linux-kernel - Re: [RFC PATCH 12/23] kernel/watchdog: Introduce a struct for NMI watchdog operations

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Message-ID: <20180615022112.GE11625@voyager>
Date:   Thu, 14 Jun 2018 19:21:12 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Nicholas Piggin <npiggin@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Ashok Raj <ashok.raj@...el.com>, Borislav Petkov <bp@...e.de>,
        Tony Luck <tony.luck@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>, x86@...nel.org,
        sparclinux@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org, Jacob Pan <jacob.jun.pan@...el.com>,
        Don Zickus <dzickus@...hat.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Frederic Weisbecker <frederic@...nel.org>,
        Babu Moger <babu.moger@...cle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Colin Ian King <colin.king@...onical.com>,
        "Luis R. Rodriguez" <mcgrof@...nel.org>,
        iommu@...ts.linux-foundation.org
Subject: Re: [RFC PATCH 12/23] kernel/watchdog: Introduce a struct for NMI
 watchdog operations

On Thu, Jun 14, 2018 at 12:32:50PM +1000, Nicholas Piggin wrote:
> On Wed, 13 Jun 2018 18:31:17 -0700
> Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> wrote:
> 
> > On Wed, Jun 13, 2018 at 09:52:25PM +1000, Nicholas Piggin wrote:
> > > On Wed, 13 Jun 2018 11:26:49 +0200 (CEST)
> > > Thomas Gleixner <tglx@...utronix.de> wrote:
> > >   
> > > > On Wed, 13 Jun 2018, Peter Zijlstra wrote:  
> > > > > On Wed, Jun 13, 2018 at 05:41:41PM +1000, Nicholas Piggin wrote:    
> > > > > > On Tue, 12 Jun 2018 17:57:32 -0700
> > > > > > Ricardo Neri <ricardo.neri-calderon@...ux.intel.com> wrote:
> > > > > >     
> > > > > > > Instead of exposing individual functions for the operations of the NMI
> > > > > > > watchdog, define a common interface that can be used across multiple
> > > > > > > implementations.
> > > > > > > 
> > > > > > > The struct nmi_watchdog_ops is defined for such operations. These initial
> > > > > > > definitions include the enable, disable, start, stop, and cleanup
> > > > > > > operations.
> > > > > > > 
> > > > > > > Only a single NMI watchdog can be used in the system. The operations of
> > > > > > > this NMI watchdog are accessed via the new variable nmi_wd_ops. This
> > > > > > > variable is set to point the operations of the first NMI watchdog that
> > > > > > > initializes successfully. Even though at this moment, the only available
> > > > > > > NMI watchdog is the perf-based hardlockup detector. More implementations
> > > > > > > can be added in the future.    
> > > > > > 
> > > > > > Cool, this looks pretty nice at a quick glance. sparc and powerpc at
> > > > > > least have their own NMI watchdogs, it would be good to have those
> > > > > > converted as well.    
> > > > > 
> > > > > Yeah, agreed, this looks like half a patch.    
> > > > 
> > > > Though I'm not seeing the advantage of it. That kind of NMI watchdogs are
> > > > low level architecture details so having yet another 'ops' data structure
> > > > with a gazillion of callbacks, checks and indirections does not provide
> > > > value over the currently available weak stubs.  
> > > 
> > > The other way to go of course is librify the perf watchdog and make an
> > > x86 watchdog that selects between perf and hpet... I also probably
> > > prefer that for code such as this, but I wouldn't strongly object to
> > > ops struct if I'm not writing the code. It's not that bad is it?  
> > 
> > My motivation to add the ops was that the hpet and perf watchdog share
> > significant portions of code.
> 
> Right, a good motivation.
> 
> > I could look into creating the library for
> > common code and relocate the hpet watchdog into arch/x86 for the hpet-
> > specific parts.
> 
> If you can investigate that approach, that would be appreciated. I hope
> I did not misunderstand you there, Thomas.
> 
> Basically you would have perf infrastructure and hpet infrastructure,
> and then the x86 watchdog driver will use one or the other of those. The
> generic watchdog driver will be just a simple shim that uses the perf
> infrastructure. Then hopefully the powerpc driver would require almost
> no change.

Sure, I will try to structure code to minimize the changes to the powerpc
watchdog... without breaking the sparc one.

Thanks and BR,
Ricardo