linux-kernel - Re: [PATCH v2 1/2] watchdog: Introduce arch_watchdog_nmi_enable and arch_watchdog_nmi

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Date:   Fri, 21 Oct 2016 11:11:14 -0400
From:   Don Zickus <dzickus@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Babu Moger <babu.moger@...cle.com>, mingo@...nel.org,
        ak@...ux.intel.com, jkosina@...e.cz,
        baiyaowei@...s.chinamobile.com, atomlin@...hat.com,
        uobergfe@...hat.com, tj@...nel.org, hidehiro.kawai.ez@...achi.com,
        johunt@...mai.com, davem@...emloft.net, sparclinux@...r.kernel.org,
        linux-kernel@...r.kernel.org, sam@...nborg.org
Subject: Re: [PATCH v2 1/2] watchdog: Introduce arch_watchdog_nmi_enable and
 arch_watchdog_nmi_disable

On Thu, Oct 20, 2016 at 08:25:27PM -0700, Andrew Morton wrote:
> On Thu, 20 Oct 2016 12:14:14 -0400 Don Zickus <dzickus@...hat.com> wrote:
> 
> > > > -static int watchdog_nmi_enable(unsigned int cpu) { return 0; }
> > > > -static void watchdog_nmi_disable(unsigned int cpu) { return; }
> > > > +/*
> > > > + * These two functions are mostly architecture specific
> > > > + * defining them as weak here.
> > > > + */
> > > > +int __weak arch_watchdog_nmi_enable(unsigned int cpu) { return 0; }
> > > > +void __weak arch_watchdog_nmi_disable(unsigned int cpu) { return; }
> > > > +
> > > >  #endif /* CONFIG_HARDLOCKUP_DETECTOR */
> > > 
> > > This is a strange way of using __weak.
> > > 
> > > Take a look at (one of many examples) kernel/module.c:module_alloc(). 
> > > We simply provide a default implementation and some other compilation
> > > unit can override (actually replace) that at link time.  No strange
> > > ifdeffing needed.
> > 
> > Yeah, this is mostly because of how we enable the hardlockup detector.
> > 
> > Some arches use the perf hw and enable CONFIG_HARDLOCKUP_DETECTOR.  Other
> > arches just use their own variant of nmi and set CONFIG_HAVE_NMI_WATCHDOG and
> > the rest of the arches do not use this.
> > 
> > So the thought was if CONFIG_HARDLOCKUP_DETECTOR use that implementation,
> > everyone else use the __weak version.  Then the arches like sparc can override
> > the weak version with their own nmi enablement.
> > 
> > I don't know how to represent those 3 states correctly and the above is what
> > we end up with.
> 
> <head spins>
> 
> Is there a suitable site where we could capture these considerations in
> a code comment?

Hi Andrew,

I am not sure I understand your question.  When you say 'site', are you
referring to the kernel/watchdog.c file?

The other approach that might help de-clutter this file, is to pull out the
HARDLOCKUP_DETECTOR changes (as they are arch specific) and move it to say
kernel/watchdog_hw_ld.c.  Then all the nmi hooks in kernel/watchdog.c can be
__weak and overridden by the kernel_watchdog_hw_ld.c file or the sparc
files.

This would leave kernel/watchdog.c with just a framework and the
arch-agnostic softlockup detector.  Probably easier to read and digest.

Thoughts?

Cheers,
Don