Don't preempt without good reason

kernel/src/lib.rs

@@ -93,8 +93,6 @@ pub fn main() {
         ThreadOptions::new(init_thread)
             .priority(Priority::new(PriorityRange::new(PriorityRange::MAX))),
     );
-    // Spawning functions in the bootstrap context will not return.
-    unreachable!()
 }
 
 pub fn init() {
@@ -110,7 +108,7 @@ pub fn init() {
     process::init();
 }
 
-fn ap_init() -> ! {
+fn ap_init() {
     fn ap_idle_thread() {
         let preempt_guard = ostd::task::disable_preempt();
         let cpu_id = preempt_guard.current_cpu();
@@ -129,8 +127,6 @@ fn ap_init() -> ! {
             .cpu_affinity(cpu_id.into())
             .priority(Priority::new(PriorityRange::new(PriorityRange::MAX))),
     );
-    // Spawning functions in the bootstrap context will not return.
-    unreachable!()
 }
 
 fn init_thread() {

kernel/src/sched/priority_scheduler.rs

@@ -104,6 +104,9 @@ impl<T: Sync + Send + PreemptSchedInfo + FromTask<U>, U: Sync + Send + CommonSch
         if still_in_rq && let Err(_) = entity.task.cpu().set_if_is_none(target_cpu) {
             return None;
         }
+
+        let new_priority = entity.thread.priority();
+
         if entity.thread.is_real_time() {
             rq.real_time_entities.push_back(entity);
         } else if entity.thread.is_lowest() {
@@ -112,7 +115,17 @@ impl<T: Sync + Send + PreemptSchedInfo + FromTask<U>, U: Sync + Send + CommonSch
             rq.normal_entities.push_back(entity);
         }
 
-        Some(target_cpu)
+        // Preempt the current task, but only if the newly queued task has a strictly higher
+        // priority (i.e., a lower value returned by the `priority` method) than the current task.
+        if rq
+            .current
+            .as_ref()
+            .is_some_and(|current| new_priority < current.thread.priority())
+        {
+            Some(target_cpu)
+        } else {
+            None
+        }
     }
 
     fn local_rq_with(&self, f: &mut dyn FnMut(&dyn LocalRunQueue<U>)) {

osdk/test-kernel/src/lib.rs

@@ -49,9 +49,7 @@ fn main() {
         };
     };
 
-    let _ = TaskOptions::new(test_task).data(()).spawn();
-
-    unreachable!("The spawn method will NOT return in the boot context")
+    TaskOptions::new(test_task).data(()).spawn().unwrap();
 }
 
 #[ostd::ktest::panic_handler]

ostd/libs/ostd-macros/src/lib.rs

@@ -30,8 +30,10 @@ pub fn main(_attr: TokenStream, item: TokenStream) -> TokenStream {
         #[cfg(not(ktest))]
         #[no_mangle]
         extern "Rust" fn __ostd_main() -> ! {
-            #main_fn_name();
-            ostd::prelude::abort();
+            let _: () = #main_fn_name();
+
+            ostd::task::Task::yield_now();
+            unreachable!("`yield_now` in the boot context should not return");
         }
 
         #[allow(unused)]
@@ -52,8 +54,10 @@ pub fn test_main(_attr: TokenStream, item: TokenStream) -> TokenStream {
     quote!(
         #[no_mangle]
         extern "Rust" fn __ostd_main() -> ! {
-            #main_fn_name();
-            ostd::prelude::abort();
+            let _: () = #main_fn_name();
+
+            ostd::task::Task::yield_now();
+            unreachable!("`yield_now` in the boot context should not return");
         }
 
         #main_fn

ostd/src/boot/smp.rs

@@ -15,6 +15,7 @@ use crate::{
         page::{self, meta::KernelMeta, ContPages},
         PAGE_SIZE,
     },
+    task::Task,
 };
 
 pub(crate) static AP_BOOT_INFO: Once<ApBootInfo> = Once::new();
@@ -37,7 +38,7 @@ struct PerApInfo {
     boot_stack_pages: ContPages<KernelMeta>,
 }
 
-static AP_LATE_ENTRY: Once<fn() -> !> = Once::new();
+static AP_LATE_ENTRY: Once<fn()> = Once::new();
 
 /// Boot all application processors.
 ///
@@ -104,7 +105,7 @@ pub fn boot_all_aps() {
 ///
 /// Once the entry function is registered, all the application processors
 /// will jump to the entry function immediately.
-pub fn register_ap_entry(entry: fn() -> !) {
+pub fn register_ap_entry(entry: fn()) {
     AP_LATE_ENTRY.call_once(|| entry);
 }
 
@@ -150,6 +151,9 @@ fn ap_early_entry(local_apic_id: u32) -> ! {
 
     let ap_late_entry = AP_LATE_ENTRY.wait();
     ap_late_entry();
+
+    Task::yield_now();
+    unreachable!("`yield_now` in the boot context should not return");
 }
 
 fn wait_for_all_aps_started() {

ostd/src/task/scheduler/fifo_scheduler.rs

@@ -58,7 +58,8 @@ impl<T: CommonSchedInfo + Send + Sync> Scheduler<T> for FifoScheduler<T> {
         }
         rq.queue.push_back(runnable);
 
-        Some(target_cpu)
+        // All tasks are important. Do not preempt the current task without good reason.
+        None
     }
 
     fn local_rq_with(&self, f: &mut dyn FnMut(&dyn LocalRunQueue<T>)) {