jlaitine commented on code in PR #16673:
URL: https://github.com/apache/nuttx/pull/16673#discussion_r2193976614
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sched/sched/sched_addreadytorun.c:
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@@ -114,7 +114,87 @@ bool nxsched_add_readytorun(FAR struct tcb_s *btcb)
return ret;
}
-#endif /* !CONFIG_SMP */
+
+#else /* !CONFIG_SMP */
+
+/****************************************************************************
+ * Name: nxsched_switch_running
+ *
+ * Description:
+ * This function switches the head of the current CPU's assigned tasks
+ * list to the TCB given as parameter. The idle task is not switched out.
+ * If the running task can't be swapped out, the btcb is pushed to
+ * the ready-to-run list.
+ *
+ * Input Parameters:
+ * btcb - Points to the TCB that is ready-to-run
+ * cpu - Always this_cpu(). Given as argument only for optimization
+ *
+ * Returned Value:
+ * true if the currently active task is switched to the btcb
+ *
+ * Assumptions:
+ * - The caller has established a critical section
+ * - The caller has already removed the input rtcb from whatever list it
+ * was in.
+ * - The caller handles the condition that occurs if the head of the
+ * assigned tasks list has changed.
+ *
+ ****************************************************************************/
+
+bool nxsched_switch_running(FAR struct tcb_s *btcb, int cpu)
+{
+ FAR struct tcb_s *rtcb = current_task(cpu);
+ bool ret = true;
+
+ DEBUGASSERT(cpu == this_cpu());
+ DEBUGASSERT(btcb->task_state != TSTATE_TASK_RUNNING);
+
+ /* If the currently running task is locked, or is on higher priority than
+ * the requested one. Just add the btcb into ready-to-run list (not
+ * running) and return false
+ */
+
+ if (nxsched_islocked_tcb(rtcb) ||
+ btcb->sched_priority < rtcb->sched_priority)
+ {
+ nxsched_add_prioritized(btcb, list_readytorun());
+ btcb->task_state = TSTATE_TASK_READYTORUN;
+ ret = false;
+ }
+ else
+ {
+ FAR dq_queue_t *tasklist = list_assignedtasks(cpu);
+
+ /* Only the idle task and one other are kept in g_assignedtasks.
+ * return the other tasks to unassigned list, so that they can be
+ * picked up by other CPUs
+ */
+
+ if (!is_idle_task(rtcb))
+ {
+ rtcb->task_state = TSTATE_TASK_READYTORUN;
+ dq_rem((FAR struct dq_entry_s *)rtcb, tasklist);
+ nxsched_add_prioritized(rtcb, list_readytorun());
+
+ /* We should now have only the idle task assigned */
+
Review Comment:
This crossed my mind as well.
I'll rather try to find a way where a task is not assigned to a CPU running
a sched locked task; even though the sched lock is a transitional state, it
would be ok to check it for other cpus as well during scheduling (in critical
section), since when a task exists sched lock, it will try to reschedule anyhow.
Leaving a task in assigned list will cause a kind of priority inheritance -
if the sched lock lasts long (this happens for example in process startups in
memory protected builds), the task left in the assigned list would be "stuck"
(as long as the other task keeps the sched lock) and not eglible for scheduling
for other CPUs.
You are right that there is no need for having a list for assigned tasks
(i.e. running tasks). I just left it there. It could be removed as well, if
continuing on the path described above.
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