On Fri, Aug 28, 2015 at 10:32:59AM +0200, Ingo Molnar wrote: > > * Frederic Weisbecker <[email protected]> wrote: > > > On Tue, Aug 25, 2015 at 10:29:04AM +0200, Ingo Molnar wrote: > > > > > > * Frederic Weisbecker <[email protected]> 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.
Now this will likely evolve in the future, I can easily imagine that timekeeping becomes balanced among housekeepers when we'll have one per node. It's not yet the priority but we may come to that one day. > > Mind sending an updated series with all patches? Sure, I'm cooking that. Thanks! -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
