linux-kernel - [PATCH 9/9] Documentations/cpu-hotplug.tx, kernel-parameters.txt: Add x86 CPU0 online/offline feature

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Message-Id: <1320444241-834-10-git-send-email-fenghua.yu@intel.com>
Date:	Fri,  4 Nov 2011 15:04:01 -0700
From:	"Fenghua Yu" <fenghua.yu@...el.com>
To:	"Thomas Gleixner" <tglx@...utronix.de>,
	"H Peter Anvin" <hpa@...or.com>, "Ingo Molnar" <mingo@...e.hu>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Tony Luck" <tony.luck@...el.com>,
	"Suresh B Siddha" <suresh.b.siddha@...el.com>,
	"Len Brown" <len.brown@...el.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	"linux-kernel" <linux-kernel@...r.kernel.org>,
	"Fenghua Yu" <fenghua.yu@...el.com>
Subject: [PATCH 9/9] Documentations/cpu-hotplug.tx, kernel-parameters.txt: Add x86 CPU0 online/offline feature

From: Fenghua Yu <fenghua.yu@...el.com>

Add x86 CPU0 online/offline feature in the two documentations.

By default BSP is not pluggable. Kernel parameter bsp_hotplug enables BSP
online/offline feature.

The documentations point out two BSP dependencies. First, resume from hibernate
or suspend always starts from BSP. So hibernate and suspend are prevented if BSP
is offline. Another dependency is PIC interrupts always go to BSP.

Signed-off-by: Fenghua Yu <fenghua.yu@...el.com>
---
---
 Documentation/cpu-hotplug.txt       |   19 +++++++++++++++++++
 Documentation/kernel-parameters.txt |   13 +++++++++++++
 2 files changed, 32 insertions(+), 0 deletions(-)

diff --git a/Documentation/cpu-hotplug.txt b/Documentation/cpu-hotplug.txt
index a20bfd4..ebd41e7 100644
--- a/Documentation/cpu-hotplug.txt
+++ b/Documentation/cpu-hotplug.txt
@@ -207,6 +207,25 @@ 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 given kernel option bsp_hotplug, CPU0 is removable.
+
+But some features may depend on BSP. Known dependencies are:
+1. Hibernate/suspend depends on BSP. Hibernate/suspend will fail if BSP is
+offline and you need to online BSP before hibernate/suspend can continue.
+2. PIC interrupts also depend on BSP. BSP can't be removed if a PIC interrupt
+is detected.
+
+It's said poweroff/reboot may depend on BSP on some machines although I haven't
+seen any poweroff/reboot failure so far after BSP is offline on a few tested
+machines.
+
+Please let me know if you know or see any other dependencies of BSP.
+
+If the dependencies are under your control, you can turn on bsp_hotplug.
+
+--Fenghua Yu
+
 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
diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index a0c5c5f..b1c7e68 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -1842,6 +1842,19 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	nox2apic	[X86-64,APIC] Do not enable x2APIC mode.
 
+	bsp_hotplug	[X86] BSP (aka CPU0) is hotpluggable.
+			Some features may depend on BSP. Known dependencies are
+			1. Suspend/resume depends on BSP. Suspend will fail if
+			BSP is offline and you need to online BSP before
+			suspend/resume.
+			2. PIC interrupts also depend on BSP. BSP can't be
+			removed if a PIC interrupt is detected.
+			It's said poweroff/reboot may depend on BSP on some
+			machines although I haven't seen such issues so far
+			after BSP is offline on a few tested machines.
+			If the dependencies are under your control, you can
+			turn on bsp_hotplug.
+
 	nptcg=		[IA-64] Override max number of concurrent global TLB
 			purges which is reported from either PAL_VM_SUMMARY or
 			SAL PALO.
-- 
1.6.0.3

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