linux-kernel - Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups

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Message-ID: <20170629162618.exjllikxeac3ujih@redhat.com>
Date:   Thu, 29 Jun 2017 12:26:18 -0400
From:   Don Zickus <dzickus@...hat.com>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     "Liang, Kan" <kan.liang@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "babu.moger@...cle.com" <babu.moger@...cle.com>,
        "atomlin@...hat.com" <atomlin@...hat.com>,
        "prarit@...hat.com" <prarit@...hat.com>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "eranian@...gle.com" <eranian@...gle.com>,
        "acme@...hat.com" <acme@...hat.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH V2] kernel/watchdog: fix spurious hard lockups

On Thu, Jun 29, 2017 at 09:12:20AM -0700, Andi Kleen wrote:
> On Thu, Jun 29, 2017 at 11:44:06AM -0400, Don Zickus wrote:
> > On Wed, Jun 28, 2017 at 01:14:04PM -0700, Andi Kleen wrote:
> > > It can be a useful debugging tool for a specific class of bugs: 
> > > when kernel software is looping forever.
> > > 
> > > But if that happens does it really matter how many iterations the
> > > loop does before it is stopped?
> > > 
> > > Even the current timeout is essentially eternity in CPU time, and 3x
> > > eternity is still eternity.
> > 
> > That isn't true.  We have customers that test the accuracy and file bugs.  I
> > had to write a RHEL whitepaper a number of years ago explaining why the
> > softlockup took 62 seconds to fire instead of 60.
> 
> Ok that makes sense.
> 
> It seems like a broken QA test from your customer, not a real issue, 

Agreed.

> but yes explaining and documenting that can be difficult.

Yes.

> 
> > 
> > The question is, if the real solution is going to take a while, what is the
> > least sucky solution for now?  Or how do we minimize it to a specific class
> > of Intel boxes.
> 
> You can't minimize it because there's no forward looking solution
> to detect a large turbo range, and also whatever issue you have in the
> generic case would apply to them too.
> 
> Thomas' patch to modulate the frequency seemed reasonable to me.
> It made the NMI watchdog depend on accurate ktime, but that's probably ok.

Ok, did Kan finish testing this patch (with the small fix on top)?

Cheers,
Don