linux-kernel - Re: [PATCH 2/2] workqueue: implement lockup detector

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Message-ID: <20151204080226.GA25880@gmail.com>
Date:	Fri, 4 Dec 2015 09:02:26 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	Ulrich Obergfell <uobergfe@...hat.com>,
	Don Zickus <dzickus@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 2/2] workqueue: implement lockup detector

* Tejun Heo <tj@...nel.org> wrote:

> Hello, Ulrich.
> 
> On Thu, Dec 03, 2015 at 03:12:20PM -0500, Ulrich Obergfell wrote:
> > I share Don's concern about connecting the soft lockup detector and the
> > workqueue watchdog to the same kernel parameter in /proc. I would feel
> > more comfortable if the workqueue watchdog had its dedicated parameter.
> 
> Sure, separating the knobs out isn't difficult.  I still don't like
> the idea of having multiple set of similar knobs controlling about the
> same thing tho.
> 
> For example, let's say there's a user who boots with "nosoftlockup"
> explicitly.  I'm pretty sure the user wouldn't be intending to keep
> workqueue watchdog running.  The same goes for threshold adjustments,
> so here's my question.  What are the reasons for the concern?  What
> are we worrying about?

As Don mentioned it already, we went through similar arguments (and pain) with the 
hard/soft lockup detectors and its various control knobs, it would be better to 
have new control knobs separated.

As for the ease of use argument, we can add a new, obviously named control knob 
that controls _all_ lockup detectors:

  boot param: nolockupdetectors
  matching Kconfig knob: CONFIG_BOOTPARAM_NO_LOCKUP_DETECTORS=0

but please don't artificially couple the control knobs of these various lockup 
detectors, as these internal assumptions are less than obvious to users. With 
(effectively) 4 lockup detectors such coupling of interfaces is even more 
confusing and damaging.

but ease of use is not a big concern in any case, since new debug features are 
disabled by default, so only those people will see it (and may want to disable it 
via a boot parameter, hopefully only temporarily) who enable it intentionally.

Thanks,

	Ingo
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