linux-kernel - Re: [PATCH] locking/hung_task: Defer showing held locks

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Date:   Tue, 20 Dec 2016 09:14:40 +0100
From:   Vegard Nossum <vegard.nossum@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mandeep Singh Baines <msb@...omium.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] locking/hung_task: Defer showing held locks

On 13 December 2016 at 15:45, Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
> When I was running my testcase which may block hundreds of threads
> on fs locks, I got lockup due to output from debug_show_all_locks()
> added by commit b2d4c2edb2e4f89a ("locking/hung_task: Show all locks").
>
> I think we don't need to call debug_show_all_locks() on each blocked
> thread. Let's defer calling debug_show_all_locks() till before panic()
> or leaving for_each_process_thread() loop.

First of all, sorry for not answering earlier.

I'm not sure I fully understand the problem, you say the "output from
debug_show_all_locks()" caused a lockup, but was the problem simply
that the amount of output caused it to stall for a long time?

Could we instead

1) move the debug_show_all_locks() into the if
(sysctl_hung_task_panic) bit unconditionally

2) call something (touch_nmi_watchdog()?) inside debug_show_all_locks()

3) in another way make debug_show_all_locks() more robust so it doesn't "lockup"

?


Vegard