kernel/sched: Clamp minimum timeslice when TICKLESS

When the kernel is TICKLESS, timeouts are set as needed, and drivers
all have some minimum amount of time before which they can reliably
schedule an interrupt.  When this happens, drivers will kick the
requested interrupt out by one tick.  This means that it's not
reliably possible to get a timeout set for "one tick in the
future"[1].

And attempting to do that is dangerous anyway.  If the driver will
delay a one-tick interrupt, then code that repeatedly tries to
schedule an imminent interrupt may end up in a state where it is
constantly pushing the interrupt out into the future, and timer
interrupts stop arriving!  The timeout layer actually has protection
against this case.

Finally getting to the point: in recent changes, the timeslice layer
lost its integration with the "imminent" test in the timeout code, so
it's now able to run into this situation: very rapidly context
switching code (or rapidly arriving interrupts) will have the effect
of infinitely[2] delaying timeouts and stalling the whole timeout
subsystem.

Don't try to be fancy.  Just clamp timeslice duration such that a
slice is 2 ticks at minimum and we'll never hit the problem.  Adjust
the two tests that were explicitly requesting very short slice rates.

[1] Of course, the tradeoff is that the tick rate can be 100x higher
or more, so on balance tickless is a huge win.

[2] Actually it only lasts until a 31 bit signed rollover in the HPET
cycle count in practice.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>

kernel/sched.c

@@ -361,6 +361,12 @@ void k_sched_time_slice_set(int32_t slice, int prio)
 	LOCKED(&sched_spinlock) {
 		_current_cpu->slice_ticks = 0;
 		slice_time = k_ms_to_ticks_ceil32(slice);
+		if (IS_ENABLED(CONFIG_TICKLESS_KERNEL) && slice > 0) {
+			/* It's not possible to reliably set a 1-tick
+			 * timeout if ticks aren't regular.
+			 */
+			slice_time = MAX(2, slice_time);
+		}
 		slice_max_prio = prio;
 		z_reset_time_slice();
 	}

tests/kernel/mem_protect/syscalls/prj.conf

@@ -1,5 +1,5 @@
 CONFIG_ZTEST=y
 CONFIG_TEST_USERSPACE=y
 CONFIG_TIMESLICING=y
-CONFIG_TIMESLICE_SIZE=10
+CONFIG_TIMESLICE_SIZE=20
 CONFIG_APPLICATION_DEFINED_SYSCALL=y

tests/kernel/workq/critical/src/main.c

@@ -166,7 +166,7 @@ void regression_thread(void *arg1, void *arg2, void *arg3)
 		      "Unexpected value for <critical_var>");
 
 	TC_PRINT("Enable timeslicing at %u\n", k_uptime_get_32());
-	k_sched_time_slice_set(10, 10);
+	k_sched_time_slice_set(20, 10);
 
 	k_sem_give(&ALT_SEM);   /* Re-activate alternate_thread() */