Messages in this thread | | | Date | Fri, 4 Jul 2014 12:59:13 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86,cpu-hotplug: clear llc_shared_mask at CPU hotplug |
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On Fri, Jul 04, 2014 at 09:14:57AM +0900, Yasuaki Ishimatsu wrote: > If so, why following maps are cleared by CPU offline? > - cpu_sigling_map > - cpu_core_map
I'll let you figure that out on your own by doing some quality code staring. Hint: search for usages outside of the CPU hotplug path.
In any case, let me try to explain it to you one more time: if the core numbers are static and don't change across physical hotplug, the cores which share last level cache also remain the same. This is a static property which doesn't simply change.
For example, if cores 6-11 and 34-41 have been sharing the last level cache, then if the node containing them gets unplugged and replugged back again - then the same cores share that exact same cache and nothing has changed.
Except the core numbers, as you've shown. Which would be not only a problem for the llc_shared_mask but also a big annoyance for sysadmins and users having to realize that the topology enumeration has changed and trying to make sense of what node went where and why isn't it the same as when the machine booted.
So what I'm trying to say is, we should keep the core numbering stable across hotplug to avoid unnecessary confusion; the llc_shared_mask is just a small issue which results from not doing that.
-- Regards/Gruss, Boris.
Sent from a fat crate under my desk. Formatting is fine. --
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