dynticks: CONFIG_VIRT_CPU_ACCOUNTING + CONFIG_CONTEXT_TRACKINGbreaks accounting on core2 CPUs only

[Mike Galbraith]

Greetings,

Turning on new NO_HZ feature on my Q6600 box in master, I see that tasks
accrue zero utime/stime.  However, the same exact kernel on E5620 box
works fine, so it would appear there's a CPU dependency somewhere.

Is core2 expected to go dysfunctional with context tracking enabled?
CONFIG_VIRT_CPU_ACCOUNTING alone works fine in 3.9-stable, turn on
CONFIG_CONTEXT_TRACKING_FORCE, and CPU accounting stops working on core2
boxen only, same exact kernel continues to work just fine on E5620
(Westmere) box.

-Mike 

marge:/usr/local/src/kernel/linux-3.9 # egrep 'NO_HR|CPU_ACCOUNTING|RCU|
CONTEXT' .config
CONFIG_VIRT_CPU_ACCOUNTING=y
# CONFIG_TICK_CPU_ACCOUNTING is not set
CONFIG_VIRT_CPU_ACCOUNTING_GEN=y
# RCU Subsystem
CONFIG_TREE_RCU=y
# CONFIG_PREEMPT_RCU is not set
CONFIG_RCU_STALL_COMMON=y
CONFIG_CONTEXT_TRACKING=y
# CONFIG_RCU_USER_QS is not set
CONFIG_CONTEXT_TRACKING_FORCE=y
CONFIG_RCU_FANOUT=64
CONFIG_RCU_FANOUT_LEAF=16
# CONFIG_RCU_FANOUT_EXACT is not set
# CONFIG_RCU_FAST_NO_HZ is not set
# CONFIG_TREE_RCU_TRACE is not set
# CONFIG_RCU_NOCB_CPU is not set
CONFIG_HAVE_CONTEXT_TRACKING=y
# RCU Debugging
# CONFIG_SPARSE_RCU_POINTER is not set
# CONFIG_RCU_TORTURE_TEST is not set
CONFIG_RCU_CPU_STALL_TIMEOUT=21
# CONFIG_RCU_CPU_STALL_INFO is not set
# CONFIG_RCU_TRACE is not set
CONFIG_CONTEXT_SWITCH_TRACER=y

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