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Date: Mon, 3 Dec 2007 09:57:36 -0800
From: Andrew Morton <akpm@linux-foundation.org>
To: Ingo Molnar <mingo@elte.hu>
Cc: andi@firstfloor.org, penberg@cs.helsinki.fi, lkml@astralstorm.puszkin.org,
       arjan@infradead.org, linux-kernel@vger.kernel.org, tglx@linutronix.de
Subject: Re: [feature] automatically detect hung TASK_UNINTERRUPTIBLE tasks
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On Mon, 3 Dec 2007 15:19:25 +0100
Ingo Molnar <mingo@elte.hu> wrote:

> this patch extends the soft-lockup detector to automatically
> detect hung TASK_UNINTERRUPTIBLE tasks. Such hung tasks are
> printed the following way:
> 
>  ------------------>
>  INFO: task prctl:3042 blocked for more than 120 seconds.
>  "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message
>  prctl         D fd5e3793     0  3042   2997
>         f6050f38 00000046 00000001 fd5e3793 00000009 c06d8264 c06dae80 00000286
>         f6050f40 f6050f00 f7d34d90 f7d34fc8 c1e1be80 00000001 f6050000 00000000
>         f7e92d00 00000286 f6050f18 c0489d1a f6050f40 00006605 00000000 c0133a5b
>  Call Trace:
>   [<c04883a5>] schedule_timeout+0x6d/0x8b
>   [<c04883d8>] schedule_timeout_uninterruptible+0x15/0x17
>   [<c0133a76>] msleep+0x10/0x16
>   [<c0138974>] sys_prctl+0x30/0x1e2
>   [<c0104c52>] sysenter_past_esp+0x5f/0xa5
>   =======================
>  2 locks held by prctl/3042:
>  #0:  (&sb->s_type->i_mutex_key#5){--..}, at: [<c0197d11>] do_fsync+0x38/0x7a
>  #1:  (jbd_handle){--..}, at: [<c01ca3d2>] journal_start+0xc7/0xe9
>  <------------------
> 
> the current default timeout is 120 seconds. Such messages are printed
> up to 10 times per bootup. If the system has crashed already then the
> messages are not printed.
> 
> if lockdep is enabled then all held locks are printed as well.
> 
> this feature is a natural extension to the softlockup-detector (kernel
> locked up without scheduling) and to the NMI watchdog (kernel locked up
> with IRQs disabled).

This feature will save one full reporter-developer round-trip during
investigation of a significant number of bug reports.

It might be more practical if it were to dump the traces for _all_
D-state processes when it fires - basically an auto-triggered sysrq-W.
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