Re: [Linux PPC] Disable PREEMPT

["Michal Suchánek" <msuchanek@suse.de>]

On Thu, Feb 26, 2026 at 02:24:50PM +0100, Christophe Leroy (CS GROUP) wrote:
> Hi Christian,
> 
> Le 26/02/2026 à 06:37, Christian Zigotzky a écrit :
> > On 25/02/26 08:16, Christophe Leroy (CS GROUP) wrote:
> > > +Peter
> > > 
> > > Hi Christian,
> > > 
> > > Le 25/02/2026 à 06:54, Christian Zigotzky a écrit :
> > > > Hello,
> > > > 
> > > > We were previously able to disable PREEMPT in the kernel
> > > > configuration, but the latest kernels now enable it by default
> > > > and it is no longer possible to disable it.
> > > > 
> > > > Is it possible to let us decide whether to activate PREEMPT or not?
> > > 
> > > Can you give more details on why you want to be able to deactivate
> > > PREEMPT ?
> > > 
> > > Read https://eur01.safelinks.protection.outlook.com/? url=https%3A%2F%2Fgithub.com%2Ftorvalds%2Flinux%2Fcommit%2F7dadeaa6e851e&data=05%7C02%7Cchristophe.leroy2%40cs-soprasteria.com%7C7006c8b4b98c4315a7e508de74f9388a%7C8b87af7d86474dc78df45f69a2011bb5%7C0%7C0%7C639076810956939768%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=6xLB%2FOVSkmgZRamaBM9zaxm%2FUTk9ssf4409QJdiXkDg%3D&reserved=0
> > I use PREEMPT_NONE kernels on my Linux PPC servers. (Without desktop)
> 
> You are doing that today because it is possible. Tomorrow it will not be
> possible anymore, for the reasons explained by Peter.
> 
> My question was not to know what you are doing but why, and why you think
> you still need PREEMPT_NONE and why the modern PREEMPT_LAZY is not good
> enough for you.

Hello,

I tried to compile a kernel with preemption for pSeries, and while
PREEMPT_VOLUNTARY works for me PREEMPT_LAZY crashes.

The kernel crashing is certainly a reason why a preemtion model would
not be good enough for me.

This is not the latest kernel so it might have been fixed in the
meantime for the platform in question. Will need to check that.

Nonethless, there are quite a few platforms. Originally no preemption
was the only option, and that's the reason why many people run that.
It's the conservative, known working option. And that's the reason a lot
of platfrom code does not get tested with more aggressive preemtion
models, and never gets fixed to work with them.

Simply disabling the no preemtion option does not make the platform code
ready.

Thanks

Michal

> 
> Christophe
> 
> > > 
> > > 
> > > > 
> > > > Currently we use the following revert patch for our non-preempt kernels.
> > > > 
> > > > --- a/kernel/Kconfig.preempt    2026-02-14 10:06:32.000000000 +0000
> > > > +++ b/kernel/Kconfig.preempt    2026-02-08 21:03:27.000000000 +0000
> > > > @@ -16,13 +16,11 @@ config ARCH_HAS_PREEMPT_LAZY
> > > > 
> > > >   choice
> > > >       prompt "Preemption Model"
> > > > -    default PREEMPT_LAZY if ARCH_HAS_PREEMPT_LAZY
> > > >       default PREEMPT_NONE
> > > > 
> > > >   config PREEMPT_NONE
> > > >       bool "No Forced Preemption (Server)"
> > > >       depends on !PREEMPT_RT
> > > > -    depends on ARCH_NO_PREEMPT
> > > >       select PREEMPT_NONE_BUILD if !PREEMPT_DYNAMIC
> > > >       help
> > > >         This is the traditional Linux preemption model, geared towards
> > > > @@ -37,7 +35,6 @@ config PREEMPT_NONE
> > > > 
> > > >   config PREEMPT_VOLUNTARY
> > > >       bool "Voluntary Kernel Preemption (Desktop)"
> > > > -    depends on !ARCH_HAS_PREEMPT_LAZY
> > > >       depends on !ARCH_NO_PREEMPT
> > > >       depends on !PREEMPT_RT
> > > >       select PREEMPT_VOLUNTARY_BUILD if !PREEMPT_DYNAMIC
> > > > 
> > > > ---
> > > > 
> > > > Thanks in advance,
> > > > 
> > > > Christian
> > > > 
> > > 
> > 
> > 
>