Messages in this thread | | | Date | Tue, 11 May 2004 16:16:53 -0700 | From | Paul Jackson <> | Subject | Re: (resend) take3: Updated CPU Hotplug patches for IA64 (pj blessed) Patch [6/7] |
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Being somewhat thick in the head at times, it took me a lengthy private email thread with Ashok to understand why he needed to add a cpu_present_map for his cpu hotplug work, and just how that new map related to the existing ones.
After patient and repeated explanations from Ashok, I worked out the following explanation that he found accurate. On the off chance that others find this way of phrasing it helpful, I post it here for the record.
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Ashok's cpu hot plug patch adds a cpu_present_map, resulting in the following cpu maps being available. All the following maps are fixed size bitmaps of size NR_CPUS.
#ifdef CONFIG_HOTPLUG_CPU cpu_possible_map - map with all NR_CPUS bits set cpu_present_map - map with bit 'cpu' set iff cpu is populated cpu_online_map - map with bit 'cpu' set iff cpu available to scheduler #else cpu_possible_map - map with bit 'cpu' set iff cpu is populated cpu_present_map - copy of cpu_possible_map cpu_online_map - map with bit 'cpu' set iff cpu available to scheduler #endif
In either case, NR_CPUS is fixed at compile time, as the static size of these bitmaps. The cpu_possible_map 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_map is dynamic(*), representing which CPUs are currently plugged in. And cpu_online_map is the dynamic subset of cpu_present_map, indicating those CPUs available for scheduling.
If HOTPLUG is enabled, then cpu_possible_map is forced to have all NR_CPUS bits set, otherwise it is just the set of CPUs that ACPI reports present at boot.
If HOTPLUG is enabled, then cpu_present_map varies dynamically, depending on what ACPI reports as currently plugged in, otherwise cpu_present_map is just a copy of cpu_possible_map.
(*) Well, cpu_present_map is dynamic in the hotplug case. If not hotplug, it's the same as cpu_possible_map, hence fixed at boot.
-- I won't rest till it's the best ... Programmer, Linux Scalability Paul Jackson <pj@sgi.com> 1.650.933.1373 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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