lx_emul: adjust start & stop tick behaviour

Do not start and stop idle ticking within the timer interrupt
routine, but do it around the whole Lx_kit scheduling, which
is always called when Linux code gets active again, either
because of backend signals, interrupts, or timing signals.

This commit implicitly reverts the (incomplete) solution of
issue #4550

Ref genodelabs/genode#4778

repos/dde_linux/src/include/lx_emul/task.h

@@ -44,10 +44,10 @@ void lx_emul_task_name(struct task_struct * task, const char * name);
 
 void *lx_emul_task_stack(struct task_struct const * task);
 
-char lx_emul_task_another_runnable(void);
-
 void lx_emul_task_mark_for_removal(struct task_struct const * task);
 
+void lx_emul_task_set_idle(void);
+
 #ifdef __cplusplus
 }
 #endif

repos/dde_linux/src/include/lx_kit/scheduler.h

@@ -31,13 +31,18 @@ class Lx_kit::Scheduler
 
 		List<Task> _present_list { };
 		Task     * _current      { nullptr };
+		Task     * _idle         { nullptr };
 
 		Genode::Entrypoint &ep;
 
+		void _idle_pre_post_process();
+
 	public:
 
 		Task & current();
 
+		void idle(Task & idle) { _idle = &idle; }
+
 		bool active() const;
 
 		void add(Task & task);
@@ -47,8 +52,6 @@ class Lx_kit::Scheduler
 		void unblock_irq_handler();
 		void unblock_time_handler();
 
-		bool another_runnable(Task *);
-
 		Task & task(void * t);
 
 		template <typename FN>

repos/dde_linux/src/lib/lx_emul/spec/arm/irqchip.c

@@ -17,7 +17,6 @@
 #include <linux/of_irq.h>
 #include <linux/slab.h>
 #include <linux/irqchip.h>
-#include <linux/tick.h>
 #include <../kernel/irq/internals.h>
 
 static int dde_irq_set_wake(struct irq_data *d, unsigned int on)
@@ -173,9 +172,6 @@ int lx_emul_irq_task_function(void * data)
 		if (!dde_irq_domain)
 			continue;
 
-		/* check restarting ticking which may stopped in idle task */
-		tick_nohz_idle_restart_tick();
-
 		irq_enter();
 
 		irq = irq_find_mapping(dde_irq_domain, lx_emul_irq_last());

repos/dde_linux/src/lib/lx_emul/spec/x86/irqchip.c

@@ -15,7 +15,6 @@
 #include <lx_emul/irq.h>
 #include <linux/irq.h>
 #include <linux/irqchip.h>
-#include <linux/tick.h>
 #include <../kernel/irq/internals.h>
 
 
@@ -53,9 +52,6 @@ int lx_emul_irq_task_function(void * data)
 	for (;;) {
 		lx_emul_task_schedule(true);
 
-		/* check restarting ticking which may stopped in idle task */
-		tick_nohz_idle_restart_tick();
-
 		irq_enter();
 
 		irq = lx_emul_irq_last();

repos/dde_linux/src/lib/lx_emul/start.c

@@ -69,22 +69,40 @@ static int kernel_init(void * args)
 }
 
 
-static void timer_loop(void)
+static int kernel_idle(void * args)
 {
-	for (;;) {
+	struct task_struct *tsk = current;
-		tick_nohz_idle_enter();
+	set_task_comm(tsk, "idle");
 
-		if (!lx_emul_task_another_runnable())
+	/* set this current task to be the idle task */
+	lx_emul_task_set_idle();
+
+	/*
+	 * Idle task always gets run in the end of each schedule
+	 * and again at the beginning of each schedule
+	 */
+	for (;;) {
+		lx_emul_task_schedule(true);
+
+		tick_nohz_idle_enter();
 		tick_nohz_idle_stop_tick();
 
 		lx_emul_task_schedule(true);
-		lx_emul_time_handle();
 
-		/* check restarting ticking */
 		tick_nohz_idle_restart_tick();
-
 		tick_nohz_idle_exit();
 	}
+
+	return 0;
+}
+
+
+static void timer_loop(void)
+{
+	for (;;) {
+		lx_emul_task_schedule(true);
+		lx_emul_time_handle();
+	}
 }
 
 
@@ -125,6 +143,7 @@ int lx_emul_init_task_function(void * dtb)
 	sched_clock_init();
 
 	kernel_thread(kernel_init, NULL, CLONE_FS);
+	kernel_thread(kernel_idle, NULL, CLONE_FS);
 	pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
 	kthreadd_task = find_task_by_pid_ns(pid, NULL);;
 

repos/dde_linux/src/lib/lx_emul/task.cc

@@ -102,15 +102,13 @@ extern "C" void * lx_emul_task_stack(struct task_struct const * t)
 }
 
 
-extern "C" char lx_emul_task_another_runnable()
-{
-	Lx_kit::Task & task = Lx_kit::env().scheduler.current();
-
-	return Lx_kit::env().scheduler.another_runnable(&task);
-}
-
-
 extern "C" void lx_emul_task_mark_for_removal(struct task_struct const *t)
 {
 	Lx_kit::env().scheduler.task((void*)t).mark_for_destruction();
 }
+
+
+extern "C" void lx_emul_task_set_idle(void)
+{
+	return Lx_kit::env().scheduler.idle(Lx_kit::env().scheduler.current());
+}

repos/dde_linux/src/lib/lx_kit/scheduler.cc

@@ -27,6 +27,16 @@
 using namespace Genode;
 using namespace Lx_kit;
 
+void Scheduler::_idle_pre_post_process()
+{
+	if (!_idle)
+		return;
+
+	_current = _idle;
+	_idle->run();
+}
+
+
 Task & Scheduler::current()
 {
 	if (!_current) {
@@ -104,6 +114,8 @@ void Scheduler::schedule()
 		Genode::sleep_forever();
 	}
 
+	_idle_pre_post_process();
+
 	/*
 	 * Iterate over all tasks and run first runnable.
 	 *
@@ -116,9 +128,6 @@ void Scheduler::schedule()
 	while (true) {
 		bool at_least_one = false;
 
-		/* update jiffies before running task */
-		//Lx::timer_update_jiffies();
-
 		for (Task * t = _present_list.first(); t; ) {
 
 			Task *tmp = t;
@@ -148,23 +157,8 @@ void Scheduler::schedule()
 			break;
 	}
 
+	_idle_pre_post_process();
+
 	/* clear current as no task is running */
 	_current = nullptr;
 }
-
-
-bool Scheduler::another_runnable(Task * skip)
-{
-	for (Task * t = _present_list.first(); t; t = t->next()) {
-
-		if (!t->runnable())
-			continue;
-
-		if (skip && t == skip)
-			continue;
-
-		return true;
-	};
-
-	return false;
-}