* Frederic Weisbecker <fweis...@gmail.com> wrote:
> On Tue, Aug 25, 2015 at 10:29:04AM +0200, Ingo Molnar wrote:
> >
> > * Frederic Weisbecker <fweis...@gmail.com> wrote:
> >
> > > On Mon, Aug 24, 2015 at 08:44:12AM +0200, Ingo Molnar wrote:
> > > > 2)
> > > >
> > > > What happens if the boot CPU is offlined? (under
> > > > CONFIG_BOOTPARAM_HOTPLUG_CPU0=y)
> > > >
> > > > I don't see CPU hotplug callbacks fixing up the housekeeping_mask if
> > > > the boot CPU
> > > > is offlined.
> > >
> > > We have tick_nohz_cpu_down_callback() which makes sure that the
> > > timekeeper, which
> > > is the boot CPU in nohz full, never gets offlined.
> >
> > That solution really sucks - it essentially regresses a feature the user
> > explicitly asked for! I also see no way for the user to migrate the
> > timekeeping
> > functionality over to another CPU without rebooting.
> >
> > If this is the last timekeeping CPU then it should migrate the timekeeping
> > functionality to another CPU, and perhaps printk a warning if all other
> > CPUs are
> > nohz-full and we have to mark one of them as the timekeeper.
> >
> > Also, the nohz-full and timekeeper functionality should not be a boot
> > parameter
> > only thing, but should be runtime configurable.
>
> When I tried to allow moving the timekeeping duty over all housekeeping CPUs,
> Thomas got angry because it broke the KISS current nohz full code. Indeed,
> there
> must be at least one running all the time on behalf of nohz full CPUs that
> can
> run anytime. Thus balancing the timekeeping duty over housekeepers is a bit
> more
> complicated than in normal configurations.
>
> Now surely we can do that using an IPI from CPU_DOWN_PREPARE to a housekeeper
> if
> any remains or to a nohz full one. Then we must make sure the new timekeeper
> never goes to idle.
>
> But nohz_full is a corner usecase and I'm not sure it's worth the complexity.
> If
> a nohz full user came and complained about CPU0 hotplog not working, I would
> definetly retry it but I haven't heard about that yet. Besides, hotplug is
> very
> isolation-unfriendly in general due to stop machine.
Ok, I guess we can live with this.
Mind sending an updated series with all patches?
Thanks,
Ingo
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