Re: CONFIG_ISOLATION=y (was: [PATCH 0/6] support "dataplane" mode for nohz_full)

Frederic Weisbecker
2015-05-12T15:36:31Z

On Tue, May 12, 2015 at 02:34:40PM +0200, Ingo Molnar wrote:
>
> * Peter Zijlstra <peterz@infradead.org> wrote:
>
> > On Tue, May 12, 2015 at 11:10:32AM +0200, Ingo Molnar wrote:
> > >
> > > So I'd vote for Frederic's CONFIG_ISOLATION=y, mostly because this
> > > is a high level kernel feature, so it won't conflict with
> > > isolation concepts in lower level subsystems such as IOMMU
> > > isolation - and other higher level features like scheduler
> > > isolation are basically another partial implementation we want to
> > > merge with all this...
> >
> > But why do we need a CONFIG flag for something that has no content?
> >
> > That is, I do not see anything much; except the 'I want to stay in
> > userspace and kill me otherwise' flag, and I'm not sure that
> > warrants a CONFIG flag like this.
> >
> > Other than that, its all a combination of NOHZ_FULL and
> > cpusets/isolcpus and whatnot.
>
> Yes, that's what I meant: CONFIG_ISOLATION would trigger what is
> NO_HZ_FULL today - we could possibly even remove CONFIG_NO_HZ_FULL as
> an individual Kconfig option?

Right, we could return to what we had previously: CONFIG_NO_HZ. A config
that enables dynticks-idle by default and allows full dynticks if nohz_full=
boot option is passed (or something driven by higher level isolation interface).

Because eventually, distros enable NO_HZ_FULL so that their 0.0001% users
can use it. Well at least Red Hat does.

>
> CONFIG_ISOLATION=y would express the guarantee from the kernel that
> it's possible for user-space to configure itself to run undisturbed -
> instead of the current inconsistent set of options and facilities.
>
> A bit like CONFIG_PREEMPT_RT is more than just preemptable spinlocks,
> it also tries to offer various facilities and tune the defaults to
> turn the kernel hard-rt.
>
> Does that make sense to you?

Right although distros tend to want features to be enabled dynamically
so that they have a single kernel to maintain. Things like PREEMPT_RT
really need to be a different kernel because fundamental primitives like
spinlocks must be implemented statically.

But isolation can be a boot-enabled, or even runtime-enabled, as it's only
about timer,irq,task affinity. Full Nohz is more complicated but it can
be runtime toggled in the future.

So we can bring CONFIG_CPU_ISOLATION, at least for distros that are really
not interested in that so they can disable it. CONFIG_CPU_ISOLATION=y would
bring an ability which is default-disabled and driven dynamically through whatever
interface.