[patch] x86: Fix incorrect __read_mostly on _boot_cpu_pda

[Ravikiran G Thirumalai]

The pda rework (commit 3461b0af025251bbc6b3d56c821c6ac2de6f7209)
to remove static boot cpu pdas introduced a bug.  _boot_cpu_pda is the
actual pda used by the boot cpu and is definitely not "__read_mostly" and
ended up polluting the read mostly section with writes.  This bug caused
regression of about 8-10% on certain syscall intensive workloads.

Signed-off-by: Ravikiran Thirumalai <kiran@xxxxxxxxxxxx>

Index: linux-2.6/arch/x86/kernel/head64.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/head64.c	2008-12-30 15:28:08.000000000 -0800
+++ linux-2.6/arch/x86/kernel/head64.c	2008-12-31 10:36:21.000000000 -0800
@@ -27,7 +27,7 @@
 #include <asm/trampoline.h>
 
 /* boot cpu pda */
-static struct x8664_pda _boot_cpu_pda __read_mostly;
+static struct x8664_pda _boot_cpu_pda;
 
 #ifdef CONFIG_SMP
 /*
--
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