sched: timeout: Do not miss slice timeouts

Time slices don't have a timeout struct associated and stored in
timeout_list. Time slice timeout is direct programmed in the system
clock and tracked in _current_cpu->slice_ticks.

There is one issue where the time slice timeout can be missed because
the system clock is re-programmed to a longer timeout. To this happens,
it is only necessary that the timeout_list is empty (any timeout set)
and a new timeout longer than remaining time slice is set. This is cause
because z_add_timeout does not check for the slice ticks.

The following example spots the issue:

K_THREAD_STACK_DEFINE(tstack, STACK_SIZE);
K_THREAD_STACK_ARRAY_DEFINE(tstacks, NUM_THREAD, STACK_SIZE);
K_SEM_DEFINE(sema, 0, NUM_THREAD);

static inline void spin_for_ms(int ms)
{
	uint32_t t32 = k_uptime_get_32();

	while (k_uptime_get_32() - t32 < ms) {
	}
}

static void thread_time_slice(void *p1, void *p2, void *p3)
{
	printk("thread[%d] - Before spin\n", (int)(uintptr_t)p1);

	/* Spinning for longer than slice */
	spin_for_ms(SLICE_SIZE + 20);

	/* The following print should not happen before another
	 * same priority thread starts.
	 */
	printk("thread[%d] - After spinning\n", (int)(uintptr_t)p1);
	k_sem_give(&sema);
}

void main(void)
{
	k_tid_t tid[NUM_THREAD];
	struct k_thread t[NUM_THREAD];
	uint32_t slice_ticks = k_ms_to_ticks_ceil32(SLICE_SIZE);
	int old_prio = k_thread_priority_get(k_current_get());

	/* disable timeslice */
	k_sched_time_slice_set(0, K_PRIO_PREEMPT(0));

	for (int j = 0; j < 2; j++) {
		k_sem_reset(&sema);

		/* update priority for current thread */
		k_thread_priority_set(k_current_get(), K_PRIO_PREEMPT(j));

		/* synchronize to tick boundary */
		k_usleep(1);

		/* create delayed threads with equal preemptive priority */
		for (int i = 0; i < NUM_THREAD; i++) {
			tid[i] = k_thread_create(&t[i], tstacks[i], STACK_SIZE,
						 thread_time_slice, (void *)i, NULL,
						 NULL, K_PRIO_PREEMPT(j), 0,
						 K_NO_WAIT);
		}

		/* enable time slice (and reset the counter!) */
		k_sched_time_slice_set(SLICE_SIZE, K_PRIO_PREEMPT(0));

		/* Spins for while to spend this thread time but not longer */
		/* than a slice. This is important  */
		spin_for_ms(100);

		printk("before sleep\n");
		/* relinquish CPU and wait for each thread to complete */
		k_sleep(K_TICKS(slice_ticks * (NUM_THREAD + 1)));

		for (int i = 0; i < NUM_THREAD; i++) {
			k_sem_take(&sema, K_FOREVER);
		}

		/* test case teardown */
		for (int i = 0; i < NUM_THREAD; i++) {
			k_thread_abort(tid[i]);
		}
		/* disable time slice */
		k_sched_time_slice_set(0, K_PRIO_PREEMPT(0));
	}
	k_thread_priority_set(k_current_get(), old_prio);
}

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

kernel/timeout.c

@@ -123,7 +123,23 @@ void z_add_timeout(struct _timeout *to, _timeout_func_t fn,
 		}
 
 		if (to == first()) {
+#if CONFIG_TIMESLICING
+			/*
+			 * This is not ideal, since it does not
+			 * account the time elapsed since the the
+			 * last announcement, and slice_ticks is based
+			 * on that. It means the that time remaining for
+			 * the next announcement can be lesser than
+			 * slice_ticks.
+			 */
+			int32_t next_time = next_timeout();
+
+			if (_current_cpu->slice_ticks != next_time) {
+				z_clock_set_timeout(next_time, false);
+			}
+#else
 			z_clock_set_timeout(next_timeout(), false);
+#endif	/* CONFIG_TIMESLICING */
 		}
 	}
 }