nohz: Rename CONFIG_NO_HZ to CONFIG_NO_HZ_COMMON

We are planning to convert the dynticks Kconfig options layout
into a choice menu. The user must be able to easily pick
any of the following implementations: constant periodic tick,
idle dynticks, full dynticks.

As this implies a mutual exclusion, the two dynticks implementions
need to converge on the selection of a common Kconfig option in order
to ease the sharing of a common infrastructure.

It would thus seem pretty natural to reuse CONFIG_NO_HZ to
that end. It already implements all the idle dynticks code
and the full dynticks depends on all that code for now.
So ideally the choice menu would propose CONFIG_NO_HZ_IDLE and
CONFIG_NO_HZ_EXTENDED then both would select CONFIG_NO_HZ.

On the other hand we want to stay backward compatible: if
CONFIG_NO_HZ is set in an older config file, we want to
enable CONFIG_NO_HZ_IDLE by default.

But we can't afford both at the same time or we run into
a circular dependency:

1) CONFIG_NO_HZ_IDLE and CONFIG_NO_HZ_EXTENDED both select
   CONFIG_NO_HZ
2) If CONFIG_NO_HZ is set, we default to CONFIG_NO_HZ_IDLE

We might be able to support that from Kconfig/Kbuild but it
may not be wise to introduce such a confusing behaviour.

So to solve this, create a new CONFIG_NO_HZ_COMMON option
which gathers the common code between idle and full dynticks
(that common code for now is simply the idle dynticks code)
and select it from their referring Kconfig.

Then we'll later create CONFIG_NO_HZ_IDLE and map CONFIG_NO_HZ
to it for backward compatibility.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.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: Namhyung Kim <namhyung.kim@lge.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, 9 insertions, 4 deletions

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 726c33e00da2..c88fc43494c9 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -64,16 +64,21 @@ config GENERIC_CMOS_UPDATE
 if GENERIC_CLOCKEVENTS
 menu "Timers subsystem"
 
-# Core internal switch. Selected by NO_HZ / HIGH_RES_TIMERS. This is
+# Core internal switch. Selected by NO_HZ_COMMON / HIGH_RES_TIMERS. This is
 # only related to the tick functionality. Oneshot clockevent devices
 # are supported independ of this.
 config TICK_ONESHOT
         bool
 
+config NO_HZ_COMMON
+        bool
+        depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
+        select TICK_ONESHOT
+
 config NO_HZ
         bool "Tickless System (Dynamic Ticks)"
         depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
-        select TICK_ONESHOT
+        select NO_HZ_COMMON
         help
           This option enables a tickless system: timer interrupts will
           only trigger on an as-needed basis both when the system is
@@ -81,14 +86,14 @@ config NO_HZ
 
 config NO_HZ_EXTENDED
         bool "Full dynticks system"
-        # NO_HZ dependency
+        # NO_HZ_COMMON dependency
         depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS
         # RCU_USER_QS
         depends on HAVE_CONTEXT_TRACKING && SMP
         # RCU_NOCB_CPU dependency
         depends on TREE_RCU || TREE_PREEMPT_RCU
         depends on VIRT_CPU_ACCOUNTING_GEN
-        select NO_HZ
+        select NO_HZ_COMMON
         select RCU_USER_QS
         select RCU_NOCB_CPU
         select CONTEXT_TRACKING_FORCE