutdown code.
 //
-// Copyright (C) 2016-2021  Kevin O'Connor <kevin@koconnor.net>
+// Copyright (C) 2016-2024  Kevin O'Connor <kevin@koconnor.net>
 //
 // This file may be distributed under the terms of the GNU GPLv3 license.
 
@@ -19,7 +19,7 @@ static struct timer periodic_timer, sentinel_timer, deleted_timer;
 
 static struct {
     struct timer *timer_list, *last_insert;
-    int8_t tasks_status;
+    int8_t tasks_status, tasks_busy;
     uint8_t shutdown_status, shutdown_reason;
 } SchedStatus = {.timer_list = &periodic_timer, .last_insert = &periodic_timer};
 
@@ -205,11 +205,15 @@ sched_wake_tasks(void)
     SchedStatus.tasks_status = TS_REQUESTED;
 }
 
-// Check if tasks need to be run
+// Check if tasks busy (called from low-level timer dispatch code)
 uint8_t
-sched_tasks_busy(void)
+sched_check_set_tasks_busy(void)
 {
-    return SchedStatus.tasks_status >= TS_REQUESTED;
+    // Return busy if tasks never idle between two consecutive calls
+    if (SchedStatus.tasks_busy >= TS_REQUESTED)
+        return 1;
+    SchedStatus.tasks_busy = SchedStatus.tasks_status;
+    return 0;
 }
 
 // Note that a task is ready to run
@@ -243,7 +247,7 @@ run_tasks(void)
             irq_disable();
             if (SchedStatus.tasks_status != TS_REQUESTED) {
                 // Sleep processor (only run timers) until tasks woken
-                SchedStatus.tasks_status = TS_IDLE;
+                SchedStatus.tasks_status = SchedStatus.tasks_busy = TS_IDLE;
                 do {
                     irq_wait();
                 } while (SchedStatus.tasks_status != TS_REQUESTED);