Halt the idle CPUs

ostd/src/arch/riscv/cpu/mod.rs

@@ -4,3 +4,17 @@
 
 pub mod context;
 pub mod local;
+
+/// Halts the CPU.
+///
+/// This function halts the CPU until the next interrupt is received. By
+/// halting, the CPU might consume less power. Internally it is implemented
+/// using the `wfi` instruction.
+///
+/// Since the function sleeps the CPU, it should not be used within an atomic
+/// mode ([`crate::task::atomic_mode`]).
+#[track_caller]
+pub fn sleep_for_interrupt() {
+    crate::task::atomic_mode::might_sleep();
+    riscv::asm::wfi();
+}

ostd/src/arch/x86/cpu/mod.rs

@@ -4,3 +4,16 @@
 
 pub mod context;
 pub mod local;
+
+/// Halts the CPU.
+///
+/// This function halts the CPU until the next interrupt is received. By
+/// halting, the CPU will enter the C-0 state and consume less power.
+///
+/// Since the function sleeps the CPU, it should not be used within an atomic
+/// mode ([`crate::task::atomic_mode`]).
+#[track_caller]
+pub fn sleep_for_interrupt() {
+    crate::task::atomic_mode::might_sleep();
+    x86_64::instructions::hlt();
+}

ostd/src/task/mod.rs

@@ -2,7 +2,7 @@
 
 //! Tasks are the unit of code execution.
 
-pub(crate) mod atomic_mode;
+pub mod atomic_mode;
 mod kernel_stack;
 mod preempt;
 mod processor;

ostd/src/task/scheduler/mod.rs

@@ -12,7 +12,7 @@ use spin::Once;
 
 use super::{preempt::cpu_local, processor, Task};
 use crate::{
-    cpu::{CpuId, PinCurrentCpu},
+    cpu::{CpuId, CpuSet, PinCurrentCpu},
     prelude::*,
     task::disable_preempt,
     timer,
@@ -195,7 +195,9 @@ fn set_need_preempt(cpu_id: CpuId) {
     if preempt_guard.current_cpu() == cpu_id {
         cpu_local::set_need_preempt();
     } else {
-        // TODO: Send IPIs to set remote CPU's `need_preempt`
+        crate::smp::inter_processor_call(&CpuSet::from(cpu_id), || {
+            cpu_local::set_need_preempt();
+        });
     }
 }