[sched] [patch] idle-thread-fixes-2.5.3-pre2-A1

[Ingo Molnar <mingo@elte.hu>]

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the attached, bigger patch cleans up a particular aspect of scheduler
initialization that is the source of two unclean pieces of code within the
timer interrupt:

in idle_tick():

        if ((jiffies % IDLE_REBALANCE_TICK) ||
                        likely(this_rq()->curr == NULL))
                return;

in scheduler_tick():

        if (p == rq->idle || !rq->idle)
                return idle_tick();

both the 'this_rq()->curr == NULL' and '!rq->idle' tests are only there to
fix a temporary bootup-only situation where the idle threads are not
initialized yet.

the source of the problem is that idle thread initialization on SMP is not
'atomic'. Eg., on x86 the idle threads first do timer calibration, for
which period of time it's possible for timer interrupts to arrive on that
(partly initialized) CPU.

the attached patch solves this problem by splitting up init_idle() into
two parts: init_idle() and idle_startup_done(). After init_idle() is
called, it's possible to receive timer interrupts on a CPU. But schedule()
can only be called after all idle threads have done idle_startup_done().

	Ingo

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--- linux/init/main.c.orig	Sun Jan 20 11:36:05 2002
+++ linux/init/main.c	Sun Jan 20 11:38:59 2002
@@ -290,8 +290,6 @@
 extern void setup_arch(char **);
 extern void cpu_idle(void);
 
-unsigned long wait_init_idle;
-
 #ifndef CONFIG_SMP
 
 #ifdef CONFIG_X86_LOCAL_APIC
@@ -305,6 +303,16 @@
 
 #else
 
+static unsigned long __initdata wait_init_idle;
+
+void __init idle_startup_done(void)
+{
+	clear_bit(smp_processor_id(), &wait_init_idle);
+	while (wait_init_idle) {
+		cpu_relax();
+		barrier();
+	}
+}
 
 /* Called by boot processor to activate the rest. */
 static void __init smp_init(void)
@@ -315,6 +323,7 @@
 
 	smp_threads_ready=1;
 	smp_commence();
+	idle_startup_done();
 }
 
 #endif
@@ -411,20 +420,13 @@
 	check_bugs();
 	printk("POSIX conformance testing by UNIFIX\n");
 
+	init_idle();
 	/* 
 	 *	We count on the initial thread going ok 
 	 *	Like idlers init is an unlocked kernel thread, which will
 	 *	make syscalls (and thus be locked).
 	 */
 	smp_init();
-
-	/*
-	 * Finally, we wait for all other CPU's, and initialize this
-	 * thread that will become the idle thread for the boot CPU.
-	 * After this, the scheduler is fully initialized, and we can
-	 * start creating and running new threads.
-	 */
-	init_idle();
 
 	/* Do the rest non-__init'ed, we're now alive */
 	rest_init();
--- linux/kernel/sched.c.orig	Sun Jan 20 11:38:38 2002
+++ linux/kernel/sched.c	Sun Jan 20 11:38:59 2002
@@ -489,8 +489,7 @@
 
 static inline void idle_tick(void)
 {
-	if ((jiffies % IDLE_REBALANCE_TICK) ||
-			likely(this_rq()->curr == NULL))
+	if (jiffies % IDLE_REBALANCE_TICK)
 		return;
 	spin_lock(&this_rq()->lock);
 	load_balance(this_rq(), 1);
@@ -506,7 +505,7 @@
 	unsigned long now = jiffies;
 	runqueue_t *rq = this_rq();
 
-	if (p == rq->idle || !rq->idle)
+	if (p == rq->idle)
 		return idle_tick();
 	/* Task might have expired already, but not scheduled off yet */
 	if (p->array != rq->active) {
@@ -1221,8 +1220,6 @@
 	read_unlock(&tasklist_lock);
 }
 
-extern unsigned long wait_init_idle;
-
 static inline void double_rq_lock(runqueue_t *rq1, runqueue_t *rq2)
 {
 	if (rq1 == rq2)
@@ -1257,16 +1254,11 @@
 	this_rq->curr = this_rq->idle = current;
 	deactivate_task(current, rq);
 	current->array = NULL;
-	current->prio = MAX_PRIO-1;
+	current->prio = MAX_PRIO;
 	current->state = TASK_RUNNING;
-	clear_bit(smp_processor_id(), &wait_init_idle);
 	double_rq_unlock(this_rq, rq);
-	while (wait_init_idle) {
-		cpu_relax();
-		barrier();
-	}
 	current->need_resched = 1;
-	__sti();
+	__restore_flags(flags);
 }
 
 extern void init_timervecs(void);
@@ -1305,7 +1297,7 @@
 	 */
 	rq = this_rq();
 	rq->curr = current;
-	rq->idle = NULL;
+	rq->idle = current;
 	wake_up_process(current);
 
 	init_timervecs();
--- linux/include/linux/sched.h.orig	Sun Jan 20 11:36:05 2002
+++ linux/include/linux/sched.h	Sun Jan 20 11:38:59 2002
@@ -141,6 +141,7 @@
 
 extern void sched_init(void);
 extern void init_idle(void);
+extern void idle_startup_done(void);
 extern void show_state(void);
 extern void cpu_init (void);
 extern void trap_init(void);
--- linux/arch/i386/kernel/smpboot.c.orig	Sun Jan 20 11:36:02 2002
+++ linux/arch/i386/kernel/smpboot.c	Sun Jan 20 11:38:59 2002
@@ -462,6 +462,7 @@
 	 * things done here to the most necessary things.
 	 */
 	cpu_init();
+	init_idle();
 	smp_callin();
 	while (!atomic_read(&smp_commenced))
 		rep_nop();
@@ -470,8 +471,8 @@
 	 * the local TLBs too.
 	 */
 	local_flush_tlb();
+	idle_startup_done();
 
-	init_idle();
 	return cpu_idle();
 }