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

[Ingo Molnar]

* Frederic Weisbecker <fweisbec@xxxxxxxxx> 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.

Thanks,

	Ingo
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