On Tue, 2019-11-19 at 09:17 -0800, James Smart wrote: > > > Actually, the behaviour of num_possible_cpus() under cpu hotplug seems > > to be implementation-specific. > > One might want to set it to the max value at all times, which has the > > drawback that you'd need to scale per-cpu values with that number, > > leading to a massive memory overhead as only very few installation > > will ever reach that number. > > Others might want to set to the max value at the current > > configuration, implying that it might change under CPU hotplug. > > Which seems to be the case for PowerPC, which was the case which > > triggered the issue at hand. > > But maybe it was a plain bug in the CPU hotplug implementation; one > > should be asking BenH for details here. > > > > Cheers, > > > > Hannes > > That sure seems to be at odds with this comment from > include/linux/cpumask.h: > > * The cpu_possible_mask is fixed at boot time, as the set of CPU id's > * that it is possible might ever be plugged in at anytime during the > * life of that system boot. The cpu_present_mask is dynamic(*), > * representing which CPUs are currently plugged in. And > * cpu_online_mask is the dynamic subset of cpu_present_mask, > * indicating those CPUs available for scheduling. > > and > #define num_possible_cpus() cpumask_weight(cpu_possible_mask) Right, it should be fixed at boot. Where do you see powerpc change it ? Cheers, Ben.
