linux-kernel - [PATCH tip/core/rcu 3/9] nohz_full: Add Kconfig parameter for scalable detection of all-idle state

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Message-Id: <1376790584-28120-3-git-send-email-paulmck@linux.vnet.ibm.com>
Date:	Sat, 17 Aug 2013 18:49:38 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org
Cc:	mingo@...e.hu, laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, dhowells@...hat.com,
	edumazet@...gle.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: [PATCH tip/core/rcu 3/9] nohz_full: Add Kconfig parameter for scalable detection of all-idle state

From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>

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@...ux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@...il.com>
Cc: Steven Rostedt <rostedt@...dmis.org>
[ paulmck: Update help text per Frederic Weisbecker. ]
---
 kernel/time/Kconfig | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 70f27e8..c7d2fd6 100644
--- 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
-- 
1.8.1.5

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