linux-kernel - Re: [PATCH] hung_task: Allow printing warnings every check interval

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Date:   Thu, 25 Jul 2019 19:38:13 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org
Cc:     Dmitry Safonov <0x7f454c46@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Ingo Molnar <mingo@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Vasiliy Khoruzhick <vasilykh@...sta.com>,
        linux-doc@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] hung_task: Allow printing warnings every check interval

On 2019/07/25 2:02, Dmitry Safonov wrote:
> Hung task detector has one timeout and has two associated actions on it:
> - issuing warnings with names and stacks of blocked tasks
> - panic()
> 
> We want switches to panic (and reboot) if there's a task
> in uninterruptible sleep for some minutes - at that moment something
> ugly has happened and the box needs a reboot.
> But we also want to detect conditions that are "out of range"
> or approaching the point of failure. Under such conditions we want
> to issue an "early warning" of an impending failure, minutes before
> the switch is going to panic.

Can't we do it by extending sysctl_hung_task_panic to accept values larger
than 1, and decrease by one when at least one thread was reported by each
check_hung_uninterruptible_tasks() check, and call panic() when
sysctl_hung_task_panic reached to 0 (or maybe 1 is simpler) ?

Hmm, might have the same problem regarding how/when to reset the counter.
If some userspace process can reset the counter, such process can trigger
SysRq-c when some period expired...

> It seems rather easy to add printing tasks and their stacks for
> notification and debugging purposes into hung task detector without
> complicating the code or major cost (prints are with KERN_INFO loglevel
> and so don't go on console, only into dmesg log).

Well, I don't think so. Might be noisy for systems without "quiet" kernel
command line option, and we can't pass KERN_DEBUG to e.g. sched_show_task()...