nohz: Select VIRT_CPU_ACCOUNTING_GEN from full dynticks config

Turn the full dynticks passive dependency on VIRT_CPU_ACCOUNTING_GEN
to an active one.

The full dynticks Kconfig is currently hidden behind the full dynticks
cputime accounting, which is an awkward and counter-intuitive layout:
the user first has to select the dynticks cputime accounting in order
to make the full dynticks feature to be visible.

We definetly want it the other way around. The usual way to perform
this kind of active dependency is use "select" on the depended target.
Now we can't use the Kconfig "select" instruction when the target is
a "choice".

So this patch inspires on how the RCU subsystem Kconfig interact
with its dependencies on SMP and PREEMPT: we make sure that cputime
accounting can't propose another option than VIRT_CPU_ACCOUNTING_GEN
when NO_HZ_FULL is selected by using the right "depends on" instruction
for each cputime accounting choices.

v2: Keep full dynticks cputime accounting available even without
full dynticks, as per Paul McKenney's suggestion.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Kevin Hilman <khilman@linaro.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 3 insertions, 1 deletions

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 1ea2bba4a686..a2ddd650cb92 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -104,11 +104,13 @@ config NO_HZ_FULL
         depends on SMP
         # RCU_USER_QS dependency
         depends on HAVE_CONTEXT_TRACKING
-        depends on VIRT_CPU_ACCOUNTING_GEN
+        # VIRT_CPU_ACCOUNTING_GEN dependency
+        depends on 64BIT
         select NO_HZ_COMMON
         select RCU_USER_QS
         select RCU_NOCB_CPU
         select RCU_NOCB_CPU_ALL
+        select VIRT_CPU_ACCOUNTING_GEN
         select CONTEXT_TRACKING_FORCE
         select IRQ_WORK
         help