Re: PREEMPT_RT support

[Michael Schmitz <schmitzmic@gmail.com>]

Hi Adrian,

Am 30.09.2024 um 21:38 schrieb John Paul Adrian Glaubitz:
> Hello Jean-Michel,
>
> On Mon, 2024-09-30 at 10:19 +0200, Jean-Michel Hautbois wrote:
>> I wanted to try PREEMPT_RT. I added the support for it:
>> diff --git a/arch/m68k/Kconfig b/arch/m68k/Kconfig
>> index c7880f375518..b0360167574d 100644
>> --- a/arch/m68k/Kconfig
>> +++ b/arch/m68k/Kconfig
>> @@ -39,6 +39,7 @@ config M68K
>>          select OLD_SIGSUSPEND3
>>          select UACCESS_MEMCPY if !MMU
>>          select ZONE_DMA
>> +       select ARCH_SUPPORTS_RT
>>
>>   config CPU_BIG_ENDIAN
>>          def_bool y
>>
>> And set PREEMPT_RT then it fails to boot:
>> [    6.517000] BUG: scheduling while atomic: init/1/0x00010001
>> (...)
>> Any idea, thoughts :-) ?
>
> From what I know an architecture has to be explicitly ported to support
> a realtime kernel. Simply enabling the feature therefore won't work.

Making sure softirqs are run from a separate kernel thread and not from 
atomic context would be a good start. Adding preempt_disable() / 
preempt_enable() around more critcal sections may be needed as well.

Git history of those architectures that support PREEMPT_RT might be 
informative.

Cheers,

	Michael


>
> Adrian
>