Re: Linux v2.6.16

[Ashok Raj <ashok.raj@intel.com>]

On Wed, Mar 22, 2006 at 09:40:00PM +0100, Rafael J. Wysocki wrote:
> > 
> > with that patch, try
> > 
> > CONFIG_X86_PC=n
> > CONFIG_GENERICARCH=y
> > CONFIG_HOTPLUG_CPU=y
> 
> Well, there's nothing like CONFIG_GENERICARCH on x86_64 or I'm obviously
> missing something. :-)
> 
> On x86_64 I can choose between X86_PC and X86_VSMP and I'm not sure I'd like
> to set X86_VSMP just in order to be able to suspend a box with a dual-core CPU.
> IMHO that would be over the top.
> 

This change is only for i386.. check the patch, its introduced only for arch/i386/Kconfig

There is no change to x86_64, we anyway choose physflat mode in x86_64 which is exactly 
same as bigsmp in i386.

We didnt change anything in x86_64...Why this speculation :(

Iam attaching that patch for your reference here... in case you lost it and looking at 
some other patch. :-)

Could you _please_ really check this and tell me if there is real concern... its pretty
simple patch... no other arch is affected...


This patch makes CONFIG_HOTPLUG_CPU depend on !X86_PC, so we need to turn on 
either CONFIG_GENERICARCH, CONFIG_BIGSMP or any other subarch except X86_PC

With 2.6.15+ kernels when CONFIG_HOTPLUG_CPU is turned on we switch to bigsmp mode for
sending IPI's and ioapic configurations that caused the following error message.

>> More than 8 CPUs detected and CONFIG_X86_PC cannot handle it.
>> Use CONFIG_X86_GENERICARCH or CONFIG_X86_BIGSMP.

Originally bigsmp was added just to handle >8 cpus, but now with hotplug cpu support
we need to use bigsmp mode (why? see below), that cause the above error message even 
if there were less than 8 cpus in the system.

The message is bogus, but we are cannot use logical flat mode due to issues with
broadcast IPI can confuse a CPU just comming up. We use flat physical mode just like x86_64
case. More details on why bigsmp now uses flat physical mode (vs. cluster mode)
in following link.

http://marc.theaimsgroup.com/?l=linux-kernel&m=113261865814107&w=2


Signed-off-by: Ashok Raj <ashok.raj@intel.com>
---------------------------------------------------------

 arch/i386/Kconfig |    2 +-
 1 files changed, 1 insertion(+), 1 deletion(-)

Index: linux-2.6.16-rc6-mm1/arch/i386/Kconfig
===================================================================
--- linux-2.6.16-rc6-mm1.orig/arch/i386/Kconfig
+++ linux-2.6.16-rc6-mm1/arch/i386/Kconfig
@@ -760,7 +760,7 @@ config PHYSICAL_START
 
 config HOTPLUG_CPU
 	bool "Support for hot-pluggable CPUs (EXPERIMENTAL)"
-	depends on SMP && HOTPLUG && EXPERIMENTAL && !X86_VOYAGER
+	depends on SMP && HOTPLUG && EXPERIMENTAL && !X86_VOYAGER && !X86_PC
 	---help---
 	  Say Y here to experiment with turning CPUs off and on.  CPUs
 	  can be controlled through /sys/devices/system/cpu.