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Date:   Tue, 12 Jul 2022 09:25:32 -0700
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Subject: Re: [PATCH v4 4/4] pseries/mobility: set NMI watchdog factor during
 LPM
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To:     Laurent Dufour <ldufour@linux.ibm.com>, mpe@ellerman.id.au,
        npiggin@gmail.com, christophe.leroy@csgroup.eu,
        wim@linux-watchdog.org, linux@roeck-us.net, nathanl@linux.ibm.com
Cc:     haren@linux.vnet.ibm.com, hch@infradead.org,
        linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org,
        linux-watchdog@vger.kernel.org
References: <20220712143202.23144-1-ldufour@linux.ibm.com>
 <20220712143202.23144-5-ldufour@linux.ibm.com>
From:   Randy Dunlap <rdunlap@infradead.org>
In-Reply-To: <20220712143202.23144-5-ldufour@linux.ibm.com>
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Hi--

On 7/12/22 07:32, Laurent Dufour wrote:
> During a LPM, while the memory transfer is in progress on the arrival side,
> some latencies is generated when accessing not yet transferred pages on the

                 are

> arrival side. Thus, the NMI watchdog may be triggered too frequently, which
> increases the risk to hit a NMI interrupt in a bad place in the kernel,

                            an NMI

> leading to a kernel panic.
> 
> Disabling the Hard Lockup Watchdog until the memory transfer could be a too
> strong work around, some users would want this timeout to be eventually
> triggered if the system is hanging even during LPM.
> 
> Introduce a new sysctl variable nmi_watchdog_factor. It allows to apply
> a factor to the NMI watchdog timeout during a LPM. Just before the CPU are

                                              an LPM.            the CPU is

> stopped for the switchover sequence, the NMI watchdog timer is set to
>  watchdog_tresh + factor%

   watchdog_thresh

> 
> A value of 0 has no effect. The default value is 200, meaning that the NMI
> watchdog is set to 30s during LPM (based on a 10s watchdog_tresh value).

                                                    watchdog_thresh

> Once the memory transfer is achieved, the factor is reset to 0.
> 
> Setting this value to a high number is like disabling the NMI watchdog
> during a LPM.

         an LPM.

> 
> Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> ---
>  Documentation/admin-guide/sysctl/kernel.rst | 12 ++++++
>  arch/powerpc/platforms/pseries/mobility.c   | 43 +++++++++++++++++++++
>  2 files changed, 55 insertions(+)
> 
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index ddccd1077462..0bb0b7f27e96 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -592,6 +592,18 @@ to the guest kernel command line (see
>  Documentation/admin-guide/kernel-parameters.rst).
>  

This entire block should be in kernel-parameters.txt, not .rst,
and it should be formatted like everything else in the .txt file.

>  
> +nmi_watchdog_factor (PPC only)
> +==================================
> +
> +Factor apply to to the NMI watchdog timeout (only when ``nmi_watchdog`` is

   Factor to apply to the NMI

> +set to 1). This factor represents the percentage added to
> +``watchdog_thresh`` when calculating the NMI watchdog timeout during a

                                                                 during an

> +LPM. The soft lockup timeout is not impacted.
> +
> +A value of 0 means no change. The default value is 200 meaning the NMI
> +watchdog is set to 30s (based on ``watchdog_thresh`` equal to 10).
> +
> +
>  numa_balancing
>  ==============
>  


-- 
~Randy