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Date: Sun, 2 Aug 2009 11:45:45 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: paulmck@linux.vnet.ibm.com, mingo@redhat.com, hpa@zytor.com,
       linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
       a.p.zijlstra@chello.nl, torvalds@linux-foundation.org,
       tglx@linutronix.de, mingo@elte.hu
Cc: tip-bot for Ingo Molnar <mingo@elte.hu>, linux-tip-commits@vger.kernel.org,
       linux-kernel@vger.kernel.org, hpa@zytor.com, mingo@redhat.com,
       torvalds@linux-foundation.org, a.p.zijlstra@chello.nl,
       paulmck@linux.vnet.ibm.com, tglx@linutronix.de
Subject: Re: [tip:core/debug] debug lockups: Improve lockup detection
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	<tip-c1dc0b9c0c8979ce4d411caadff5c0d79dee58bc@git.kernel.org>
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On Sun, 2 Aug 2009 13:09:34 GMT tip-bot for Ingo Molnar <mingo@elte.hu> wrote:

> Commit-ID:  c1dc0b9c0c8979ce4d411caadff5c0d79dee58bc
> Gitweb:     http://git.kernel.org/tip/c1dc0b9c0c8979ce4d411caadff5c0d79dee58bc
> Author:     Ingo Molnar <mingo@elte.hu>
> AuthorDate: Sun, 2 Aug 2009 11:28:21 +0200
> Committer:  Ingo Molnar <mingo@elte.hu>
> CommitDate: Sun, 2 Aug 2009 13:27:17 +0200
> 
> --- a/drivers/char/sysrq.c
> +++ b/drivers/char/sysrq.c
> @@ -24,6 +24,7 @@
>  #include <linux/sysrq.h>
>  #include <linux/kbd_kern.h>
>  #include <linux/proc_fs.h>
> +#include <linux/nmi.h>
>  #include <linux/quotaops.h>
>  #include <linux/perf_counter.h>
>  #include <linux/kernel.h>
> @@ -222,12 +223,7 @@ static DECLARE_WORK(sysrq_showallcpus, sysrq_showregs_othercpus);
>  
>  static void sysrq_handle_showallcpus(int key, struct tty_struct *tty)
>  {
> -	struct pt_regs *regs = get_irq_regs();
> -	if (regs) {
> -		printk(KERN_INFO "CPU%d:\n", smp_processor_id());
> -		show_regs(regs);
> -	}
> -	schedule_work(&sysrq_showallcpus);
> +	trigger_all_cpu_backtrace();
>  }

I think this just broke all non-x86 non-sparc SMP architectures.

>  static struct sysrq_key_op sysrq_showallcpus_op = {
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index 7717b95..9c5fa9f 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -35,6 +35,7 @@
>  #include <linux/rcupdate.h>
>  #include <linux/interrupt.h>
>  #include <linux/sched.h>
> +#include <linux/nmi.h>
>  #include <asm/atomic.h>
>  #include <linux/bitops.h>
>  #include <linux/module.h>
> @@ -469,6 +470,8 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
>  	}
>  	printk(" (detected by %d, t=%ld jiffies)\n",
>  	       smp_processor_id(), (long)(jiffies - rsp->gp_start));
> +	trigger_all_cpu_backtrace();

Be aware that trigger_all_cpu_backtrace() is a PITA when you have a lot
of CPUs.

If a callsite is careful to ensure that the most important information
is emitted last then that might improve things.

otoh, log buffer overflow will truncate, I think.  So that info needs
to be emitted first too ;)

It's a PITA.