Messages in this thread | | | Date | Fri, 28 Aug 2015 10:32:59 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH RESEND] sched/nohz: Affine unpinned timers to housekeepers |
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* 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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