Re: Arches that don't support PREEMPT

[Anton Ivanov <anton.ivanov@cambridgegreys.com>]

On 19/09/2023 15:16, Peter Zijlstra wrote:
> On Tue, Sep 19, 2023 at 03:48:09PM +0200, John Paul Adrian Glaubitz wrote:
>> On Tue, 2023-09-19 at 15:42 +0200, Peter Zijlstra wrote:
>>>> The agreement to kill off ia64 wasn't an invitation to kill off other stuff
>>>> that people are still working on! Can we please not do this?
>>>
>>> If you're working on one of them, then surely it's a simple matter of
>>> working on adding CONFIG_PREEMPT support :-)
>>
>> As Geert poined out, I'm not seeing anything particular problematic with the
>> architectures lacking CONFIG_PREEMPT at the moment. This seems to be more
>> something about organizing KConfig files.
> 
> The plan in the parent thread is to remove PREEMPT_NONE and
> PREEMPT_VOLUNTARY and only keep PREEMPT_FULL.
> 
>> I find it a bit unfair that maintainers of architectures that have huge companies
>> behind them use their manpower to urge less popular architectures for removal just
>> because they don't have 150 people working on the port so they can keep up with
>> design changes quickly.
> 
> PREEMPT isn't something new. Also, I don't think the arch part for
> actually supporting it is particularly hard, mostly it is sticking the
> preempt_schedule_irq() call in return from interrupt code path.

That calls local_irq_enable() which does various signal related/irq pending work on UML. That in turn does no like being invoked again (as you may have already been invoked out of that) in the IRQ return path.

So it is likely to end up being slightly more difficult than that for UML - it will need to be wrapped so it can be invoked from the "host" side signal code as well as invoked with some additional checks to avoid making a hash out of the IRQ handling.

It may be necessary to modify some of the existing reentrancy prevention logic in the signal handlers as well and change it to make use of the preempt count instead of its own flags/counters.

> 
> If you convert the arch to generic-entry (a much larger undertaking)
> then you get this for free.
> 
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-- 
Anton R. Ivanov
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