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From:   Nathan Lynch <nathanl@linux.ibm.com>
To:     Laurent Dufour <ldufour@linux.ibm.com>
Cc:     linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
        mpe@ellerman.id.au, benh@kernel.crashing.org, paulus@samba.org,
        haren@linux.vnet.ibm.com, npiggin@gmail.com
Subject: Re: [PATCH 0/2] Disabling NMI watchdog during LPM's memory transfer
In-Reply-To: <20220601155315.35109-1-ldufour@linux.ibm.com>
References: <20220601155315.35109-1-ldufour@linux.ibm.com>
Date:   Thu, 02 Jun 2022 12:58:31 -0500
Message-ID: <87a6av0wxk.fsf@linux.ibm.com>
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Laurent Dufour <ldufour@linux.ibm.com> writes:
> When a partition is transferred, once it arrives at the destination node,
> the partition is active but much of its memory must be transferred from the
> start node.
>
> It depends on the activity in the partition, but the more CPU the partition
> has, the more memory to be transferred is likely to be. This causes latency
> when accessing pages that need to be transferred, and often, for large
> partitions, it triggers the NMI watchdog.

It also triggers warnings from other watchdogs and subsystems that
have soft latency requirements  - softlockup, RCU, workqueue. The issue
is more general than the NMI watchdog.

> The NMI watchdog causes the CPU stack to dump where it appears to be
> stuck. In this case, it does not bring much information since it can happen
> during any memory access of the kernel.

When the site of a watchdog backtrace shows a thread stuck on a routine
memory access as opposed to something like a lock acquisition, that is
actually useful information that shouldn't be discarded. It tells us the
platform is failing to adequately virtualize partition memory. This
isn't a benign situation and it's likely to unacceptably affect real
workloads. The kernel is ideally situated to detect and warn about this.

> In addition, the NMI interrupt mechanism is not secure and can generate a
> dump system in the event that the interruption is taken while
> MSR[RI]=0.

This sounds like a general problem with that facility that isn't
specific to partition migration? Maybe it should be disabled altogether
until that can be fixed?

> Given how often hard lockups are detected when transferring large
> partitions, it seems best to disable the watchdog NMI until the memory
> transfer from the start node is complete.

At this time, I'm far from convinced. Disabling the watchdog is going to
make the underlying problems in the platform and/or network harder to
understand.