Re: Strange hang on ia64 with CONFIG_PRINTK_TIME=y

["Tony Luck" <tony.luck@gmail.com>]

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> Perhaps what is happening is that cpu0 comes online ... safely skips
> over the early printk calls.  Calls cpu_init() which sets up the resources
> *it* needs (ar.k3 points to per-cpu space), and then executes
> sched_init() which marks it safe for all printk's. Then cpu1 comes
> up and does a printk before it gets to cpu_init().

I just tried Ingo's patch[1] on a 2.6.25-rc2 kernel with printk timestamps
turned on ... and it booted just fine on my tiger4.  The default path
for non-boot cpus is from head.S to start_secondary(), and that
calls cpu_init() pretty quickly.  There shouldn't normally[2] be any
printk() calls on the non-boot cpu before it is safe to do so.

-Tony

[1] Attached
[2] If you set #define SMP_DEBUG in arch/ia64/kernel/smpboot.c
that enables at least one printk() that will cause problems if you have
also configured timestamps.

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 kernel/sched.c |   14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

Index: linux-x86.q/kernel/sched.c
===================================================================
--- linux-x86.q.orig/kernel/sched.c
+++ linux-x86.q/kernel/sched.c
@@ -666,6 +666,8 @@ const_debug unsigned int sysctl_sched_rt
  */
 const_debug unsigned int sysctl_sched_rt_ratio = 62259;
 
+static __read_mostly int scheduler_running;
+
 /*
  * For kernel-internal use: high-speed (but slightly incorrect) per-cpu
  * clock constructed from sched_clock():
@@ -676,14 +678,16 @@ unsigned long long cpu_clock(int cpu)
 	unsigned long flags;
 	struct rq *rq;
 
-	local_irq_save(flags);
-	rq = cpu_rq(cpu);
 	/*
 	 * Only call sched_clock() if the scheduler has already been
 	 * initialized (some code might call cpu_clock() very early):
 	 */
-	if (rq->idle)
-		update_rq_clock(rq);
+	if (unlikely(!scheduler_running))
+		return 0;
+
+	local_irq_save(flags);
+	rq = cpu_rq(cpu);
+	update_rq_clock(rq);
 	now = rq->clock;
 	local_irq_restore(flags);
 
@@ -7255,6 +7259,8 @@ void __init sched_init(void)
 	 * During early bootup we pretend to be a normal task:
 	 */
 	current->sched_class = &fair_sched_class;
+
+	scheduler_running = 1;
 }
 
 #ifdef CONFIG_DEBUG_SPINLOCK_SLEEP
--