sched_ext: Documentation: Clarify time slice handling in task lifecycle

It is not always obvious how a task's time slice can be refilled, either
explicitly from ops.dispatch() or automatically by the sched_ext core,
to skip subsequent ops.enqueue() and ops.dispatch() calls. This
typically happens when the task is the only one running on a CPU.

To make this behavior easier to understand, update the task lifecycle
diagram to explicitly document how time slice handling works in such
cases.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: arighi

Documentation/scheduler/sched-ext.rst

@@ -313,16 +313,21 @@ by a sched_ext scheduler:
         ops.runnable();         /* Task becomes ready to run */
 
         while (task is runnable) {
-            if (task is not in a DSQ) {
+            if (task is not in a DSQ && task->scx.slice == 0) {
                 ops.enqueue();  /* Task can be added to a DSQ */
 
-                /* A CPU becomes available */
+                /* Any usable CPU becomes available */
 
                 ops.dispatch(); /* Task is moved to a local DSQ */
             }
             ops.running();      /* Task starts running on its assigned CPU */
-            ops.tick();         /* Called every 1/HZ seconds */
+            while (task->scx.slice > 0 && task is runnable)
+                ops.tick();     /* Called every 1/HZ seconds */
             ops.stopping();     /* Task stops running (time slice expires or wait) */
+
+            /* Task's CPU becomes available */
+
+            ops.dispatch();     /* task->scx.slice can be refilled */
         }
 
         ops.quiescent();        /* Task releases its assigned CPU (wait) */