kernel: Add k_reschedule()

The routine k_reschedule() allows an application to manually force
a schedule point. Although similar to k_yield(), it has different
properties. The most significant difference is that k_yield() if
invoked from a cooperative thread will voluntarily give up execution
control to the next thread of equal or higher priority while
k_reschedule() will not.

Applications that play with EDF deadlines via k_thread_deadline_set()
may need to use k_reschedule() to force a reschedule.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>

Author: peter-mitsis

include/zephyr/kernel.h

@@ -946,6 +946,26 @@ __syscall void k_thread_priority_set(k_tid_t thread, int prio);
 __syscall void k_thread_deadline_set(k_tid_t thread, int deadline);
 #endif
 
+/**
+ * @brief Invoke the scheduler
+ *
+ * This routine invokes the scheduler to force a schedule point on the current
+ * CPU. If invoked from within a thread, the scheduler will be invoked
+ * immediately (provided interrupts were not locked when invoked). If invoked
+ * from within an ISR, the scheduler will be invoked upon exiting the ISR.
+ *
+ * Invoking the scheduler allows the kernel to make an immediate determination
+ * as to what the next thread to execute should be. Unlike yielding, this
+ * routine is not guaranteed to switch to a thread of equal or higher priority
+ * if any are available. For example, if the current thread is cooperative and
+ * there is a still higher priority cooperative thread that is ready, then
+ * yielding will switch to that higher priority thread whereas this routine
+ * will not.
+ *
+ * Most applications will never use this routine.
+ */
+__syscall void k_reschedule(void);
+
 #ifdef CONFIG_SCHED_CPU_MASK
 /**
  * @brief Sets all CPU enable masks to zero

kernel/sched.c

@@ -1050,6 +1050,24 @@ static inline void z_vrfy_k_thread_deadline_set(k_tid_t tid, int deadline)
 #endif /* CONFIG_USERSPACE */
 #endif /* CONFIG_SCHED_DEADLINE */
 
+void z_impl_k_reschedule(void)
+{
+	k_spinlock_key_t key;
+
+	key = k_spin_lock(&_sched_spinlock);
+
+	update_cache(0);
+
+	z_reschedule(&_sched_spinlock, key);
+}
+
+#ifdef CONFIG_USERSPACE
+static inline void z_vrfy_k_reschedule(void)
+{
+	z_impl_k_reschedule();
+}
+#endif /* CONFIG_USERSPACE */
+
 bool k_can_yield(void)
 {
 	return !(k_is_pre_kernel() || k_is_in_isr() ||