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

Peter Zijlstra
2020-10-09T09:47:52Z

On Wed, Oct 07, 2020 at 02:35:53PM +0200, Michal Hocko wrote:
> On Wed 07-10-20 14:21:44, Peter Zijlstra wrote:
> > On Wed, Oct 07, 2020 at 02:04:01PM +0200, Michal Hocko wrote:
> > > I wanted to make sure that the idea is sound for maintainers first. The
> > > next step would be extending the command line to support full preemption
> > > as well but there is much more work in that area. Frederic has promissed
> > > to look into that.
> >
> > The sanest way there is to static_call() __preempt_schedule() I think.
>
> Yes, I have checked the code and identified few other places like
> irqentry_exit_cond_resched. We also need unconditional
> CONFIG_PREEMPT_COUNT IIUC and there are quite some places guarded by
> CONFIG_PREEMPTION that would need to be examined. Some of them are
> likely pretending to be more clever than they really are/should be -
> e.g. mm/slub.c. So there is likely a lot of leg work.

The easiest way might be to introduce PREEMPT_DYNAMIC that
depends/selects PREEMPT. That way you're basically running a PREEMPT=y
kernel.

Then have PREEMPT_DYNAMIC allow disabling the __preempt_schedule /
preempt_schedule_irq() callsites using static_call/static_branch
respectively.

That is, work backwards (from PREEMPT back to VOLUNTARY) instead of the
other way around.