Re: [PATCH] kernel/hung_task.c: use timeout diff when timeout is updated

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 17 Dec 2015 21:23:03 +0900 Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp> wrote:

> >From 529ff00b556e110c6e801c39e94b06f559307136 Mon Sep 17 00:00:00 2001
> From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> Date: Thu, 17 Dec 2015 16:27:08 +0900
> Subject: [PATCH] kernel/hung_task.c: use timeout diff when timeout is updated
> 
> When new timeout is written to /proc/sys/kernel/hung_task_timeout_secs,
> khungtaskd is interrupted and again sleeps for full timeout duration.
> 
> This means that hang task will not be checked if new timeout is written
> periodically within old timeout duration and/or checking of hang task
> will be delayed for up to previous timeout duration.
> Fix this by remembering last time khungtaskd checked hang task.
> 
> This change will allow other watchdog tasks (if any) to share khungtaskd
> by sleeping for minimal timeout diff of all watchdog tasks. Doing more
> watchdog tasks from khungtaskd will reduce the possibility of printk()
> collisions by multiple watchdog threads.

This seems like reasonable behaviour, but it is a
non-backward-compatible change.  I don't know how important that is -
probably "not very".

Please let's fully describe this behaviour in the documentation. 
Documentation/sysctl/kernel.txt.

It appears that if userspace writes a new timeout which has already
expired, a check is triggered immediately, correct?  Let's ensure that
this is documented as well.   And tested!

And it would be helpful to add a comment to hung_timeout_jiffies()
which describes the behaviour and explains the reasons for it.