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Subject: Re: SMP lockup at boot on Freescale/NXP T2080 (powerpc 64)
To: Chris Packham <Chris.Packham@alliedtelesis.co.nz>,
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From: Christophe Leroy <christophe.leroy@c-s.fr>
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Date: Thu, 8 Aug 2019 15:12:03 +0200
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Le 08/08/2019 à 10:46, Christophe Leroy a écrit :
> 
> 
> 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 ?
> 

Reproduced on QEMU. CONFIG_DEBUG_PREEMPT is the trigger. Due to 
smp_processor_id() being called from early_init_this_mmu(), when 
CONFIG_DEBUG_PREEMPT is set debug_smp_processor_id() is called instead 
of raw_smp_processor_id(), but this is too early for 
debug_smp_processor_id()

As this call is useless, just drop it.

Can you test patch at https://patchwork.ozlabs.org/patch/1144005/ ?

Thanks
Christophe