linux-kernel - Re: Strange hang on ia64 with CONFIG_PRINTK

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Date:	Wed, 13 Feb 2008 14:02:36 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	rdreier@...co.com, linux-kernel@...r.kernel.org
Subject: Re: Strange hang on ia64 with CONFIG_PRINTK_TIME=y


* Ingo Molnar <mingo@...e.hu> wrote:

> so .. how about the patch below? Note that we already had an "early 
> bootup" special (the rq->idle check), it's now just made explicit via 
> the scheduler_running flag.

the one below even builds. (untested otherwise)

	Ingo

----------->
Subject: sched: make sched_clock() early-bootup capable
From: Ingo Molnar <mingo@...e.hu>
Date: Wed Feb 13 13:49:36 CET 2008

do not call sched_clock() too early. Not only might rq->idle
not be set up - but pure per-cpu data might not be accessible
either.

Signed-off-by: Ingo Molnar <mingo@...e.hu>
---
 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
--
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