core: make idle thread optional

Makefile.dep

@@ -1074,6 +1074,9 @@ FEATURES_REQUIRED += $(filter arch_%,$(FEATURES_PROVIDED))
 # always select CPU core features
 FEATURES_REQUIRED += $(filter cpu_core_%,$(FEATURES_PROVIDED))
 
+# don't use idle thread if architecture has needed support
+FEATURES_OPTIONAL += no_idle_thread
+
 ifneq (,$(filter ecc_%,$(USEMODULE)))
   USEMODULE += ecc
 endif

core/include/sched.h

@@ -203,8 +203,22 @@ extern clist_node_t sched_runqueues[SCHED_PRIO_LEVELS];
  */
 NORETURN void sched_task_exit(void);
 
-#if IS_USED(MODULE_SCHED_CB) || defined(DOXYGEN)
+/**
+ * @brief  Set CPU to idle mode (CPU dependent)
+ *
+ * Only used when there's no idle thread.
+ *
+ * This function will be called by the scheduler when there's no runnable thread.
+ * It will be called from ISR context, and *must* allow other ISR handlers to be run.
+ * E.g., on Cortex-M, the PendSV priority is temporarily lowered (set to higher
+ * value) in order to enable other exceptions to be run.
+ *
+ * This function should also invoke setting a low power mode, e.g., by calling
+ * 'pm_set_lowest()'.
+ */
+void sched_arch_idle(void);
 
+#if IS_USED(MODULE_SCHED_CB) || defined(DOXYGEN)
 /**
  * @brief   Scheduler run callback
  *

core/init.c

@@ -53,6 +53,9 @@ static void *main_trampoline(void *arg)
     return NULL;
 }
 
+static char main_stack[THREAD_STACKSIZE_MAIN];
+static char idle_stack[THREAD_STACKSIZE_IDLE];
+
 static void *idle_thread(void *arg)
 {
     (void)arg;
@@ -64,17 +67,17 @@ static void *idle_thread(void *arg)
     return NULL;
 }
 
-static char main_stack[THREAD_STACKSIZE_MAIN];
-static char idle_stack[THREAD_STACKSIZE_IDLE];
 
 void kernel_init(void)
 {
     irq_disable();
 
-    thread_create(idle_stack, sizeof(idle_stack),
-                  THREAD_PRIORITY_IDLE,
-                  THREAD_CREATE_WOUT_YIELD | THREAD_CREATE_STACKTEST,
-                  idle_thread, NULL, "idle");
+    if (IS_USED(MODULE_CORE_IDLE_THREAD)) {
+        thread_create(idle_stack, sizeof(idle_stack),
+                      THREAD_PRIORITY_IDLE,
+                      THREAD_CREATE_WOUT_YIELD | THREAD_CREATE_STACKTEST,
+                      idle_thread, NULL, "idle");
+    }
 
     thread_create(main_stack, sizeof(main_stack),
                   THREAD_PRIORITY_MAIN,

core/sched.c

@@ -76,15 +76,45 @@ static void (*sched_cb) (kernel_pid_t active_thread,
                          kernel_pid_t next_thread) = NULL;
 #endif
 
+static void _unschedule(thread_t *active_thread)
+{
+    if (active_thread->status == STATUS_RUNNING) {
+        active_thread->status = STATUS_PENDING;
+    }
+
+#ifdef SCHED_TEST_STACK
+    if (*((uintptr_t *)active_thread->stack_start) !=
+        (uintptr_t)active_thread->stack_start) {
+        LOG_WARNING(
+            "scheduler(): stack overflow detected, pid=%" PRIkernel_pid "\n",
+            active_thread->pid);
+    }
+#endif
+#ifdef MODULE_SCHED_CB
+    if (sched_cb) {
+        sched_cb(active_thread->pid, KERNEL_PID_UNDEF);
+    }
+#endif
+}
+
 int __attribute__((used)) sched_run(void)
 {
     sched_context_switch_request = 0;
-
     thread_t *active_thread = (thread_t *)sched_active_thread;
 
-    /* The bitmask in runqueue_bitcache is never empty,
-     * since the threading should not be started before at least the idle thread was started.
-     */
+    if (!IS_USED(MODULE_CORE_IDLE_THREAD)) {
+        if (!runqueue_bitcache) {
+            if (active_thread) {
+                _unschedule(active_thread);
+                active_thread = NULL;
+            }
+
+            while (!runqueue_bitcache) {
+                sched_arch_idle();
+            }
+        }
+    }
+
     int nextrq = bitarithm_lsb(runqueue_bitcache);
     thread_t *next_thread = container_of(sched_runqueues[nextrq].next->next,
                                          thread_t, rq_entry);
@@ -102,26 +132,12 @@ int __attribute__((used)) sched_run(void)
     }
 
     if (active_thread) {
-        if (active_thread->status == STATUS_RUNNING) {
-            active_thread->status = STATUS_PENDING;
-        }
-
-#ifdef SCHED_TEST_STACK
-        if (*((uintptr_t *)active_thread->stack_start) !=
-            (uintptr_t)active_thread->stack_start) {
-            LOG_WARNING(
-                "scheduler(): stack overflow detected, pid=%" PRIkernel_pid "\n",
-                active_thread->pid);
-        }
-#endif
+        _unschedule(active_thread);
     }
 
 #ifdef MODULE_SCHED_CB
     if (sched_cb) {
-        /* Use `sched_active_pid` instead of `active_thread` since after `sched_task_exit()` is
-           called `active_thread` is set to NULL while `sched_active_thread` isn't updated until
-           `next_thread` is scheduled*/
-        sched_cb(sched_active_pid, next_thread->pid);
+        sched_cb(KERNEL_PID_UNDEF, next_thread->pid);
     }
 #endif
 

cpu/cortexm_common/Kconfig

@@ -13,11 +13,13 @@ config CPU_ARCH_ARMV7M
     bool
     select HAS_ARCH_ARM
     select HAS_ARCH_32BIT
+    select HAS_NO_IDLE_THREAD
 
 config CPU_ARCH_ARMV8M
     bool
     select HAS_ARCH_ARM
     select HAS_ARCH_32BIT
+    select HAS_NO_IDLE_THREAD
 
 config CPU_ARCH
     default "armv6m" if CPU_ARCH_ARMV6M

cpu/cortexm_common/Makefile.features

@@ -30,3 +30,8 @@ else ifeq ($(CPU_CORE),cortex-m23)
 else
   $(error Unkwnown cortexm core: $(CPU_CORE))
 endif
+
+# cortex-m3 and higher don't need the idle thread
+ifneq (,$(filter armv7m armv8m,$(CPU_ARCH)))
+  FEATURES_PROVIDED += no_idle_thread
+endif

cpu/cortexm_common/thread_arch.c

@@ -446,3 +446,24 @@ void __attribute__((used)) isr_svc(void)
     SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
 }
 #endif /* MODULE_CORTEXM_SVC */
+
+void sched_arch_idle(void)
+{
+    /* by default, PendSV has the same priority as other ISRs.
+     * In this function, we temporarily lower the priority (set higher value),
+     * allowing other ISRs to interrupt.
+     *
+     * According to [this](http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0321a/BIHJICIE.html),
+     * dynamically changing the priority is not supported on CortexM0(+).
+     */
+    NVIC_SetPriority(PendSV_IRQn, CPU_CORTEXM_PENDSV_IRQ_PRIO + 1);
+    __DSB();
+    __ISB();
+#ifdef MODULE_PM_LAYERED
+    void pm_set_lowest(void);
+    pm_set_lowest();
+#else
+    __WFI();
+#endif
+    NVIC_SetPriority(PendSV_IRQn, CPU_CORTEXM_PENDSV_IRQ_PRIO);
+}

kconfigs/Kconfig.features

@@ -67,6 +67,11 @@ config HAS_ETHERNET
     help
         Indicates that Ethernet connectivity is present.
 
+config HAS_NO_IDLE_THREAD
+    bool
+    help
+        Indicates that this MCU doesn't need the idle thread
+
 config HAS_MOTOR_DRIVER
     bool
     help

makefiles/features_modules.inc.mk

@@ -28,3 +28,8 @@ endif
 
 # select cortexm_svc pseudomodule if the corresponding feature is used
 USEMODULE += $(filter cortexm_svc, $(FEATURES_USED))
+
+# select core_idle_thread if the feature no_idle_thread is *not* used
+ifeq (, $(filter no_idle_thread, $(FEATURES_USED)))
+  USEMODULE += core_idle_thread
+endif

sys/schedstatistics/schedstatistics.c

@@ -31,15 +31,18 @@ void sched_statistics_cb(kernel_pid_t active_thread, kernel_pid_t next_thread)
 {
     uint32_t now = xtimer_now().ticks32;
 
-    /* Update active thread runtime, there is always an active thread since
-       first sched_run happens when main_trampoline gets scheduled */
-    schedstat_t *active_stat = &sched_pidlist[active_thread];
-    active_stat->runtime_ticks += now - active_stat->laststart;
+    /* Update active thread stats */
+    if (active_thread != KERNEL_PID_UNDEF) {
+        schedstat_t *active_stat = &sched_pidlist[active_thread];
+        active_stat->runtime_ticks += now - active_stat->laststart;
+    }
 
     /* Update next_thread stats */
-    schedstat_t *next_stat = &sched_pidlist[next_thread];
-    next_stat->laststart = now;
-    next_stat->schedules++;
+    if (next_thread != KERNEL_PID_UNDEF) {
+        schedstat_t *next_stat = &sched_pidlist[next_thread];
+        next_stat->laststart = now;
+        next_stat->schedules++;
+    }
 }
 
 void init_schedstatistics(void)

tests/shell/tests/01-run.py

@@ -24,8 +24,7 @@
 
 EXPECTED_PS = (
     '\tpid | state    Q | pri',
-    '\t  1 | pending  Q |  15',
-    '\t  2 | running  Q |   7'
+    '\t  \d | running  Q |   7'
 )
 
 # In native we are directly executing the binary (no terminal program). We must
@@ -102,6 +101,8 @@
     ('end_test', '[TEST_END]'),
 )
 
+CMDS_REGEX = {'ps'}
+
 BOARD = os.environ['BOARD']
 
 
@@ -112,10 +113,14 @@ def print_error(message):
 
 
 def check_cmd(child, cmd, expected):
+    regex = cmd in CMDS_REGEX
     child.expect(PROMPT)
     child.sendline(cmd)
     for line in expected:
-        child.expect_exact(line)
+        if regex:
+            child.expect(line)
+        else:
+            child.expect_exact(line)
 
 
 def check_startup(child):

tests/unittests/Makefile

@@ -28,6 +28,9 @@ BASELIBS += $(UNIT_TESTS:%=$(BINDIR)/%.a)
 
 INCLUDES += -I$(RIOTBASE)/tests/unittests/common
 
+# some tests need more stack
+CFLAGS += -DTHREAD_STACKSIZE_MAIN=THREAD_STACKSIZE_LARGE
+
 include $(RIOTBASE)/Makefile.include
 
 .PHONY: $(UNIT_TESTS)