feat: Round robin for FIFO preemption

runtime/include/local_runqueue_list.h

@@ -1,3 +1,4 @@
 #pragma once
 
 void local_runqueue_list_initialize();
+void local_runqueue_list_rotate();

runtime/include/ps_list.h

@@ -82,6 +82,18 @@ ps_list_ll_empty(struct ps_list *l)
 	return l->n == l;
 }
 
+static inline int
+ps_list_ll_one_node(struct ps_list *l)
+{
+	return l->n != l && l->n == l->p;
+}
+
+static inline int
+ps_list_head_one_node(struct ps_list_head *lh)
+{
+	return ps_list_ll_one_node(&lh->l);
+}
+
 static inline int
 ps_list_head_empty(struct ps_list_head *lh)
 {

runtime/include/scheduler.h

@@ -68,18 +68,23 @@ static inline struct sandbox *
 scheduler_fifo_get_next()
 {
 	struct sandbox *sandbox = local_runqueue_get_next();
+
 	struct sandbox_request *sandbox_request = NULL;
 
-	/* If the local runqueue is empty, pull from global request scheduler */
 	if (sandbox == NULL) {
-		struct sandbox_request *sandbox_request;
+		/* If the local runqueue is empty, pull from global request scheduler */
 		if (global_request_scheduler_remove(&sandbox_request) < 0) goto err;
 
 		sandbox = sandbox_allocate(sandbox_request);
 		if (!sandbox) goto err_allocate;
 
 		sandbox_set_as_runnable(sandbox, SANDBOX_INITIALIZED);
-	};
+	} else if (sandbox == current_sandbox_get()) {
+		/* Execute Round Robin Scheduling Logic if the head is the current sandbox */
+		local_runqueue_list_rotate();
+		sandbox = local_runqueue_get_next();
+	}
+
 
 done:
 	return sandbox;
@@ -143,11 +148,6 @@ scheduler_runqueue_initialize()
 static inline void
 scheduler_preempt(ucontext_t *user_context)
 {
-	/* 	If FIFO, just return
-	 * TODO: Should this RR? */
-	if (scheduler == SCHEDULER_FIFO) return;
-
-	assert(scheduler == SCHEDULER_EDF);
 	assert(user_context != NULL);
 
 	/* Process epoll to make sure that all runnable jobs are considered for execution */
@@ -160,7 +160,7 @@ scheduler_preempt(ucontext_t *user_context)
 	struct sandbox *next = scheduler_get_next();
 	assert(next != NULL);
 
-	/* If current equals return, we are already running earliest deadline, so resume execution */
+	/* If current equals next, no switch is necessary, so resume execution */
 	if (current == next) return;
 
 #ifdef LOG_PREEMPTION
@@ -175,10 +175,37 @@ scheduler_preempt(ucontext_t *user_context)
 	assert(next->state == SANDBOX_RUNNABLE);
 	sandbox_set_as_running(next, SANDBOX_RUNNABLE);
 
-	/* A sandbox cannot be preempted by a slow context because this was in the
-	 * runqueue during the last scheduling decision. */
-	assert(next->ctxt.variant == ARCH_CONTEXT_VARIANT_FAST);
+	switch (next->ctxt.variant) {
+	case ARCH_CONTEXT_VARIANT_FAST: {
 		arch_context_restore_new(&user_context->uc_mcontext, &next->ctxt);
+		break;
+	}
+	case ARCH_CONTEXT_VARIANT_SLOW: {
+		/* Our scheduler restores a fast context when switching to a sandbox that cooperatively yielded
+		 * (probably by blocking) or when switching to a freshly allocated sandbox that hasn't yet run.
+		 * These conditions can occur in either EDF or FIFO.
+		 *
+		 * A scheduler restores a slow context when switching to a sandbox that was preempted previously.
+		 * Under EDF, a sandbox is only ever preempted by an earlier deadline that either had blocked and since
+		 * become runnable or was just freshly allocated. This means that such EDF preemption context switches
+		 * should always use a fast context.
+		 *
+		 * This is not true under FIFO, where there is no innate ordering between sandboxes. A runqueue is
+		 * normally only a single sandbox, but it may have multiple sandboxes when one blocks and the worker
+		 * pulls an addition request. When the blocked sandbox becomes runnable, the executing sandbox can be
+		 * preempted yielding a slow context. This means that FIFO preemption context switches might cause
+		 * either a fast or a slow context to be restored during "round robin" execution.
+		 */
+		assert(scheduler != SCHEDULER_EDF);
+
+		arch_mcontext_restore(&user_context->uc_mcontext, &next->ctxt);
+		break;
+	}
+	default: {
+		panic("Unexpectedly tried to switch to a context in %s state\n",
+		      arch_context_variant_print(next->ctxt.variant));
+	}
+	}
 }
 
 static inline char *

runtime/src/local_runqueue_list.c

@@ -1,4 +1,5 @@
 #include "client_socket.h"
+#include "current_sandbox.h"
 #include "global_request_scheduler.h"
 #include "local_runqueue_list.h"
 #include "local_runqueue.h"
@@ -20,7 +21,7 @@ local_runqueue_list_get_head()
 }
 
 /**
- * Removes the thread from the thread-local runqueue
+ * Removes the sandbox from the thread-local runqueue
  * @param sandbox sandbox
  */
 void
@@ -38,32 +39,39 @@ local_runqueue_list_remove_and_return()
 }
 
 /**
- * Get the next sandbox and then insert at tail to "round robin"
- * @return the sandbox to execute or NULL if none are available
+ * Append a sandbox to the tail of the runqueue
+ * @returns the appended sandbox
  */
-struct sandbox *
-local_runqueue_list_get_next()
+void
+local_runqueue_list_append(struct sandbox *sandbox_to_append)
 {
-	if (local_runqueue_list_is_empty()) return NULL;
+	assert(sandbox_to_append != NULL);
+	assert(ps_list_singleton_d(sandbox_to_append));
+	ps_list_head_append_d(&local_runqueue_list, sandbox_to_append);
+}
 
-	/* Execute Round Robin Scheduling Logic */
-	struct sandbox *next_sandbox = local_runqueue_list_remove_and_return();
-	assert(next_sandbox->state == SANDBOX_RUNNABLE);
+/* Remove sandbox from head of runqueue and add it to tail */
+void
+local_runqueue_list_rotate()
+{
+	/* If runqueue is size one, skip round robin logic since tail equals head */
+	if (ps_list_head_one_node(&local_runqueue_list)) return;
 
-	return next_sandbox;
+	struct sandbox *sandbox_at_head = local_runqueue_list_remove_and_return();
+	assert(sandbox_at_head->state == SANDBOX_RUNNING || sandbox_at_head->state == SANDBOX_RUNNABLE);
+	local_runqueue_list_append(sandbox_at_head);
 }
 
-
 /**
- * Append a sandbox to the runqueue
- * @returns the appended sandbox
+ * Get the next sandbox
+ * @return the sandbox to execute or NULL if none are available
  */
-void
-local_runqueue_list_append(struct sandbox *sandbox_to_append)
+struct sandbox *
+local_runqueue_list_get_next()
 {
-	assert(sandbox_to_append != NULL);
-	assert(ps_list_singleton_d(sandbox_to_append));
-	ps_list_head_append_d(&local_runqueue_list, sandbox_to_append);
+	if (local_runqueue_list_is_empty()) return NULL;
+
+	return local_runqueue_list_get_head();
 }
 
 void