linux-kernel - Re: Intermittent early panic in try_to_wake

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Date:	Fri, 23 Oct 2009 11:23:01 +0200
From:	Mike Galbraith <efault@....de>
To:	Kevin Winchester <kjwinchester@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: Intermittent early panic in try_to_wake_up

On Thu, 2009-10-22 at 20:33 -0300, Kevin Winchester wrote:
> Hi,
> 
> A week or two ago I saw a panic on boot in try_to_wake_up, but it was not
> reproducible and I had not written down any trace information.  This
> evening I saw it twice more, but then on the third boot things worked fine.
> This time I copied down the stack trace:
> 
> try_to_wake_up+0x2e/0x102
> wake_up_process+0x10/0x12
> kthread_create+0x88/0x12c
> ?ksoftirqd+0x00/0xb7
> cpu_callback+0x42/0x8f
> ?spawn_ksoftirqd+0x0/0x39
> spawn_ksoftirqd+0x17/0x39
> do_one_initcall+0x58/0x147
> 
> The first time it happened, I remember checking the git logs and it was
> shortly after:
> 
> commit f5dc37530ba8a35aae0f7f4f13781d1904f71e94
> Author: Mike Galbraith <efault@....de>
> Date:   Fri Oct 9 08:35:03 2009 +0200
> 
>     sched: Update the clock of runqueue select_task_rq() selected
>     
>     In try_to_wake_up(), we update the runqueue clock, but
>     select_task_rq() may select a different runqueue than the one we
>     updated, leaving the new runqueue's clock stale for a bit.
>     
>     This patch cures occasional huge latencies reported by latencytop
>     when coming out of idle on a mostly idle NO_HZ box.
>     
>     Signed-off-by: Mike Galbraith <efault@....de>
>     Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
>     LKML-Reference: <1255070103.7639.30.camel@...ge.simson.net>
>     Signed-off-by: Ingo Molnar <mingo@...e.hu>
> 
> 
> ...so perhaps that has something to do with it.

I don't think that's very likely.  Box did explode near my grubby
fingerprints though.

> Config below.  Any help would be appreciated.

Building with your config, try_to_wake_up+0x2e is around..

(gdb) list *try_to_wake_up+0x2e
0xffffffff81029107 is in try_to_wake_up (kernel/sched.c:2324).
2319            this_cpu = get_cpu();
2320
2321            smp_wmb();
2322            rq = orig_rq = task_rq_lock(p, &flags);
2323            update_rq_clock(rq);
2324            if (!(p->state & state))
2325                    goto out;
2326
2327            if (p->se.on_rq)
2328                    goto out_running;

I don't see how any of that can explode without something very bad
having happened to ksoftirqd before we tried to wake it.

	-Mike

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