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SubjectRe: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers

* Frederic Weisbecker <fweisbec@gmail.com> wrote:

> On Tue, Aug 25, 2015 at 10:29:04AM +0200, Ingo Molnar wrote:
> >
> > * Frederic Weisbecker <fweisbec@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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