RedBear-OS/local/patches/kernel/P5-sched-rt-policy.patch

diff --git a/src/context/switch.rs b/src/context/switch.rs
index 86684c8..aeb29c9 100644
--- a/src/context/switch.rs
+++ b/src/context/switch.rs
@@ -5,7 +5,7 @@
 use crate::{
     context::{
         self, arch, idle_contexts, idle_contexts_try, run_contexts, ArcContextLockWriteGuard,
-        Context, ContextLock, WeakContextRef,
+        Context, ContextLock, SchedPolicy, WeakContextRef,
     },
     cpu_set::LogicalCpuId,
     cpu_stats::{self, CpuState},
@@ -33,35 +33,17 @@ const SCHED_PRIO_TO_WEIGHT: [usize; 40] = [
     70, 56, 45, 36, 29, 23, 18, 15,
 ];
 
-/// Determines if a given context is eligible to be scheduled on a given CPU (in
-/// principle, the current CPU).
-///
-/// # Safety
-/// This function is unsafe because it modifies the `context`'s state directly without synchronization.
-///
-/// # Parameters
-/// - `context`: The context (process/thread) to be checked.
-/// - `cpu_id`: The logical ID of the CPU on which the context is being scheduled.
-///
-/// # Returns
-/// - `UpdateResult::CanSwitch`: If the context can be switched to.
-/// - `UpdateResult::Skip`: If the context should be skipped (e.g., it's running on another CPU).
 unsafe fn update_runnable(
     context: &mut Context,
     cpu_id: LogicalCpuId,
     switch_time: u128,
 ) -> UpdateResult {
-    // Ignore contexts that are already running.
     if context.running {
         return UpdateResult::Skip;
     }
-
-    // Ignore contexts assigned to other CPUs.
     if !context.sched_affinity.contains(cpu_id) {
         return UpdateResult::Skip;
     }
-
-    // If context is soft-blocked and has a wake-up time, check if it should wake up.
     if context.status.is_soft_blocked()
         && let Some(wake) = context.wake
         && switch_time >= wake
@@ -69,8 +51,6 @@ unsafe fn update_runnable(
         context.wake = None;
         context.unblock_no_ipi();
     }
-
-    // If the context is runnable, indicate it can be switched to.
     if context.status.is_runnable() {
         UpdateResult::CanSwitch
     } else {
@@ -95,7 +75,7 @@ pub fn tick(token: &mut CleanLockToken) {
     let new_ticks = ticks_cell.get() + 1;
     ticks_cell.set(new_ticks);
 
-    // Trigger a context switch after every 3 ticks (approx. 6.75 ms).
+    // Trigger a context switch after every 3 ticks.
     if new_ticks >= 3 {
         switch(token);
         crate::context::signal::signal_handler(token);
@@ -167,10 +147,7 @@ pub fn switch(token: &mut CleanLockToken) -> SwitchResult {
     let mut prev_context_guard = unsafe { prev_context_lock.write_arc() };
 
     if !prev_context_guard.is_preemptable() {
-        // Unset global lock
         arch::CONTEXT_SWITCH_LOCK.store(false, Ordering::SeqCst);
-
-        // Pretend to have finished switching, so CPU is not idled
         return SwitchResult::Switched;
     }
 
@@ -377,6 +354,71 @@ fn select_next_context(
     let total_contexts: usize = contexts_list.iter().map(|q| q.len()).sum();
     let mut skipped_contexts = 0;
 
+    // PASS 0: SCHED_FIFO and SCHED_RR — scan for RT contexts to schedule.
+    // When a runnable RT context is found, it takes priority over all SCHED_OTHER.
+    for prio in 0..40 {
+        let rt_contexts = contexts_list
+            .get_mut(prio)
+            .expect("prio should be between [0, 39]");
+        let len = rt_contexts.len();
+        for _ in 0..len {
+            let (rt_ref, rt_lock) = match rt_contexts.pop_front() {
+                Some(lock) => match lock.upgrade() {
+                    Some(l) => (lock, l),
+                    None => {
+                        skipped_contexts += 1;
+                        continue;
+                    }
+                },
+                None => break,
+            };
+            if Arc::ptr_eq(&rt_lock, &idle_context) {
+                rt_contexts.push_back(rt_ref);
+                continue;
+            }
+            // Current RT thread: if runnable with no higher-prio RT found yet,
+            // keep it running (no demotion to SCHED_OTHER)
+            if Arc::ptr_eq(&rt_lock, &prev_context_lock) {
+                let mut rt_guard = unsafe { rt_lock.write_arc() };
+                if rt_guard.status.is_runnable()
+                    && (rt_guard.sched_policy == SchedPolicy::Fifo
+                        || rt_guard.sched_policy == SchedPolicy::RoundRobin)
+                {
+                    percpu.balance.set(balance);
+                    percpu.last_queue.set(i);
+                    return Ok(Some(rt_guard));
+                }
+                rt_contexts.push_back(rt_ref);
+                continue;
+            }
+            let mut rt_guard = unsafe { rt_lock.write_arc() };
+            if !rt_guard.status.is_runnable() || rt_guard.running
+                || !rt_guard.sched_affinity.contains(cpu_id)
+            {
+                rt_contexts.push_back(rt_ref);
+                continue;
+            }
+            if rt_guard.sched_policy == SchedPolicy::Fifo
+                || rt_guard.sched_policy == SchedPolicy::RoundRobin
+            {
+                percpu.balance.set(balance);
+                percpu.last_queue.set(i);
+                if !Arc::ptr_eq(&prev_context_lock, &idle_context) {
+                    let prev_ctx = WeakContextRef(Arc::downgrade(&prev_context_lock));
+                    if prev_context_guard.status.is_runnable() {
+                        contexts_list[prev_context_guard.prio].push_back(prev_ctx);
+                    } else {
+                        idle_contexts(token.token()).push_back(prev_ctx);
+                    }
+                }
+                return Ok(Some(rt_guard));
+            }
+            rt_contexts.push_back(rt_ref);
+        }
+    }
+
+    // PASS 1: SCHED_OTHER — existing DWRR deficit tracking
+
     'priority: loop {
         i = (i + 1) % 40;
         total_iters += 1;