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Subject: Re: SMP lockup at boot on Freescale/NXP T2080 (powerpc 64)
To:     Chris Packham <Chris.Packham@alliedtelesis.co.nz>,
        "linuxppc-dev@lists.ozlabs.org" <linuxppc-dev@lists.ozlabs.org>,
        "mpe@ellerman.id.au" <mpe@ellerman.id.au>,
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Cc:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Grant McEwan <grant.mcewan@alliedtelesis.co.nz>
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From:   Christophe Leroy <christophe.leroy@c-s.fr>
Message-ID: <b0ad453c-3a2b-6dd5-7bfc-5e275aa7bf62@c-s.fr>
Date:   Thu, 8 Aug 2019 10:46:35 +0200
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Le 07/08/2019 à 03:24, Chris Packham a écrit :
> On Wed, 2019-08-07 at 11:13 +1000, Michael Ellerman wrote:
>> Chris Packham <Chris.Packham@alliedtelesis.co.nz> writes:
>>>
>>> On Tue, 2019-08-06 at 21:32 +1000, Michael Ellerman wrote:
>>> The difference between a working and non working defconfig is
>>> CONFIG_PREEMPT specifically CONFIG_PREEMPT=y makes my system hang
>>> at
>>> boot.
>>>
>>> Is that now intentionally prohibited on 64-bit powerpc?
>> It's not prohibitied, but it probably should be because no one really
>> tests it properly. I have a handful of IBM machines where I boot a
>> PREEMPT kernel but that's about it.
>>
>> The corenet configs don't have PREEMPT enabled, which suggests it was
>> never really supported on those machines.
>>
>> But maybe someone from NXP can tell me otherwise.
>>
> 
> I think our workloads need CONFIG_PREEMPT=y because our systems have
> switch ASIC drivers implemented in userland and we need to be able to
> react quickly to network events in order to prevent loops. We have seen
> instances of this not happening simply because some other process is in
> the middle of a syscall.
> 
> One thing I am working on here is a setup with a few vendor boards and
> some of our own kit that we can test the upstream kernels on. Hopefully
> that'd make these kinds of reports more timely rather than just
> whenever we decide to move to a new kernel version.
> 
> 


The defconfig also sets CONFIG_DEBUG_PREEMPT. Have you tried without 
CONFIG_DEBUG_PREEMPT ?

Christophe