linux-kernel - Re: [PATCH AUTOSEL for 3.18 36/40] powerpc/xmon: Avoid tripping SMP hardlockup watchdog

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Date:   Sat, 3 Feb 2018 15:58:22 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Michael Ellerman <mpe@...erman.id.au>
Cc:     alexander.levin@...izon.com,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable\@vger.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH AUTOSEL for 3.18 36/40] powerpc/xmon: Avoid tripping SMP
 hardlockup watchdog

On Tue, 30 Jan 2018 15:35:54 +1100
Michael Ellerman <mpe@...erman.id.au> wrote:

> alexander.levin@...izon.com writes:
> 
> > On Thu, Dec 14, 2017 at 12:10:39AM +1100, Michael Ellerman wrote:  
> >>alexander.levin@...izon.com writes:
> >>  
> >>> From: Nicholas Piggin <npiggin@...il.com>
> >>>
> >>> [ Upstream commit 064996d62a33ffe10264b5af5dca92d54f60f806 ]
> >>>
> >>> The SMP hardlockup watchdog cross-checks other CPUs for lockups, which
> >>> causes xmon headaches because it's assuming interrupts hard disabled
> >>> means no watchdog troubles. Try to improve that by calling
> >>> touch_nmi_watchdog() in obvious places where secondaries are spinning.
> >>>
> >>> Also annotate these spin loops with spin_begin/end calls.  
> >>
> >>These macros didn't exist until 4.13, and haven't been backported AFAIK.  
> >
> > But the touch_nmi_watchdog() bits are something we want in stable, right?  
> 
> I don't think you need them unless you've also back ported
> arch/powerpc/kernel/watchdog.c, which I don't think you have.
> 
> Maybe Nick can confirm?

I'm not 100% sure. The CPUs only check themselves for lockups. They will
blow their threshold when in xmon, but when they come out of xmon, I think
by a quirk of our local_irq_enable() implementation that actually checks
timers explicitly and runs them first before re-enabling hard interrupts,
then our heartbeat starts up again just before the perf interrupt would
come in to report the lockup.

I think.

Given that we've had no reports of misbehaviour of the old perf watchdog,
I would say you can skip the backport.

Thanks,
Nick