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

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Date:	Fri, 04 Nov 2011 16:27:06 -0700
From:	Randy Dunlap <rdunlap@...otime.net>
To:	Fenghua Yu <fenghua.yu@...el.com>
CC:	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>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 9/9] Documentations/cpu-hotplug.tx, kernel-parameters.txt:
 Add x86 CPU0 online/offline feature

On 11/04/2011 03:04 PM, Fenghua Yu wrote:
> 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:

Drop "may".

> +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

Drop "may".  Then add ":" after "are".  I.e.,

			Some features 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.


-- 
~Randy
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