nohz_full: Add Kconfig parameter for scalable detection of all-idle state

At least one CPU must keep the scheduling-clock tick running for
timekeeping purposes whenever there is a non-idle CPU.  However, with
the new nohz_full adaptive-idle machinery, it is difficult to distinguish
between all CPUs really being idle as opposed to all non-idle CPUs being
in adaptive-ticks mode.  This commit therefore adds a Kconfig parameter
as a first step towards enabling a scalable detection of full-system
idle state.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
[ paulmck: Update help text per Frederic Weisbecker. ]
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 70f27e89012b29d18b4aeb9c6e5c367d1b3955a5..c7d2fd67799e917e2117abc92870d5cc7c0cf3b1 100644 (file)
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -134,6 +134,29 @@ config NO_HZ_FULL_ALL
         Note the boot CPU will still be kept outside the range to
         handle the timekeeping duty.
 
+config NO_HZ_FULL_SYSIDLE
+       bool "Detect full-system idle state for full dynticks system"
+       depends on NO_HZ_FULL
+       default n
+       help
+        At least one CPU must keep the scheduling-clock tick running for
+        timekeeping purposes whenever there is a non-idle CPU, where
+        "non-idle" also includes dynticks CPUs as long as they are
+        running non-idle tasks.  Because the underlying adaptive-tick
+        support cannot distinguish between all CPUs being idle and
+        all CPUs each running a single task in dynticks mode, the
+        underlying support simply ensures that there is always a CPU
+        handling the scheduling-clock tick, whether or not all CPUs
+        are idle.  This Kconfig option enables scalable detection of
+        the all-CPUs-idle state, thus allowing the scheduling-clock
+        tick to be disabled when all CPUs are idle.  Note that scalable
+        detection of the all-CPUs-idle state means that larger systems
+        will be slower to declare the all-CPUs-idle state.
+
+        Say Y if you would like to help debug all-CPUs-idle detection.
+
+        Say N if you are unsure.
+
 config NO_HZ
        bool "Old Idle dynticks config"
        depends on !ARCH_USES_GETTIMEOFFSET && GENERIC_CLOCKEVENTS