Re: [Linux PPC] Disable PREEMPT

Peter Zijlstra
2026-02-25T11:37:09Z

On Wed, Feb 25, 2026 at 08:16:46AM +0100, 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://github.com/torvalds/linux/commit/7dadeaa6e851e

Right. So to recap, there are and will be more cases where relying on
preemption is mandatory.

The typical example is huge page memset/memcpy. Currently that is done
in single page chunks with cond_resched() in between, but it is much
better (and faster) to do a full huge page in one go.

Combine this with the endless stream of mostly random cond_resched()
placements due to $reason, and it is all a bit of a maintenance
nightmare.

I suppose the question is; why do you think you require non-preempt
kernels?