linux-kernel - Re: [tip:core/debug] debug lockups: Improve lockup detection

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Message-Id: <20090802114545.f1520c81.akpm@linux-foundation.org>
Date:	Sun, 2 Aug 2009 11:45:45 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	paulmck@...ux.vnet.ibm.com, mingo@...hat.com, hpa@...or.com,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	a.p.zijlstra@...llo.nl, torvalds@...ux-foundation.org,
	tglx@...utronix.de, mingo@...e.hu
Cc:	tip-bot for Ingo Molnar <mingo@...e.hu>,
	linux-tip-commits@...r.kernel.org, linux-kernel@...r.kernel.org,
	hpa@...or.com, mingo@...hat.com, torvalds@...ux-foundation.org,
	a.p.zijlstra@...llo.nl, paulmck@...ux.vnet.ibm.com,
	tglx@...utronix.de
Subject: Re: [tip:core/debug] debug lockups: Improve lockup detection

On Sun, 2 Aug 2009 13:09:34 GMT tip-bot for Ingo Molnar <mingo@...e.hu> wrote:

> Commit-ID:  c1dc0b9c0c8979ce4d411caadff5c0d79dee58bc
> Gitweb:     http://git.kernel.org/tip/c1dc0b9c0c8979ce4d411caadff5c0d79dee58bc
> Author:     Ingo Molnar <mingo@...e.hu>
> AuthorDate: Sun, 2 Aug 2009 11:28:21 +0200
> Committer:  Ingo Molnar <mingo@...e.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.


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