Re: [RFC PATCH] kernel: allow to configure PREEMPT_NONE, PREEMPT_VOLUNTARY on kernel command line

[Peter Zijlstra]

On Fri, Oct 09, 2020 at 12:14:05PM +0200, Michal Hocko wrote:
> On Fri 09-10-20 11:47:41, Peter Zijlstra wrote:

> > That is, work backwards (from PREEMPT back to VOLUNTARY) instead of the
> > other way around.
> 
> My original idea was that the config would only define the default
> preemption mode. preempt_none parameter would then just act as an
> override. That would mean that CONFIG_PREEMPTION would be effectively
> gone from the kernel. The reason being that any code outside of the
> scheduler shouldn't really care about the preemption mode. I suspect
> this will prevent from dubious hacks and provide a more robust code in
> the end.

Sure; but the way of arriving at that destination might be easier if
you work backwards from PREEMPT=y, because while there _should_ not be
dependencies outside of the scheduler, we both know there are.

This also makes your patches independent of the series that makes
CONFIG_PREEMPTION unconditional.

It also gives Kconfig space to limit the dynamic thing to archs that
have sufficient support (we'll be relying on static_call/static_branch,
and not everybody has that implemented in a way that makes it the
dynamic change worth-while).