Re: [PATCH 0/5] improve handling of errors returned by kthread_park()

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

On Mon, 28 Sep 2015 22:44:07 +0200 Ulrich Obergfell <uobergfe@redhat.com> wrote:

> The original watchdog_park_threads() function that was introduced by
> commit 81a4beef91ba4a9e8ad6054ca9933dff7e25ff28 takes a very simple
> approach to handle errors returned by kthread_park(): It attempts to
> roll back all watchdog threads to the unparked state. However, this
> may be undesired behaviour from the perspective of the caller which
> may want to handle errors as appropriate in its specific context.
> Currently, there are two possible call chains:
> 
> - watchdog suspend/resume interface
> 
>     lockup_detector_suspend
>       watchdog_park_threads
> 
> - write to parameters in /proc/sys/kernel
> 
>     proc_watchdog_update
>       watchdog_enable_all_cpus
>         update_watchdog_all_cpus
>           watchdog_park_threads
> 
> Instead of 'blindly' attempting to unpark the watchdog threads if a 
> kthread_park() call fails, the new approach is to disable the lockup
> detectors in the above call chains. Failure becomes visible to the
> user as follows:
> 
> - error messages from lockup_detector_suspend()
>                    or watchdog_enable_all_cpus()
> 
> - the state that can be read from /proc/sys/kernel/watchdog_enabled
> 
> - the 'write' system call in the latter call chain returns an error
> 

hm, you made me look at kthread parking.  Why does it exist?  What is a
"parked" thread anyway, and how does it differ from, say, a sleeping
one?  The 2a1d446019f9a5983ec5a335b changelog is pretty useless and the
patch added no useful documentation, sigh.

Anwyay...  what inspired this patchset?  Are you experiencing
kthread_park() failures in practice?  If so, what is causing them?  And
what is the user-visible effect of these failures?  This is all pretty
important context for such a patchset.