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authorFenghua Yu <fenghua.yu@intel.com>2012-11-13 19:32:38 (GMT)
committerH. Peter Anvin <hpa@linux.intel.com>2012-11-14 17:39:44 (GMT)
commitf78cff48c3d13636705851b0a9e733db117d58ee (patch)
treeb6bd89c046057eb58719e05e4d76f96cf4439099 /Documentation/cpu-hotplug.txt
parent77b67063bb6bce6d475e910d3b886a606d0d91f7 (diff)
doc: Add x86 CPU0 online/offline feature
If CONFIG_BOOTPARAM_HOTPLUG_CPU0 is turned on, CPU0 is hotpluggable. Otherwise, by default CPU0 is not hotpluggable and kernel parameter cpu0_hotplug enables CPU0 online/offline feature. The documentations point out two known CPU0 dependencies. First, resume from hibernate or suspend always starts from CPU0. So hibernate and suspend are prevented if CPU0 is offline. Another dependency is PIC interrupts always go to CPU0. It's said that some machines may depend on CPU0 to poweroff/reboot. But I haven't seen such dependency on a few tested machines. Please let me know if you see any CPU0 dependencies on your machine. Signed-off-by: Fenghua Yu <fenghua.yu@intel.com> Link: http://lkml.kernel.org/r/1352835171-3958-2-git-send-email-fenghua.yu@intel.com Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Diffstat (limited to 'Documentation/cpu-hotplug.txt')
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diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
index 66ef8f3..9f40135 100644
--- a/Documentation/cpu-hotplug.txt
+++ b/Documentation/cpu-hotplug.txt
@@ -207,6 +207,30 @@ by making it not-removable.
In such cases you will also notice that the online file is missing under cpu0.
+Q: Is CPU0 removable on X86?
+A: Yes. If kernel is compiled with CONFIG_BOOTPARAM_HOTPLUG_CPU0=y, CPU0 is
+removable by default. Otherwise, CPU0 is also removable by kernel option
+cpu0_hotplug.
+
+But some features depend on CPU0. Two known dependencies are:
+
+1. Resume from hibernate/suspend depends on CPU0. Hibernate/suspend will fail if
+CPU0 is offline and you need to online CPU0 before hibernate/suspend can
+continue.
+2. PIC interrupts also depend on CPU0. CPU0 can't be removed if a PIC interrupt
+is detected.
+
+It's said poweroff/reboot may depend on CPU0 on some machines although I haven't
+seen any poweroff/reboot failure so far after CPU0 is offline on a few tested
+machines.
+
+Please let me know if you know or see any other dependencies of CPU0.
+
+If the dependencies are under your control, you can turn on CPU0 hotplug feature
+either by CONFIG_BOOTPARAM_HOTPLUG_CPU0 or by kernel parameter cpu0_hotplug.
+
+--Fenghua Yu <fenghua.yu@intel.com>
+
Q: How do i find out if a particular CPU is not removable?
A: Depending on the implementation, some architectures may show this by the
absence of the "online" file. This is done if it can be determined ahead of

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