[PATCH tip/core/rcu 0/9] sysidle changes for v3.12

[PATCH tip/core/rcu 0/9] sysidle changes for v3.12

[Paul E. McKenney]

Hello!

Whenever there is at least one non-idle CPU, it is necessary to
periodically update timekeeping information.  Before NO_HZ_FULL, this
updating was carried out by the scheduling-clock tick, which ran on
every non-idle CPU.  With the advent of NO_HZ_FULL, it is possible
to have non-idle CPUs that are not receiving scheduling-clock ticks.
This possibility is handled by assigning a timekeeping CPU that continues
taking scheduling-clock ticks.

Unfortunately, timekeeping CPU continues taking scheduling-clock
interrupts even when all other CPUs are completely idle, which is
not so good for energy efficiency and battery lifetime.  Clearly, it
would be good to turn off the timekeeping CPU's scheduling-clock tick
when all CPUs are completely idle.  This is conceptually simple, but
we also need good performance and scalability on large systems, which
rules out implementations based on frequently updated global counts of
non-idle CPUs as well as implementations that frequently scan all CPUs.
Nevertheless, we need a single global indicator in order to keep the
overhead of checking acceptably low.

The chosen approach is to enforce hysteresis on the non-idle to
full-system-idle transition, with the amount of hysteresis increasing
linearly with the number of CPUs, thus keeping contention acceptably low.
This approach piggybacks on RCU's existing force-quiescent-state scanning
of idle CPUs, which has the advantage of avoiding the scan entirely on
busy systems that have high levels of multiprogramming.  This scan
takes per-CPU idleness information and feeds it into a state machine
that applies the level of hysteresis required to arrive at a single
full-system-idle indicator.

The individual patches are as follows:

1.	Eliminate unused APIs that were intended for adaptive ticks.

2.	Add documentation covering the testing of nohz_full.

3.	Add a CONFIG_NO_HZ_FULL_SYSIDLE Kconfig parameter to enable
	this feature.  Kernels built with CONFIG_NO_HZ_FULL_SYSIDLE=n
	act exactly as they do today.

4.	Add new fields to the rcu_dynticks structure that track CPU-idle
	information.  These fields consider CPUs running usermode to be
	non-idle, in contrast with the existing fields in that structure.

5.	Track per-CPU idle states.

6.	Add full-system idle states and state variables.

7.	Expand force_qs_rnp(), dyntick_save_progress_counter(), and
	rcu_implicit_dynticks_qs() APIs to enable passing full-system
	idle state information.

8.	Add full-system-idle state machine.

9.	Force RCU's grace-period kthreads onto the timekeeping CPU.

Changes since v3 (https://lkml.org/lkml/2013/7/8/441):

o	Updated Kconfig help text as suggested by Frederic.

o	Added a few adaptive-ticks-related fixes (#1 and #2 above).

Changes since v2:

o	Completed removing NMI support (thanks to Frederic for spotting
	the remaining cruft).

o	Fix a state-machine bug, again spotted by Frederic.  See
	http://lists-archives.com/linux-kernel/27865835-nohz_full-add-full-system-idle-state-machine.html
	for the full details of the bug.

o	Updated commit log and comment as suggested by Josh Triplett.

Changes since v1:

o	Removed NMI support because NMI handlers cannot safely read
	the time anyway (thanks to Thomas Gleixner and Peter Zijlstra).

						Thanx, Paul


 b/Documentation/timers/NO_HZ.txt |   44 +++
 b/include/linux/rcupdate.h       |   22 +
 b/kernel/rcutree.c               |   92 +++-----
 b/kernel/rcutree.h               |   17 +
 b/kernel/rcutree_plugin.h        |  431 ++++++++++++++++++++++++++++++++++++++-
 b/kernel/time/Kconfig            |   23 ++
 6 files changed, 557 insertions(+), 72 deletions(-)

[PATCH tip/core/rcu 1/9] rcu: Eliminate unused APIs intended for adaptive ticks

[Paul E. McKenney]

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

The rcu_user_enter_after_irq() and rcu_user_exit_after_irq()
functions were intended for use by adaptive ticks, but changes
in implementation have rendered them unnecessary.  This commit
therefore removes them.

Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 include/linux/rcupdate.h |  4 ----
 kernel/rcutree.c         | 43 -------------------------------------------
 2 files changed, 47 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0c38abb..30bea9c 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -229,13 +229,9 @@ extern void rcu_irq_exit(void);
 #ifdef CONFIG_RCU_USER_QS
 extern void rcu_user_enter(void);
 extern void rcu_user_exit(void);
-extern void rcu_user_enter_after_irq(void);
-extern void rcu_user_exit_after_irq(void);
 #else
 static inline void rcu_user_enter(void) { }
 static inline void rcu_user_exit(void) { }
-static inline void rcu_user_enter_after_irq(void) { }
-static inline void rcu_user_exit_after_irq(void) { }
 static inline void rcu_user_hooks_switch(struct task_struct *prev,
 					 struct task_struct *next) { }
 #endif /* CONFIG_RCU_USER_QS */
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 338f1d1..8807019 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -444,27 +444,6 @@ void rcu_user_enter(void)
 {
 	rcu_eqs_enter(1);
 }
-
-/**
- * rcu_user_enter_after_irq - inform RCU that we are going to resume userspace
- * after the current irq returns.
- *
- * This is similar to rcu_user_enter() but in the context of a non-nesting
- * irq. After this call, RCU enters into idle mode when the interrupt
- * returns.
- */
-void rcu_user_enter_after_irq(void)
-{
-	unsigned long flags;
-	struct rcu_dynticks *rdtp;
-
-	local_irq_save(flags);
-	rdtp = &__get_cpu_var(rcu_dynticks);
-	/* Ensure this irq is interrupting a non-idle RCU state.  */
-	WARN_ON_ONCE(!(rdtp->dynticks_nesting & DYNTICK_TASK_MASK));
-	rdtp->dynticks_nesting = 1;
-	local_irq_restore(flags);
-}
 #endif /* CONFIG_RCU_USER_QS */
 
 /**
@@ -581,28 +560,6 @@ void rcu_user_exit(void)
 {
 	rcu_eqs_exit(1);
 }
-
-/**
- * rcu_user_exit_after_irq - inform RCU that we won't resume to userspace
- * idle mode after the current non-nesting irq returns.
- *
- * This is similar to rcu_user_exit() but in the context of an irq.
- * This is called when the irq has interrupted a userspace RCU idle mode
- * context. When the current non-nesting interrupt returns after this call,
- * the CPU won't restore the RCU idle mode.
- */
-void rcu_user_exit_after_irq(void)
-{
-	unsigned long flags;
-	struct rcu_dynticks *rdtp;
-
-	local_irq_save(flags);
-	rdtp = &__get_cpu_var(rcu_dynticks);
-	/* Ensure we are interrupting an RCU idle mode. */
-	WARN_ON_ONCE(rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK);
-	rdtp->dynticks_nesting += DYNTICK_TASK_EXIT_IDLE;
-	local_irq_restore(flags);
-}
 #endif /* CONFIG_RCU_USER_QS */
 
 /**
-- 
1.8.1.5

[PATCH tip/core/rcu 2/9] nohz_full: Add testing information to documentation

[Paul E. McKenney]

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

This commit adds information about testing nohz_full, and also emphasizes
the fact that you need a multi-CPU system to get any benefit from nohz_full.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 Documentation/timers/NO_HZ.txt | 44 ++++++++++++++++++++++++++++++++----------
 1 file changed, 34 insertions(+), 10 deletions(-)

diff --git a/Documentation/timers/NO_HZ.txt b/Documentation/timers/NO_HZ.txt
index 8869758..cca122f 100644
--- a/Documentation/timers/NO_HZ.txt
+++ b/Documentation/timers/NO_HZ.txt
@@ -24,8 +24,8 @@ There are three main ways of managing scheduling-clock interrupts
 	workloads, you will normally -not- want this option.
 
 These three cases are described in the following three sections, followed
-by a third section on RCU-specific considerations and a fourth and final
-section listing known issues.
+by a third section on RCU-specific considerations, a fourth section
+discussing testing, and a fifth and final section listing known issues.
 
 
 NEVER OMIT SCHEDULING-CLOCK TICKS
@@ -121,14 +121,15 @@ boot parameter specifies the adaptive-ticks CPUs.  For example,
 "nohz_full=1,6-8" says that CPUs 1, 6, 7, and 8 are to be adaptive-ticks
 CPUs.  Note that you are prohibited from marking all of the CPUs as
 adaptive-tick CPUs:  At least one non-adaptive-tick CPU must remain
-online to handle timekeeping tasks in order to ensure that system calls
-like gettimeofday() returns accurate values on adaptive-tick CPUs.
-(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no
-running user processes to observe slight drifts in clock rate.)
-Therefore, the boot CPU is prohibited from entering adaptive-ticks
-mode.  Specifying a "nohz_full=" mask that includes the boot CPU will
-result in a boot-time error message, and the boot CPU will be removed
-from the mask.
+online to handle timekeeping tasks in order to ensure that system
+calls like gettimeofday() returns accurate values on adaptive-tick CPUs.
+(This is not an issue for CONFIG_NO_HZ_IDLE=y because there are no running
+user processes to observe slight drifts in clock rate.)  Therefore, the
+boot CPU is prohibited from entering adaptive-ticks mode.  Specifying a
+"nohz_full=" mask that includes the boot CPU will result in a boot-time
+error message, and the boot CPU will be removed from the mask.  Note that
+this means that your system must have at least two CPUs in order for
+CONFIG_NO_HZ_FULL=y to do anything for you.
 
 Alternatively, the CONFIG_NO_HZ_FULL_ALL=y Kconfig parameter specifies
 that all CPUs other than the boot CPU are adaptive-ticks CPUs.  This
@@ -232,6 +233,29 @@ scheduler will decide where to run them, which might or might not be
 where you want them to run.
 
 
+TESTING
+
+So you enable all the OS-jitter features described in this document,
+but do not see any change in your workload's behavior.  Is this because
+your workload isn't affected that much by OS jitter, or is it because
+something else is in the way?  This section helps answer this question
+by providing a simple OS-jitter test suite, which is available on branch
+master of the following git archive:
+
+git://git.kernel.org/pub/scm/linux/kernel/git/frederic/dynticks-testing.git
+
+Clone this archive and follow the instructions in the README file.
+This test procedure will produce a trace that will allow you to evaluate
+whether or not you have succeeded in removing OS jitter from your system.
+If this trace shows that you have removed OS jitter as much as is
+possible, then you can conclude that your workload is not all that
+sensitive to OS jitter.
+
+Note: this test requires that your system have at least two CPUs.
+We do not currently have a good way to remove OS jitter from single-CPU
+systems.
+
+
 KNOWN ISSUES
 
 o	Dyntick-idle slows transitions to and from idle slightly.
-- 
1.8.1.5

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

[Paul E. McKenney]

From: "Paul E. McKenney" <paulmck@linux.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@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.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

[PATCH tip/core/rcu 4/9] nohz_full: Add rcu_dyntick data for scalable detection of all-idle state

[Paul E. McKenney]

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

This commit adds fields to the rcu_dyntick structure that are used to
detect idle CPUs.  These new fields differ from the existing ones in
that the existing ones consider a CPU executing in user mode to be idle,
where the new ones consider CPUs executing in user mode to be busy.
The handling of these new fields is otherwise quite similar to that for
the exiting fields.  This commit also adds the initialization required
for these fields.

So, why is usermode execution treated differently, with RCU considering
it a quiescent state equivalent to idle, while in contrast the new
full-system idle state detection considers usermode execution to be
non-idle?

It turns out that although one of RCU's quiescent states is usermode
execution, it is not a full-system idle state.  This is because the
purpose of the full-system idle state is not RCU, but rather determining
when accurate timekeeping can safely be disabled.  Whenever accurate
timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one
CPU must keep the scheduling-clock tick going.  If even one CPU is
executing in user mode, accurate timekeeping is requires, particularly for
architectures where gettimeofday() and friends do not enter the kernel.
Only when all CPUs are really and truly idle can accurate timekeeping be
disabled, allowing all CPUs to turn off the scheduling clock interrupt,
thus greatly improving energy efficiency.

This naturally raises the question "Why is this code in RCU rather than in
timekeeping?", and the answer is that RCU has the data and infrastructure
to efficiently make this determination.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/rcutree.c        |  5 +++++
 kernel/rcutree.h        |  9 +++++++++
 kernel/rcutree_plugin.h | 19 +++++++++++++++++++
 3 files changed, 33 insertions(+)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 8807019..4f27b85 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -224,6 +224,10 @@ EXPORT_SYMBOL_GPL(rcu_note_context_switch);
 DEFINE_PER_CPU(struct rcu_dynticks, rcu_dynticks) = {
 	.dynticks_nesting = DYNTICK_TASK_EXIT_IDLE,
 	.dynticks = ATOMIC_INIT(1),
+#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
+	.dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE,
+	.dynticks_idle = ATOMIC_INIT(1),
+#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
 };
 
 static long blimit = 10;	/* Maximum callbacks per rcu_do_batch. */
@@ -2904,6 +2908,7 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp, int preemptible)
 	rdp->blimit = blimit;
 	init_callback_list(rdp);  /* Re-enable callbacks on this CPU. */
 	rdp->dynticks->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
+	rcu_sysidle_init_percpu_data(rdp->dynticks);
 	atomic_set(&rdp->dynticks->dynticks,
 		   (atomic_read(&rdp->dynticks->dynticks) & ~0x1) + 1);
 	raw_spin_unlock(&rnp->lock);		/* irqs remain disabled. */
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index cbdeac6..52d1be1 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -88,6 +88,14 @@ struct rcu_dynticks {
 				    /* Process level is worth LLONG_MAX/2. */
 	int dynticks_nmi_nesting;   /* Track NMI nesting level. */
 	atomic_t dynticks;	    /* Even value for idle, else odd. */
+#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
+	long long dynticks_idle_nesting;
+				    /* irq/process nesting level from idle. */
+	atomic_t dynticks_idle;	    /* Even value for idle, else odd. */
+				    /*  "Idle" excludes userspace execution. */
+	unsigned long dynticks_idle_jiffies;
+				    /* End of last non-NMI non-idle period. */
+#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
 #ifdef CONFIG_RCU_FAST_NO_HZ
 	bool all_lazy;		    /* Are all CPU's CBs lazy? */
 	unsigned long nonlazy_posted;
@@ -545,6 +553,7 @@ static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
 static void rcu_spawn_nocb_kthreads(struct rcu_state *rsp);
 static void rcu_kick_nohz_cpu(int cpu);
 static bool init_nocb_callback_list(struct rcu_data *rdp);
+static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);
 
 #endif /* #ifndef RCU_TREE_NONCORE */
 
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index dff86f5..e5baccb 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2373,3 +2373,22 @@ static void rcu_kick_nohz_cpu(int cpu)
 		smp_send_reschedule(cpu);
 #endif /* #ifdef CONFIG_NO_HZ_FULL */
 }
+
+
+#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
+
+/*
+ * Initialize dynticks sysidle state for CPUs coming online.
+ */
+static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
+{
+	rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
+}
+
+#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
+
+static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
+{
+}
+
+#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
-- 
1.8.1.5

Re: [PATCH tip/core/rcu 4/9] nohz_full: Add rcu_dyntick data for scalable detection of all-idle state

[Josh Triplett]

On Sat, Aug 17, 2013 at 06:49:39PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> This commit adds fields to the rcu_dyntick structure that are used to
> detect idle CPUs.  These new fields differ from the existing ones in
> that the existing ones consider a CPU executing in user mode to be idle,
> where the new ones consider CPUs executing in user mode to be busy.
> The handling of these new fields is otherwise quite similar to that for
> the exiting fields.  This commit also adds the initialization required
> for these fields.
> 
> So, why is usermode execution treated differently, with RCU considering
> it a quiescent state equivalent to idle, while in contrast the new
> full-system idle state detection considers usermode execution to be
> non-idle?
> 
> It turns out that although one of RCU's quiescent states is usermode
> execution, it is not a full-system idle state.  This is because the
> purpose of the full-system idle state is not RCU, but rather determining
> when accurate timekeeping can safely be disabled.  Whenever accurate
> timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one
> CPU must keep the scheduling-clock tick going.  If even one CPU is
> executing in user mode, accurate timekeeping is requires, particularly for
> architectures where gettimeofday() and friends do not enter the kernel.
> Only when all CPUs are really and truly idle can accurate timekeeping be
> disabled, allowing all CPUs to turn off the scheduling clock interrupt,
> thus greatly improving energy efficiency.
> 
> This naturally raises the question "Why is this code in RCU rather than in
> timekeeping?", and the answer is that RCU has the data and infrastructure
> to efficiently make this determination.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>

One comment below.  With that change:
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

> +#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> +
> +/*
> + * Initialize dynticks sysidle state for CPUs coming online.
> + */
> +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> +{
> +	rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
> +}
> +
> +#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> +
> +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> +{
> +}
> +
> +#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */

Just move the ifdef around the function body:

static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
{
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
	rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
#endif /* CONFIG_NO_HZ_FULL_SYSIDLE */
}

- Josh Triplett

Re: [PATCH tip/core/rcu 4/9] nohz_full: Add rcu_dyntick data for scalable detection of all-idle state

[Paul E. McKenney]

On Sat, Aug 17, 2013 at 08:02:34PM -0700, Josh Triplett wrote:
> On Sat, Aug 17, 2013 at 06:49:39PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > 
> > This commit adds fields to the rcu_dyntick structure that are used to
> > detect idle CPUs.  These new fields differ from the existing ones in
> > that the existing ones consider a CPU executing in user mode to be idle,
> > where the new ones consider CPUs executing in user mode to be busy.
> > The handling of these new fields is otherwise quite similar to that for
> > the exiting fields.  This commit also adds the initialization required
> > for these fields.
> > 
> > So, why is usermode execution treated differently, with RCU considering
> > it a quiescent state equivalent to idle, while in contrast the new
> > full-system idle state detection considers usermode execution to be
> > non-idle?
> > 
> > It turns out that although one of RCU's quiescent states is usermode
> > execution, it is not a full-system idle state.  This is because the
> > purpose of the full-system idle state is not RCU, but rather determining
> > when accurate timekeeping can safely be disabled.  Whenever accurate
> > timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one
> > CPU must keep the scheduling-clock tick going.  If even one CPU is
> > executing in user mode, accurate timekeeping is requires, particularly for
> > architectures where gettimeofday() and friends do not enter the kernel.
> > Only when all CPUs are really and truly idle can accurate timekeeping be
> > disabled, allowing all CPUs to turn off the scheduling clock interrupt,
> > thus greatly improving energy efficiency.
> > 
> > This naturally raises the question "Why is this code in RCU rather than in
> > timekeeping?", and the answer is that RCU has the data and infrastructure
> > to efficiently make this determination.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> 
> One comment below.  With that change:
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> 
> > +#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> > +
> > +/*
> > + * Initialize dynticks sysidle state for CPUs coming online.
> > + */
> > +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> > +{
> > +	rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
> > +}
> > +
> > +#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> > +
> > +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> > +{
> > +}
> > +
> > +#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> 
> Just move the ifdef around the function body:
> 
> static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> {
> #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> 	rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
> #endif /* CONFIG_NO_HZ_FULL_SYSIDLE */
> }

This makes sense for this isolated function, and it would also
make sense if the end result had only functions that were exported.
But if I try to apply this to the result, I will end up with something
like the following.  Is that really what you want?

I suppose I could individually enclose whole functions whose definitions
are unneeded for CONFIG_NO_HZ_FULL_SYSIDLE=n, but that doesn't seem
helpful either.

Thoughts?

							Thanx, Paul


/*
 * Invoked to note exit from irq or task transition to idle.  Note that
 * usermode execution does -not- count as idle here!  After all, we want
 * to detect full-system idle states, not RCU quiescent states and grace
 * periods.  The caller must have disabled interrupts.
 */
static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq)
{
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
	unsigned long j;

	/* Adjust nesting, check for fully idle. */
	if (irq) {
		rdtp->dynticks_idle_nesting--;
		WARN_ON_ONCE(rdtp->dynticks_idle_nesting < 0);
		if (rdtp->dynticks_idle_nesting != 0)
			return;  /* Still not fully idle. */
	} else {
		if ((rdtp->dynticks_idle_nesting & DYNTICK_TASK_NEST_MASK) ==
		    DYNTICK_TASK_NEST_VALUE) {
			rdtp->dynticks_idle_nesting = 0;
		} else {
			rdtp->dynticks_idle_nesting -= DYNTICK_TASK_NEST_VALUE;
			WARN_ON_ONCE(rdtp->dynticks_idle_nesting < 0);
			return;  /* Still not fully idle. */
		}
	}

	/* Record start of fully idle period. */
	j = jiffies;
	ACCESS_ONCE(rdtp->dynticks_idle_jiffies) = j;
	smp_mb__before_atomic_inc();
	atomic_inc(&rdtp->dynticks_idle);
	smp_mb__after_atomic_inc();
	WARN_ON_ONCE(atomic_read(&rdtp->dynticks_idle) & 0x1);
#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
}

#ifdef CONFIG_NO_HZ_FULL_SYSIDLE

/*
 * Unconditionally force exit from full system-idle state.  This is
 * invoked when a normal CPU exits idle, but must be called separately
 * for the timekeeping CPU (tick_do_timer_cpu).  The reason for this
 * is that the timekeeping CPU is permitted to take scheduling-clock
 * interrupts while the system is in system-idle state, and of course
 * rcu_sysidle_exit() has no way of distinguishing a scheduling-clock
 * interrupt from any other type of interrupt.
 */
void rcu_sysidle_force_exit(void)
{
	int oldstate = ACCESS_ONCE(full_sysidle_state);
	int newoldstate;

	/*
	 * Each pass through the following loop attempts to exit full
	 * system-idle state.  If contention proves to be a problem,
	 * a trylock-based contention tree could be used here.
	 */
	while (oldstate > RCU_SYSIDLE_SHORT) {
		newoldstate = cmpxchg(&full_sysidle_state,
				      oldstate, RCU_SYSIDLE_NOT);
		if (oldstate == newoldstate &&
		    oldstate == RCU_SYSIDLE_FULL_NOTED) {
			rcu_kick_nohz_cpu(tick_do_timer_cpu);
			return; /* We cleared it, done! */
		}
		oldstate = newoldstate;
	}
	smp_mb(); /* Order initial oldstate fetch vs. later non-idle work. */
}

#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */

/*
 * Invoked to note entry to irq or task transition from idle.  Note that
 * usermode execution does -not- count as idle here!  The caller must
 * have disabled interrupts.
 */
static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
{
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
	/* Adjust nesting, check for already non-idle. */
	if (irq) {
		rdtp->dynticks_idle_nesting++;
		WARN_ON_ONCE(rdtp->dynticks_idle_nesting <= 0);
		if (rdtp->dynticks_idle_nesting != 1)
			return; /* Already non-idle. */
	} else {
		/*
		 * Allow for irq misnesting.  Yes, it really is possible
		 * to enter an irq handler then never leave it, and maybe
		 * also vice versa.  Handle both possibilities.
		 */
		if (rdtp->dynticks_idle_nesting & DYNTICK_TASK_NEST_MASK) {
			rdtp->dynticks_idle_nesting += DYNTICK_TASK_NEST_VALUE;
			WARN_ON_ONCE(rdtp->dynticks_idle_nesting <= 0);
			return; /* Already non-idle. */
		} else {
			rdtp->dynticks_idle_nesting = DYNTICK_TASK_EXIT_IDLE;
		}
	}

	/* Record end of idle period. */
	smp_mb__before_atomic_inc();
	atomic_inc(&rdtp->dynticks_idle);
	smp_mb__after_atomic_inc();
	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks_idle) & 0x1));

	/*
	 * If we are the timekeeping CPU, we are permitted to be non-idle
	 * during a system-idle state.  This must be the case, because
	 * the timekeeping CPU has to take scheduling-clock interrupts
	 * during the time that the system is transitioning to full
	 * system-idle state.  This means that the timekeeping CPU must
	 * invoke rcu_sysidle_force_exit() directly if it does anything
	 * more than take a scheduling-clock interrupt.
	 */
	if (smp_processor_id() == tick_do_timer_cpu)
		return;

	/* Update system-idle state: We are clearly no longer fully idle! */
	rcu_sysidle_force_exit();
#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
}

/*
 * Check to see if the current CPU is idle.  Note that usermode execution
 * does not count as idle.  The caller must have disabled interrupts.
 */
static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
				  unsigned long *maxj)
{
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
	int cur;
	unsigned long j;
	struct rcu_dynticks *rdtp = rdp->dynticks;

	/*
	 * If some other CPU has already reported non-idle, if this is
	 * not the flavor of RCU that tracks sysidle state, or if this
	 * is an offline or the timekeeping CPU, nothing to do.
	 */
	if (!*isidle || rdp->rsp != rcu_sysidle_state ||
	    cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
		return;
	if (rcu_gp_in_progress(rdp->rsp))
		WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);

	/* Pick up current idle and NMI-nesting counter and check. */
	cur = atomic_read(&rdtp->dynticks_idle);
	if (cur & 0x1) {
		*isidle = false; /* We are not idle! */
		return;
	}
	smp_mb(); /* Read counters before timestamps. */

	/* Pick up timestamps. */
	j = ACCESS_ONCE(rdtp->dynticks_idle_jiffies);
	/* If this CPU entered idle more recently, update maxj timestamp. */
	if (ULONG_CMP_LT(*maxj, j))
		*maxj = j;
#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
}

/*
 * Is this the flavor of RCU that is handling full-system idle?
 */
static bool is_sysidle_rcu_state(struct rcu_state *rsp)
{
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
	return rsp == rcu_sysidle_state;
#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
}

/*
 * Bind the grace-period kthread for the sysidle flavor of RCU to the
 * timekeeping CPU.
 */
static void rcu_bind_gp_kthread(void)
{
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
	int cpu = ACCESS_ONCE(tick_do_timer_cpu);

	if (cpu < 0 || cpu >= nr_cpu_ids)
		return;
	if (raw_smp_processor_id() != cpu)
		set_cpus_allowed_ptr(current, cpumask_of(cpu));
#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
}

#ifdef CONFIG_NO_HZ_FULL_SYSIDLE

/*
 * Return a delay in jiffies based on the number of CPUs, rcu_node
 * leaf fanout, and jiffies tick rate.  The idea is to allow larger
 * systems more time to transition to full-idle state in order to
 * avoid the cache thrashing that otherwise occur on the state variable.
 * Really small systems (less than a couple of tens of CPUs) should
 * instead use a single global atomically incremented counter, and later
 * versions of this will automatically reconfigure themselves accordingly.
 */
static unsigned long rcu_sysidle_delay(void)
{
	if (nr_cpu_ids <= RCU_SYSIDLE_SMALL)
		return 0;
	return DIV_ROUND_UP(nr_cpu_ids * HZ, rcu_fanout_leaf * 1000);
}

/*
 * Advance the full-system-idle state.  This is invoked when all of
 * the non-timekeeping CPUs are idle.
 */
static void rcu_sysidle(unsigned long j)
{
	/* Check the current state. */
	switch (ACCESS_ONCE(full_sysidle_state)) {
	case RCU_SYSIDLE_NOT:

		/* First time all are idle, so note a short idle period. */
		ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_SHORT;
		break;

	case RCU_SYSIDLE_SHORT:

		/*
		 * Idle for a bit, time to advance to next state?
		 * cmpxchg failure means race with non-idle, let them win.
		 */
		if (ULONG_CMP_GE(jiffies, j + rcu_sysidle_delay()))
			(void)cmpxchg(&full_sysidle_state,
				      RCU_SYSIDLE_SHORT, RCU_SYSIDLE_LONG);
		break;

	case RCU_SYSIDLE_LONG:

		/*
		 * Do an additional check pass before advancing to full.
		 * cmpxchg failure means race with non-idle, let them win.
		 */
		if (ULONG_CMP_GE(jiffies, j + rcu_sysidle_delay()))
			(void)cmpxchg(&full_sysidle_state,
				      RCU_SYSIDLE_LONG, RCU_SYSIDLE_FULL);
		break;

	default:
		break;
	}
}

/*
 * Found a non-idle non-timekeeping CPU, so kick the system-idle state
 * back to the beginning.
 */
static void rcu_sysidle_cancel(void)
{
	smp_mb();
	ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_NOT;
}

/*
 * Update the sysidle state based on the results of a force-quiescent-state
 * scan of the CPUs' dyntick-idle state.
 */
static void rcu_sysidle_report(struct rcu_state *rsp, int isidle,
			       unsigned long maxj, bool gpkt)
{
	if (rsp != rcu_sysidle_state)
		return;  /* Wrong flavor, ignore. */
	if (isidle) {
		if (gpkt && nr_cpu_ids > RCU_SYSIDLE_SMALL)
			rcu_sysidle(maxj);    /* More idle! */
	} else {
		rcu_sysidle_cancel(); /* Idle is over. */
	}
}

#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */

static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
				  unsigned long maxj)
{
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
	rcu_sysidle_report(rsp, isidle, maxj, true);
#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
}

#ifdef CONFIG_NO_HZ_FULL_SYSIDLE

/* Callback and function for forcing an RCU grace period. */
struct rcu_sysidle_head {
	struct rcu_head rh;
	int inuse;
};

static void rcu_sysidle_cb(struct rcu_head *rhp)
{
	struct rcu_sysidle_head *rshp;

	smp_mb();  /* grace period precedes setting inuse. */
	rshp = container_of(rhp, struct rcu_sysidle_head, rh);
	ACCESS_ONCE(rshp->inuse) = 0;
}

/*
 * Check to see if the system is fully idle, other than the timekeeping CPU.
 * The caller must have disabled interrupts.
 */
bool rcu_sys_is_idle(void)
{
	static struct rcu_sysidle_head rsh;
	int rss = ACCESS_ONCE(full_sysidle_state);

	if (WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu))
		return false;

	/* Handle small-system case by doing a full scan of CPUs. */
	if (nr_cpu_ids <= RCU_SYSIDLE_SMALL) {
		int oldrss = rss - 1;

		/*
		 * One pass to advance to each state up to _FULL.
		 * Give up if any pass fails to advance the state.
		 */
		while (rss < RCU_SYSIDLE_FULL && oldrss < rss) {
			int cpu;
			bool isidle = true;
			unsigned long maxj = jiffies - ULONG_MAX / 4;
			struct rcu_data *rdp;

			/* Scan all the CPUs looking for nonidle CPUs. */
			for_each_possible_cpu(cpu) {
				rdp = per_cpu_ptr(rcu_sysidle_state->rda, cpu);
				rcu_sysidle_check_cpu(rdp, &isidle, &maxj);
				if (!isidle)
					break;
			}
			rcu_sysidle_report(rcu_sysidle_state,
					   isidle, maxj, false);
			oldrss = rss;
			rss = ACCESS_ONCE(full_sysidle_state);
		}
	}

	/* If this is the first observation of an idle period, record it. */
	if (rss == RCU_SYSIDLE_FULL) {
		rss = cmpxchg(&full_sysidle_state,
			      RCU_SYSIDLE_FULL, RCU_SYSIDLE_FULL_NOTED);
		return rss == RCU_SYSIDLE_FULL;
	}

	smp_mb(); /* ensure rss load happens before later caller actions. */

	/* If already fully idle, tell the caller (in case of races). */
	if (rss == RCU_SYSIDLE_FULL_NOTED)
		return true;

	/*
	 * If we aren't there yet, and a grace period is not in flight,
	 * initiate a grace period.  Either way, tell the caller that
	 * we are not there yet.
	 */
	if (nr_cpu_ids > RCU_SYSIDLE_SMALL &&
	    !rcu_gp_in_progress(rcu_sysidle_state) &&
	    !rsh.inuse && xchg(&rsh.inuse, 1) == 0)
		call_rcu(&rsh.rh, rcu_sysidle_cb);
	return false;
}

#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */

/*
 * Initialize dynticks sysidle state for CPUs coming online.
 */
static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
{
#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
	rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
#endif /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
}

#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */

static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq)
{
}

static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
{
}

static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
				  unsigned long *maxj)
{
}

static bool is_sysidle_rcu_state(struct rcu_state *rsp)
{
	return false;
}

static void rcu_bind_gp_kthread(void)
{
}

static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
				  unsigned long maxj)
{
}

static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
{
}

#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */

Re: [PATCH tip/core/rcu 4/9] nohz_full: Add rcu_dyntick data for scalable detection of all-idle state

[Josh Triplett]

On Sun, Aug 18, 2013 at 06:22:29PM -0700, Paul E. McKenney wrote:
> On Sat, Aug 17, 2013 at 08:02:34PM -0700, Josh Triplett wrote:
> > On Sat, Aug 17, 2013 at 06:49:39PM -0700, Paul E. McKenney wrote:
> > > +#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> > > +
> > > +/*
> > > + * Initialize dynticks sysidle state for CPUs coming online.
> > > + */
> > > +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> > > +{
> > > +	rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
> > > +}
> > > +
> > > +#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> > > +
> > > +static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> > > +{
> > > +}
> > > +
> > > +#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
> > 
> > Just move the ifdef around the function body:
> > 
> > static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
> > {
> > #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> > 	rdtp->dynticks_idle_nesting = DYNTICK_TASK_NEST_VALUE;
> > #endif /* CONFIG_NO_HZ_FULL_SYSIDLE */
> > }
> 
> This makes sense for this isolated function, and it would also
> make sense if the end result had only functions that were exported.
> But if I try to apply this to the result, I will end up with something
> like the following.  Is that really what you want?
> 
> I suppose I could individually enclose whole functions whose definitions
> are unneeded for CONFIG_NO_HZ_FULL_SYSIDLE=n, but that doesn't seem
> helpful either.
> 
> Thoughts?

I see what you mean.  Short of sorting the functions to put all the
unexported ones together, which seems suboptimal, I don't see an obvious
fix.  The result you posted isn't *terrible*, but it's not great either.

I had mostly hoped to avoid having two duplicate function headers that
would then both need changing whenever changing the function signature,
and which could then potentially get out of sync without causing a
compilation error.

I'd say that if you have a single-function block like the one above, you
should use the ifdef-body approach, but if you've got a group of
functions that don't all use one or the other approach, go ahead and
wrap the whole thing in one big ifdef rather than one per function.

Use whichever approach seems most sensible on a case-by-case basis; your
call.  Feel free to add a
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
to whichever approach you go with.

- Josh Triplett

[PATCH tip/core/rcu 5/9] nohz_full: Add per-CPU idle-state tracking

[Paul E. McKenney]

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

This commit adds the code that updates the rcu_dyntick structure's
new fields to track the per-CPU idle state based on interrupts and
transitions into and out of the idle loop (NMIs are ignored because NMI
handlers cannot cleanly read out the time anyway).  This code is similar
to the code that maintains RCU's idea of per-CPU idleness, but differs
in that RCU treats CPUs running in user mode as idle, where this new
code does not.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/rcutree.c        |  4 +++
 kernel/rcutree.h        |  2 ++
 kernel/rcutree_plugin.h | 79 +++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 85 insertions(+)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 4f27b85..b0d2cc3 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -431,6 +431,7 @@ void rcu_idle_enter(void)
 
 	local_irq_save(flags);
 	rcu_eqs_enter(false);
+	rcu_sysidle_enter(&__get_cpu_var(rcu_dynticks), 0);
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_enter);
@@ -481,6 +482,7 @@ void rcu_irq_exit(void)
 		trace_rcu_dyntick(TPS("--="), oldval, rdtp->dynticks_nesting);
 	else
 		rcu_eqs_enter_common(rdtp, oldval, true);
+	rcu_sysidle_enter(rdtp, 1);
 	local_irq_restore(flags);
 }
 
@@ -549,6 +551,7 @@ void rcu_idle_exit(void)
 
 	local_irq_save(flags);
 	rcu_eqs_exit(false);
+	rcu_sysidle_exit(&__get_cpu_var(rcu_dynticks), 0);
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_exit);
@@ -600,6 +603,7 @@ void rcu_irq_enter(void)
 		trace_rcu_dyntick(TPS("++="), oldval, rdtp->dynticks_nesting);
 	else
 		rcu_eqs_exit_common(rdtp, oldval, true);
+	rcu_sysidle_exit(rdtp, 1);
 	local_irq_restore(flags);
 }
 
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 52d1be1..9dd8b17 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -553,6 +553,8 @@ static void rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp);
 static void rcu_spawn_nocb_kthreads(struct rcu_state *rsp);
 static void rcu_kick_nohz_cpu(int cpu);
 static bool init_nocb_callback_list(struct rcu_data *rdp);
+static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq);
+static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq);
 static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);
 
 #endif /* #ifndef RCU_TREE_NONCORE */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index e5baccb..eab81da 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2378,6 +2378,77 @@ static void rcu_kick_nohz_cpu(int cpu)
 #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
 
 /*
+ * Invoked to note exit from irq or task transition to idle.  Note that
+ * usermode execution does -not- count as idle here!  After all, we want
+ * to detect full-system idle states, not RCU quiescent states and grace
+ * periods.  The caller must have disabled interrupts.
+ */
+static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq)
+{
+	unsigned long j;
+
+	/* Adjust nesting, check for fully idle. */
+	if (irq) {
+		rdtp->dynticks_idle_nesting--;
+		WARN_ON_ONCE(rdtp->dynticks_idle_nesting < 0);
+		if (rdtp->dynticks_idle_nesting != 0)
+			return;  /* Still not fully idle. */
+	} else {
+		if ((rdtp->dynticks_idle_nesting & DYNTICK_TASK_NEST_MASK) ==
+		    DYNTICK_TASK_NEST_VALUE) {
+			rdtp->dynticks_idle_nesting = 0;
+		} else {
+			rdtp->dynticks_idle_nesting -= DYNTICK_TASK_NEST_VALUE;
+			WARN_ON_ONCE(rdtp->dynticks_idle_nesting < 0);
+			return;  /* Still not fully idle. */
+		}
+	}
+
+	/* Record start of fully idle period. */
+	j = jiffies;
+	ACCESS_ONCE(rdtp->dynticks_idle_jiffies) = j;
+	smp_mb__before_atomic_inc();
+	atomic_inc(&rdtp->dynticks_idle);
+	smp_mb__after_atomic_inc();
+	WARN_ON_ONCE(atomic_read(&rdtp->dynticks_idle) & 0x1);
+}
+
+/*
+ * Invoked to note entry to irq or task transition from idle.  Note that
+ * usermode execution does -not- count as idle here!  The caller must
+ * have disabled interrupts.
+ */
+static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
+{
+	/* Adjust nesting, check for already non-idle. */
+	if (irq) {
+		rdtp->dynticks_idle_nesting++;
+		WARN_ON_ONCE(rdtp->dynticks_idle_nesting <= 0);
+		if (rdtp->dynticks_idle_nesting != 1)
+			return; /* Already non-idle. */
+	} else {
+		/*
+		 * Allow for irq misnesting.  Yes, it really is possible
+		 * to enter an irq handler then never leave it, and maybe
+		 * also vice versa.  Handle both possibilities.
+		 */
+		if (rdtp->dynticks_idle_nesting & DYNTICK_TASK_NEST_MASK) {
+			rdtp->dynticks_idle_nesting += DYNTICK_TASK_NEST_VALUE;
+			WARN_ON_ONCE(rdtp->dynticks_idle_nesting <= 0);
+			return; /* Already non-idle. */
+		} else {
+			rdtp->dynticks_idle_nesting = DYNTICK_TASK_EXIT_IDLE;
+		}
+	}
+
+	/* Record end of idle period. */
+	smp_mb__before_atomic_inc();
+	atomic_inc(&rdtp->dynticks_idle);
+	smp_mb__after_atomic_inc();
+	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks_idle) & 0x1));
+}
+
+/*
  * Initialize dynticks sysidle state for CPUs coming online.
  */
 static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
@@ -2387,6 +2458,14 @@ static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
 
 #else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
 
+static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq)
+{
+}
+
+static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
+{
+}
+
 static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
 {
 }
-- 
1.8.1.5

Re: [PATCH tip/core/rcu 5/9] nohz_full: Add per-CPU idle-state tracking

[Josh Triplett]

On Sat, Aug 17, 2013 at 06:49:40PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> This commit adds the code that updates the rcu_dyntick structure's
> new fields to track the per-CPU idle state based on interrupts and
> transitions into and out of the idle loop (NMIs are ignored because NMI
> handlers cannot cleanly read out the time anyway).  This code is similar
> to the code that maintains RCU's idea of per-CPU idleness, but differs
> in that RCU treats CPUs running in user mode as idle, where this new
> code does not.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>

Same comment here about using ifdef around function bodies.

- Josh Triplett

[PATCH tip/core/rcu 6/9] nohz_full: Add full-system idle states and variables

[Paul E. McKenney]

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

This commit adds control variables and states for full-system idle.
The system will progress through the states in numerical order when
the system is fully idle (other than the timekeeping CPU), and reset
down to the initial state if any non-timekeeping CPU goes non-idle.
The current state is kept in full_sysidle_state.

A RCU_SYSIDLE_SMALL macro is defined, and systems with this number
of CPUs or fewer move through the states more aggressively.  The idea
is that the resulting memory contention is less of a problem on small
systems.  Architectures can adjust this value (which defaults to 8)
using CONFIG_ARCH_RCU_SYSIDLE_SMALL.

One flavor of RCU will be in charge of driving the state machine,
defined by rcu_sysidle_state.  This should be the busiest flavor of RCU.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
---
 kernel/rcutree_plugin.h | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index eab81da..64a05b9f 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2378,6 +2378,34 @@ static void rcu_kick_nohz_cpu(int cpu)
 #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
 
 /*
+ * Handle small systems specially, accelerating their transition into
+ * full idle state.  Allow arches to override this code's idea of
+ * what constitutes a "small" system.
+ */
+#ifdef CONFIG_ARCH_RCU_SYSIDLE_SMALL
+#define RCU_SYSIDLE_SMALL CONFIG_ARCH_RCU_SYSIDLE_SMALL
+#else /* #ifdef CONFIG_ARCH_RCU_SYSIDLE_SMALL */
+#define RCU_SYSIDLE_SMALL 8
+#endif
+
+/*
+ * Define RCU flavor that holds sysidle state.  This needs to be the
+ * most active flavor of RCU.
+ */
+#ifdef CONFIG_PREEMPT_RCU
+static struct rcu_state __maybe_unused *rcu_sysidle_state = &rcu_preempt_state;
+#else /* #ifdef CONFIG_PREEMPT_RCU */
+static struct rcu_state __maybe_unused *rcu_sysidle_state = &rcu_sched_state;
+#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
+
+static int __maybe_unused full_sysidle_state; /* Current system-idle state. */
+#define RCU_SYSIDLE_NOT		0	/* Some CPU is not idle. */
+#define RCU_SYSIDLE_SHORT	1	/* All CPUs idle for brief period. */
+#define RCU_SYSIDLE_LONG	2	/* All CPUs idle for long enough. */
+#define RCU_SYSIDLE_FULL	3	/* All CPUs idle, ready for sysidle. */
+#define RCU_SYSIDLE_FULL_NOTED	4	/* Actually entered sysidle state. */
+
+/*
  * Invoked to note exit from irq or task transition to idle.  Note that
  * usermode execution does -not- count as idle here!  After all, we want
  * to detect full-system idle states, not RCU quiescent states and grace
-- 
1.8.1.5

Re: [PATCH tip/core/rcu 6/9] nohz_full: Add full-system idle states and variables

[Josh Triplett]

On Sat, Aug 17, 2013 at 06:49:41PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> This commit adds control variables and states for full-system idle.
> The system will progress through the states in numerical order when
> the system is fully idle (other than the timekeeping CPU), and reset
> down to the initial state if any non-timekeeping CPU goes non-idle.
> The current state is kept in full_sysidle_state.
> 
> A RCU_SYSIDLE_SMALL macro is defined, and systems with this number
> of CPUs or fewer move through the states more aggressively.  The idea
> is that the resulting memory contention is less of a problem on small
> systems.  Architectures can adjust this value (which defaults to 8)
> using CONFIG_ARCH_RCU_SYSIDLE_SMALL.
> 
> One flavor of RCU will be in charge of driving the state machine,
> defined by rcu_sysidle_state.  This should be the busiest flavor of RCU.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>

One issue (and one question) below; with the issue addressed,
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

>  kernel/rcutree_plugin.h | 28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> index eab81da..64a05b9f 100644
> --- a/kernel/rcutree_plugin.h
> +++ b/kernel/rcutree_plugin.h
> @@ -2378,6 +2378,34 @@ static void rcu_kick_nohz_cpu(int cpu)
>  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
>  
>  /*
> + * Handle small systems specially, accelerating their transition into
> + * full idle state.  Allow arches to override this code's idea of
> + * what constitutes a "small" system.
> + */
> +#ifdef CONFIG_ARCH_RCU_SYSIDLE_SMALL

I don't see any Kconfig creating this new config option.

Also, why not simply define this config option unconditionally, with a
default of 8, and then use its value directly?

> +static int __maybe_unused full_sysidle_state; /* Current system-idle state. */
> +#define RCU_SYSIDLE_NOT		0	/* Some CPU is not idle. */
> +#define RCU_SYSIDLE_SHORT	1	/* All CPUs idle for brief period. */
> +#define RCU_SYSIDLE_LONG	2	/* All CPUs idle for long enough. */
> +#define RCU_SYSIDLE_FULL	3	/* All CPUs idle, ready for sysidle. */
> +#define RCU_SYSIDLE_FULL_NOTED	4	/* Actually entered sysidle state. */

Perhaps there's a kernel style rule I'm not thinking of that makes it
verboten, but: why not use an enum for a state variable like this?

- Josh Triplett

Re: [PATCH tip/core/rcu 6/9] nohz_full: Add full-system idle states and variables

[Paul E. McKenney]

On Sat, Aug 17, 2013 at 08:09:21PM -0700, Josh Triplett wrote:
> On Sat, Aug 17, 2013 at 06:49:41PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > 
> > This commit adds control variables and states for full-system idle.
> > The system will progress through the states in numerical order when
> > the system is fully idle (other than the timekeeping CPU), and reset
> > down to the initial state if any non-timekeeping CPU goes non-idle.
> > The current state is kept in full_sysidle_state.
> > 
> > A RCU_SYSIDLE_SMALL macro is defined, and systems with this number
> > of CPUs or fewer move through the states more aggressively.  The idea
> > is that the resulting memory contention is less of a problem on small
> > systems.  Architectures can adjust this value (which defaults to 8)
> > using CONFIG_ARCH_RCU_SYSIDLE_SMALL.
> > 
> > One flavor of RCU will be in charge of driving the state machine,
> > defined by rcu_sysidle_state.  This should be the busiest flavor of RCU.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> 
> One issue (and one question) below; with the issue addressed,
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> 
> >  kernel/rcutree_plugin.h | 28 ++++++++++++++++++++++++++++
> >  1 file changed, 28 insertions(+)
> > 
> > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > index eab81da..64a05b9f 100644
> > --- a/kernel/rcutree_plugin.h
> > +++ b/kernel/rcutree_plugin.h
> > @@ -2378,6 +2378,34 @@ static void rcu_kick_nohz_cpu(int cpu)
> >  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> >  
> >  /*
> > + * Handle small systems specially, accelerating their transition into
> > + * full idle state.  Allow arches to override this code's idea of
> > + * what constitutes a "small" system.
> > + */
> > +#ifdef CONFIG_ARCH_RCU_SYSIDLE_SMALL
> 
> I don't see any Kconfig creating this new config option.
> 
> Also, why not simply define this config option unconditionally, with a
> default of 8, and then use its value directly?

Good point, removing this and adding a Kconfig option in the
"nohz_full: Add full-system-idle state machine" commit, with a
default value of 8.  Architecture maintainers who want something
different can then set that up in their defconfig files.

> > +static int __maybe_unused full_sysidle_state; /* Current system-idle state. */
> > +#define RCU_SYSIDLE_NOT		0	/* Some CPU is not idle. */
> > +#define RCU_SYSIDLE_SHORT	1	/* All CPUs idle for brief period. */
> > +#define RCU_SYSIDLE_LONG	2	/* All CPUs idle for long enough. */
> > +#define RCU_SYSIDLE_FULL	3	/* All CPUs idle, ready for sysidle. */
> > +#define RCU_SYSIDLE_FULL_NOTED	4	/* Actually entered sysidle state. */
> 
> Perhaps there's a kernel style rule I'm not thinking of that makes it
> verboten, but: why not use an enum for a state variable like this?

I didn't trust enum interactions with xchg and cmpxchg, so opted for "int"
instead.  That said, enum is much more portable than when I last looked
at it.  Admittedly, the last time I looked at it was in the early 1980s...

							Thanx, Paul

Re: [PATCH tip/core/rcu 6/9] nohz_full: Add full-system idle states and variables

[Josh Triplett]

On Sun, Aug 18, 2013 at 06:39:25PM -0700, Paul E. McKenney wrote:
> On Sat, Aug 17, 2013 at 08:09:21PM -0700, Josh Triplett wrote:
> > On Sat, Aug 17, 2013 at 06:49:41PM -0700, Paul E. McKenney wrote:
> > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > 
> > > This commit adds control variables and states for full-system idle.
> > > The system will progress through the states in numerical order when
> > > the system is fully idle (other than the timekeeping CPU), and reset
> > > down to the initial state if any non-timekeeping CPU goes non-idle.
> > > The current state is kept in full_sysidle_state.
> > > 
> > > A RCU_SYSIDLE_SMALL macro is defined, and systems with this number
> > > of CPUs or fewer move through the states more aggressively.  The idea
> > > is that the resulting memory contention is less of a problem on small
> > > systems.  Architectures can adjust this value (which defaults to 8)
> > > using CONFIG_ARCH_RCU_SYSIDLE_SMALL.
> > > 
> > > One flavor of RCU will be in charge of driving the state machine,
> > > defined by rcu_sysidle_state.  This should be the busiest flavor of RCU.
> > > 
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Cc: Frederic Weisbecker <fweisbec@gmail.com>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > 
> > One issue (and one question) below; with the issue addressed,
> > Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> > 
> > >  kernel/rcutree_plugin.h | 28 ++++++++++++++++++++++++++++
> > >  1 file changed, 28 insertions(+)
> > > 
> > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > > index eab81da..64a05b9f 100644
> > > --- a/kernel/rcutree_plugin.h
> > > +++ b/kernel/rcutree_plugin.h
> > > @@ -2378,6 +2378,34 @@ static void rcu_kick_nohz_cpu(int cpu)
> > >  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> > >  
> > >  /*
> > > + * Handle small systems specially, accelerating their transition into
> > > + * full idle state.  Allow arches to override this code's idea of
> > > + * what constitutes a "small" system.
> > > + */
> > > +#ifdef CONFIG_ARCH_RCU_SYSIDLE_SMALL
> > 
> > I don't see any Kconfig creating this new config option.
> > 
> > Also, why not simply define this config option unconditionally, with a
> > default of 8, and then use its value directly?
> 
> Good point, removing this and adding a Kconfig option in the
> "nohz_full: Add full-system-idle state machine" commit, with a
> default value of 8.  Architecture maintainers who want something
> different can then set that up in their defconfig files.

Sounds good.

> > > +static int __maybe_unused full_sysidle_state; /* Current system-idle state. */
> > > +#define RCU_SYSIDLE_NOT		0	/* Some CPU is not idle. */
> > > +#define RCU_SYSIDLE_SHORT	1	/* All CPUs idle for brief period. */
> > > +#define RCU_SYSIDLE_LONG	2	/* All CPUs idle for long enough. */
> > > +#define RCU_SYSIDLE_FULL	3	/* All CPUs idle, ready for sysidle. */
> > > +#define RCU_SYSIDLE_FULL_NOTED	4	/* Actually entered sysidle state. */
> > 
> > Perhaps there's a kernel style rule I'm not thinking of that makes it
> > verboten, but: why not use an enum for a state variable like this?
> 
> I didn't trust enum interactions with xchg and cmpxchg, so opted for "int"
> instead.  That said, enum is much more portable than when I last looked
> at it.  Admittedly, the last time I looked at it was in the early 1980s...

That would make sense if this was an atomic_t, but it's an int; unless I
missed something, you don't currently use xchg or cmpxchg on it.

- Josh Triplett

Re: [PATCH tip/core/rcu 6/9] nohz_full: Add full-system idle states and variables

[Paul E. McKenney]

On Sun, Aug 18, 2013 at 07:49:14PM -0700, Josh Triplett wrote:
> On Sun, Aug 18, 2013 at 06:39:25PM -0700, Paul E. McKenney wrote:
> > On Sat, Aug 17, 2013 at 08:09:21PM -0700, Josh Triplett wrote:
> > > On Sat, Aug 17, 2013 at 06:49:41PM -0700, Paul E. McKenney wrote:
> > > > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > > 
> > > > This commit adds control variables and states for full-system idle.
> > > > The system will progress through the states in numerical order when
> > > > the system is fully idle (other than the timekeeping CPU), and reset
> > > > down to the initial state if any non-timekeeping CPU goes non-idle.
> > > > The current state is kept in full_sysidle_state.
> > > > 
> > > > A RCU_SYSIDLE_SMALL macro is defined, and systems with this number
> > > > of CPUs or fewer move through the states more aggressively.  The idea
> > > > is that the resulting memory contention is less of a problem on small
> > > > systems.  Architectures can adjust this value (which defaults to 8)
> > > > using CONFIG_ARCH_RCU_SYSIDLE_SMALL.
> > > > 
> > > > One flavor of RCU will be in charge of driving the state machine,
> > > > defined by rcu_sysidle_state.  This should be the busiest flavor of RCU.
> > > > 
> > > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > > Cc: Frederic Weisbecker <fweisbec@gmail.com>
> > > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > 
> > > One issue (and one question) below; with the issue addressed,
> > > Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> > > 
> > > >  kernel/rcutree_plugin.h | 28 ++++++++++++++++++++++++++++
> > > >  1 file changed, 28 insertions(+)
> > > > 
> > > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
> > > > index eab81da..64a05b9f 100644
> > > > --- a/kernel/rcutree_plugin.h
> > > > +++ b/kernel/rcutree_plugin.h
> > > > @@ -2378,6 +2378,34 @@ static void rcu_kick_nohz_cpu(int cpu)
> > > >  #ifdef CONFIG_NO_HZ_FULL_SYSIDLE
> > > >  
> > > >  /*
> > > > + * Handle small systems specially, accelerating their transition into
> > > > + * full idle state.  Allow arches to override this code's idea of
> > > > + * what constitutes a "small" system.
> > > > + */
> > > > +#ifdef CONFIG_ARCH_RCU_SYSIDLE_SMALL
> > > 
> > > I don't see any Kconfig creating this new config option.
> > > 
> > > Also, why not simply define this config option unconditionally, with a
> > > default of 8, and then use its value directly?
> > 
> > Good point, removing this and adding a Kconfig option in the
> > "nohz_full: Add full-system-idle state machine" commit, with a
> > default value of 8.  Architecture maintainers who want something
> > different can then set that up in their defconfig files.
> 
> Sounds good.
> 
> > > > +static int __maybe_unused full_sysidle_state; /* Current system-idle state. */
> > > > +#define RCU_SYSIDLE_NOT		0	/* Some CPU is not idle. */
> > > > +#define RCU_SYSIDLE_SHORT	1	/* All CPUs idle for brief period. */
> > > > +#define RCU_SYSIDLE_LONG	2	/* All CPUs idle for long enough. */
> > > > +#define RCU_SYSIDLE_FULL	3	/* All CPUs idle, ready for sysidle. */
> > > > +#define RCU_SYSIDLE_FULL_NOTED	4	/* Actually entered sysidle state. */
> > > 
> > > Perhaps there's a kernel style rule I'm not thinking of that makes it
> > > verboten, but: why not use an enum for a state variable like this?
> > 
> > I didn't trust enum interactions with xchg and cmpxchg, so opted for "int"
> > instead.  That said, enum is much more portable than when I last looked
> > at it.  Admittedly, the last time I looked at it was in the early 1980s...
> 
> That would make sense if this was an atomic_t, but it's an int; unless I
> missed something, you don't currently use xchg or cmpxchg on it.

The xchg and cmpxchg show up in the "Add full-system-idle state machine"
commit.  Of course, now I am trying to remember why I used int instead
of atomic_t in this case...  :-/

							Thanx, Paul

[PATCH tip/core/rcu 7/9] nohz_full: Add full-system-idle arguments to API

[Paul E. McKenney]

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

This commit adds an isidle and jiffies argument to force_qs_rnp(),
dyntick_save_progress_counter(), and rcu_implicit_dynticks_qs() to enable
RCU's force-quiescent-state process to check for full-system idle.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
[ paulmck: Use true and false for boolean constants per Lai Jiangshan. ]
---
 kernel/rcutree.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index b0d2cc3..f1a0b05 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -246,7 +246,9 @@ module_param(jiffies_till_next_fqs, ulong, 0644);
 
 static void rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
 				  struct rcu_data *rdp);
-static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
+static void force_qs_rnp(struct rcu_state *rsp,
+			 int (*f)(struct rcu_data *, bool *, unsigned long *),
+			 bool *isidle, unsigned long *maxj);
 static void force_quiescent_state(struct rcu_state *rsp);
 static int rcu_pending(int cpu);
 
@@ -727,7 +729,8 @@ static int rcu_is_cpu_rrupt_from_idle(void)
  * credit them with an implicit quiescent state.  Return 1 if this CPU
  * is in dynticks idle mode, which is an extended quiescent state.
  */
-static int dyntick_save_progress_counter(struct rcu_data *rdp)
+static int dyntick_save_progress_counter(struct rcu_data *rdp,
+					 bool *isidle, unsigned long *maxj)
 {
 	rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
 	return (rdp->dynticks_snap & 0x1) == 0;
@@ -739,7 +742,8 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp)
  * idle state since the last call to dyntick_save_progress_counter()
  * for this same CPU, or by virtue of having been offline.
  */
-static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
+static int rcu_implicit_dynticks_qs(struct rcu_data *rdp,
+				    bool *isidle, unsigned long *maxj)
 {
 	unsigned int curr;
 	unsigned int snap;
@@ -1361,16 +1365,19 @@ static int rcu_gp_init(struct rcu_state *rsp)
 int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
 {
 	int fqs_state = fqs_state_in;
+	bool isidle = false;
+	unsigned long maxj;
 	struct rcu_node *rnp = rcu_get_root(rsp);
 
 	rsp->n_force_qs++;
 	if (fqs_state == RCU_SAVE_DYNTICK) {
 		/* Collect dyntick-idle snapshots. */
-		force_qs_rnp(rsp, dyntick_save_progress_counter);
+		force_qs_rnp(rsp, dyntick_save_progress_counter,
+			     &isidle, &maxj);
 		fqs_state = RCU_FORCE_QS;
 	} else {
 		/* Handle dyntick-idle and offline CPUs. */
-		force_qs_rnp(rsp, rcu_implicit_dynticks_qs);
+		force_qs_rnp(rsp, rcu_implicit_dynticks_qs, &isidle, &maxj);
 	}
 	/* Clear flag to prevent immediate re-entry. */
 	if (ACCESS_ONCE(rsp->gp_flags) & RCU_GP_FLAG_FQS) {
@@ -2069,7 +2076,9 @@ void rcu_check_callbacks(int cpu, int user)
  *
  * The caller must have suppressed start of new grace periods.
  */
-static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
+static void force_qs_rnp(struct rcu_state *rsp,
+			 int (*f)(struct rcu_data *, bool *, unsigned long *),
+			 bool *isidle, unsigned long *maxj)
 {
 	unsigned long bit;
 	int cpu;
@@ -2093,7 +2102,7 @@ static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *))
 		bit = 1;
 		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
 			if ((rnp->qsmask & bit) != 0 &&
-			    f(per_cpu_ptr(rsp->rda, cpu)))
+			    f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
 				mask |= bit;
 		}
 		if (mask != 0) {
-- 
1.8.1.5

Re: [PATCH tip/core/rcu 7/9] nohz_full: Add full-system-idle arguments to API

[Josh Triplett]

On Sat, Aug 17, 2013 at 06:49:42PM -0700, Paul E. McKenney wrote:
> From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> 
> This commit adds an isidle and jiffies argument to force_qs_rnp(),
> dyntick_save_progress_counter(), and rcu_implicit_dynticks_qs() to enable
> RCU's force-quiescent-state process to check for full-system idle.
> 
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> [ paulmck: Use true and false for boolean constants per Lai Jiangshan. ]

One optional comment below; with or without that,
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

>  kernel/rcutree.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> index b0d2cc3..f1a0b05 100644
> --- a/kernel/rcutree.c
> +++ b/kernel/rcutree.c
> @@ -246,7 +246,9 @@ module_param(jiffies_till_next_fqs, ulong, 0644);
>  
>  static void rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
>  				  struct rcu_data *rdp);
> -static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
> +static void force_qs_rnp(struct rcu_state *rsp,
> +			 int (*f)(struct rcu_data *, bool *, unsigned long *),
> +			 bool *isidle, unsigned long *maxj);

You might consider giving the parameters of the function pointer names
(both here and in the definition), to make it more self-documenting.

- Josh Triplett

Re: [PATCH tip/core/rcu 7/9] nohz_full: Add full-system-idle arguments to API

[Paul E. McKenney]

On Sat, Aug 17, 2013 at 08:11:20PM -0700, Josh Triplett wrote:
> On Sat, Aug 17, 2013 at 06:49:42PM -0700, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > 
> > This commit adds an isidle and jiffies argument to force_qs_rnp(),
> > dyntick_save_progress_counter(), and rcu_implicit_dynticks_qs() to enable
> > RCU's force-quiescent-state process to check for full-system idle.
> > 
> > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Frederic Weisbecker <fweisbec@gmail.com>
> > Cc: Steven Rostedt <rostedt@goodmis.org>
> > Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> > [ paulmck: Use true and false for boolean constants per Lai Jiangshan. ]
> 
> One optional comment below; with or without that,
> Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> 
> >  kernel/rcutree.c | 23 ++++++++++++++++-------
> >  1 file changed, 16 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
> > index b0d2cc3..f1a0b05 100644
> > --- a/kernel/rcutree.c
> > +++ b/kernel/rcutree.c
> > @@ -246,7 +246,9 @@ module_param(jiffies_till_next_fqs, ulong, 0644);
> >  
> >  static void rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
> >  				  struct rcu_data *rdp);
> > -static void force_qs_rnp(struct rcu_state *rsp, int (*f)(struct rcu_data *));
> > +static void force_qs_rnp(struct rcu_state *rsp,
> > +			 int (*f)(struct rcu_data *, bool *, unsigned long *),
> > +			 bool *isidle, unsigned long *maxj);
> 
> You might consider giving the parameters of the function pointer names
> (both here and in the definition), to make it more self-documenting.

Forces a line break, but given that I couldn't immediately recall all
of the parameter names myself, I made the change.  ;-)

							Thanx, Paul

[PATCH tip/core/rcu 8/9] nohz_full: Add full-system-idle state machine

[Paul E. McKenney]

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

This commit adds the state machine that takes the per-CPU idle data
as input and produces a full-system-idle indication as output.  This
state machine is driven out of RCU's quiescent-state-forcing
mechanism, which invokes rcu_sysidle_check_cpu() to collect per-CPU
idle state and then rcu_sysidle_report() to drive the state machine.

The full-system-idle state is sampled using rcu_sys_is_idle(), which
also drives the state machine if RCU is idle (and does so by forcing
RCU to become non-idle).  This function returns true if all but the
timekeeping CPU (tick_do_timer_cpu) are idle and have been idle long
enough to avoid memory contention on the full_sysidle_state state
variable.  The rcu_sysidle_force_exit() may be called externally
to reset the state machine back into non-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>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
[ paulmck: Use true and false for boolean constants per Lai Jiangshan. ]
---
 include/linux/rcupdate.h |  18 +++
 kernel/rcutree.c         |  16 ++-
 kernel/rcutree.h         |   5 +
 kernel/rcutree_plugin.h  | 284 ++++++++++++++++++++++++++++++++++++++++++++++-
 4 files changed, 316 insertions(+), 7 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 30bea9c..f1f1bc3 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -1011,4 +1011,22 @@ static inline bool rcu_is_nocb_cpu(int cpu) { return false; }
 #endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */
 
 
+/* Only for use by adaptive-ticks code. */
+#ifdef CONFIG_NO_HZ_FULL_SYSIDLE
+extern bool rcu_sys_is_idle(void);
+extern void rcu_sysidle_force_exit(void);
+#else /* #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
+
+static inline bool rcu_sys_is_idle(void)
+{
+	return false;
+}
+
+static inline void rcu_sysidle_force_exit(void)
+{
+}
+
+#endif /* #else #ifdef CONFIG_NO_HZ_FULL_SYSIDLE */
+
+
 #endif /* __LINUX_RCUPDATE_H */
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index f1a0b05..3d898e5 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -733,6 +733,7 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp,
 					 bool *isidle, unsigned long *maxj)
 {
 	rdp->dynticks_snap = atomic_add_return(0, &rdp->dynticks->dynticks);
+	rcu_sysidle_check_cpu(rdp, isidle, maxj);
 	return (rdp->dynticks_snap & 0x1) == 0;
 }
 
@@ -1372,11 +1373,17 @@ int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
 	rsp->n_force_qs++;
 	if (fqs_state == RCU_SAVE_DYNTICK) {
 		/* Collect dyntick-idle snapshots. */
+		if (is_sysidle_rcu_state(rsp)) {
+			isidle = 1;
+			maxj = jiffies - ULONG_MAX / 4;
+		}
 		force_qs_rnp(rsp, dyntick_save_progress_counter,
 			     &isidle, &maxj);
+		rcu_sysidle_report_gp(rsp, isidle, maxj);
 		fqs_state = RCU_FORCE_QS;
 	} else {
 		/* Handle dyntick-idle and offline CPUs. */
+		isidle = 0;
 		force_qs_rnp(rsp, rcu_implicit_dynticks_qs, &isidle, &maxj);
 	}
 	/* Clear flag to prevent immediate re-entry. */
@@ -2101,9 +2108,12 @@ static void force_qs_rnp(struct rcu_state *rsp,
 		cpu = rnp->grplo;
 		bit = 1;
 		for (; cpu <= rnp->grphi; cpu++, bit <<= 1) {
-			if ((rnp->qsmask & bit) != 0 &&
-			    f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
-				mask |= bit;
+			if ((rnp->qsmask & bit) != 0) {
+				if ((rnp->qsmaskinit & bit) != 0)
+					*isidle = 0;
+				if (f(per_cpu_ptr(rsp->rda, cpu), isidle, maxj))
+					mask |= bit;
+			}
 		}
 		if (mask != 0) {
 
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 9dd8b17..6fd3659 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -555,6 +555,11 @@ static void rcu_kick_nohz_cpu(int cpu);
 static bool init_nocb_callback_list(struct rcu_data *rdp);
 static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq);
 static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq);
+static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
+				  unsigned long *maxj);
+static bool is_sysidle_rcu_state(struct rcu_state *rsp);
+static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
+				  unsigned long maxj);
 static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);
 
 #endif /* #ifndef RCU_TREE_NONCORE */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 64a05b9f..58573c8 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -28,7 +28,7 @@
 #include <linux/gfp.h>
 #include <linux/oom.h>
 #include <linux/smpboot.h>
-#include <linux/tick.h>
+#include "time/tick-internal.h"
 
 #define RCU_KTHREAD_PRIO 1
 
@@ -2393,12 +2393,12 @@ static void rcu_kick_nohz_cpu(int cpu)
  * most active flavor of RCU.
  */
 #ifdef CONFIG_PREEMPT_RCU
-static struct rcu_state __maybe_unused *rcu_sysidle_state = &rcu_preempt_state;
+static struct rcu_state *rcu_sysidle_state = &rcu_preempt_state;
 #else /* #ifdef CONFIG_PREEMPT_RCU */
-static struct rcu_state __maybe_unused *rcu_sysidle_state = &rcu_sched_state;
+static struct rcu_state *rcu_sysidle_state = &rcu_sched_state;
 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
 
-static int __maybe_unused full_sysidle_state; /* Current system-idle state. */
+static int full_sysidle_state;		/* Current system-idle state. */
 #define RCU_SYSIDLE_NOT		0	/* Some CPU is not idle. */
 #define RCU_SYSIDLE_SHORT	1	/* All CPUs idle for brief period. */
 #define RCU_SYSIDLE_LONG	2	/* All CPUs idle for long enough. */
@@ -2442,6 +2442,38 @@ static void rcu_sysidle_enter(struct rcu_dynticks *rdtp, int irq)
 }
 
 /*
+ * Unconditionally force exit from full system-idle state.  This is
+ * invoked when a normal CPU exits idle, but must be called separately
+ * for the timekeeping CPU (tick_do_timer_cpu).  The reason for this
+ * is that the timekeeping CPU is permitted to take scheduling-clock
+ * interrupts while the system is in system-idle state, and of course
+ * rcu_sysidle_exit() has no way of distinguishing a scheduling-clock
+ * interrupt from any other type of interrupt.
+ */
+void rcu_sysidle_force_exit(void)
+{
+	int oldstate = ACCESS_ONCE(full_sysidle_state);
+	int newoldstate;
+
+	/*
+	 * Each pass through the following loop attempts to exit full
+	 * system-idle state.  If contention proves to be a problem,
+	 * a trylock-based contention tree could be used here.
+	 */
+	while (oldstate > RCU_SYSIDLE_SHORT) {
+		newoldstate = cmpxchg(&full_sysidle_state,
+				      oldstate, RCU_SYSIDLE_NOT);
+		if (oldstate == newoldstate &&
+		    oldstate == RCU_SYSIDLE_FULL_NOTED) {
+			rcu_kick_nohz_cpu(tick_do_timer_cpu);
+			return; /* We cleared it, done! */
+		}
+		oldstate = newoldstate;
+	}
+	smp_mb(); /* Order initial oldstate fetch vs. later non-idle work. */
+}
+
+/*
  * Invoked to note entry to irq or task transition from idle.  Note that
  * usermode execution does -not- count as idle here!  The caller must
  * have disabled interrupts.
@@ -2474,6 +2506,235 @@ static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
 	atomic_inc(&rdtp->dynticks_idle);
 	smp_mb__after_atomic_inc();
 	WARN_ON_ONCE(!(atomic_read(&rdtp->dynticks_idle) & 0x1));
+
+	/*
+	 * If we are the timekeeping CPU, we are permitted to be non-idle
+	 * during a system-idle state.  This must be the case, because
+	 * the timekeeping CPU has to take scheduling-clock interrupts
+	 * during the time that the system is transitioning to full
+	 * system-idle state.  This means that the timekeeping CPU must
+	 * invoke rcu_sysidle_force_exit() directly if it does anything
+	 * more than take a scheduling-clock interrupt.
+	 */
+	if (smp_processor_id() == tick_do_timer_cpu)
+		return;
+
+	/* Update system-idle state: We are clearly no longer fully idle! */
+	rcu_sysidle_force_exit();
+}
+
+/*
+ * Check to see if the current CPU is idle.  Note that usermode execution
+ * does not count as idle.  The caller must have disabled interrupts.
+ */
+static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
+				  unsigned long *maxj)
+{
+	int cur;
+	unsigned long j;
+	struct rcu_dynticks *rdtp = rdp->dynticks;
+
+	/*
+	 * If some other CPU has already reported non-idle, if this is
+	 * not the flavor of RCU that tracks sysidle state, or if this
+	 * is an offline or the timekeeping CPU, nothing to do.
+	 */
+	if (!*isidle || rdp->rsp != rcu_sysidle_state ||
+	    cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
+		return;
+	/* WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); */
+
+	/* Pick up current idle and NMI-nesting counter and check. */
+	cur = atomic_read(&rdtp->dynticks_idle);
+	if (cur & 0x1) {
+		*isidle = false; /* We are not idle! */
+		return;
+	}
+	smp_mb(); /* Read counters before timestamps. */
+
+	/* Pick up timestamps. */
+	j = ACCESS_ONCE(rdtp->dynticks_idle_jiffies);
+	/* If this CPU entered idle more recently, update maxj timestamp. */
+	if (ULONG_CMP_LT(*maxj, j))
+		*maxj = j;
+}
+
+/*
+ * Is this the flavor of RCU that is handling full-system idle?
+ */
+static bool is_sysidle_rcu_state(struct rcu_state *rsp)
+{
+	return rsp == rcu_sysidle_state;
+}
+
+/*
+ * Return a delay in jiffies based on the number of CPUs, rcu_node
+ * leaf fanout, and jiffies tick rate.  The idea is to allow larger
+ * systems more time to transition to full-idle state in order to
+ * avoid the cache thrashing that otherwise occur on the state variable.
+ * Really small systems (less than a couple of tens of CPUs) should
+ * instead use a single global atomically incremented counter, and later
+ * versions of this will automatically reconfigure themselves accordingly.
+ */
+static unsigned long rcu_sysidle_delay(void)
+{
+	if (nr_cpu_ids <= RCU_SYSIDLE_SMALL)
+		return 0;
+	return DIV_ROUND_UP(nr_cpu_ids * HZ, rcu_fanout_leaf * 1000);
+}
+
+/*
+ * Advance the full-system-idle state.  This is invoked when all of
+ * the non-timekeeping CPUs are idle.
+ */
+static void rcu_sysidle(unsigned long j)
+{
+	/* Check the current state. */
+	switch (ACCESS_ONCE(full_sysidle_state)) {
+	case RCU_SYSIDLE_NOT:
+
+		/* First time all are idle, so note a short idle period. */
+		ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_SHORT;
+		break;
+
+	case RCU_SYSIDLE_SHORT:
+
+		/*
+		 * Idle for a bit, time to advance to next state?
+		 * cmpxchg failure means race with non-idle, let them win.
+		 */
+		if (ULONG_CMP_GE(jiffies, j + rcu_sysidle_delay()))
+			(void)cmpxchg(&full_sysidle_state,
+				      RCU_SYSIDLE_SHORT, RCU_SYSIDLE_LONG);
+		break;
+
+	case RCU_SYSIDLE_LONG:
+
+		/*
+		 * Do an additional check pass before advancing to full.
+		 * cmpxchg failure means race with non-idle, let them win.
+		 */
+		if (ULONG_CMP_GE(jiffies, j + rcu_sysidle_delay()))
+			(void)cmpxchg(&full_sysidle_state,
+				      RCU_SYSIDLE_LONG, RCU_SYSIDLE_FULL);
+		break;
+
+	default:
+		break;
+	}
+}
+
+/*
+ * Found a non-idle non-timekeeping CPU, so kick the system-idle state
+ * back to the beginning.
+ */
+static void rcu_sysidle_cancel(void)
+{
+	smp_mb();
+	ACCESS_ONCE(full_sysidle_state) = RCU_SYSIDLE_NOT;
+}
+
+/*
+ * Update the sysidle state based on the results of a force-quiescent-state
+ * scan of the CPUs' dyntick-idle state.
+ */
+static void rcu_sysidle_report(struct rcu_state *rsp, int isidle,
+			       unsigned long maxj, bool gpkt)
+{
+	if (rsp != rcu_sysidle_state)
+		return;  /* Wrong flavor, ignore. */
+	if (isidle) {
+		if (gpkt && nr_cpu_ids > RCU_SYSIDLE_SMALL)
+			rcu_sysidle(maxj);    /* More idle! */
+	} else {
+		rcu_sysidle_cancel(); /* Idle is over. */
+	}
+}
+
+static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
+				  unsigned long maxj)
+{
+	rcu_sysidle_report(rsp, isidle, maxj, true);
+}
+
+/* Callback and function for forcing an RCU grace period. */
+struct rcu_sysidle_head {
+	struct rcu_head rh;
+	int inuse;
+};
+
+static void rcu_sysidle_cb(struct rcu_head *rhp)
+{
+	struct rcu_sysidle_head *rshp;
+
+	smp_mb();  /* grace period precedes setting inuse. */
+	rshp = container_of(rhp, struct rcu_sysidle_head, rh);
+	ACCESS_ONCE(rshp->inuse) = 0;
+}
+
+/*
+ * Check to see if the system is fully idle, other than the timekeeping CPU.
+ * The caller must have disabled interrupts.
+ */
+bool rcu_sys_is_idle(void)
+{
+	static struct rcu_sysidle_head rsh;
+	int rss = ACCESS_ONCE(full_sysidle_state);
+
+	if (WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu))
+		return false;
+
+	/* Handle small-system case by doing a full scan of CPUs. */
+	if (nr_cpu_ids <= RCU_SYSIDLE_SMALL) {
+		int oldrss = rss - 1;
+
+		/*
+		 * One pass to advance to each state up to _FULL.
+		 * Give up if any pass fails to advance the state.
+		 */
+		while (rss < RCU_SYSIDLE_FULL && oldrss < rss) {
+			int cpu;
+			bool isidle = true;
+			unsigned long maxj = jiffies - ULONG_MAX / 4;
+			struct rcu_data *rdp;
+
+			/* Scan all the CPUs looking for nonidle CPUs. */
+			for_each_possible_cpu(cpu) {
+				rdp = per_cpu_ptr(rcu_sysidle_state->rda, cpu);
+				rcu_sysidle_check_cpu(rdp, &isidle, &maxj);
+				if (!isidle)
+					break;
+			}
+			rcu_sysidle_report(rcu_sysidle_state,
+					   isidle, maxj, false);
+			oldrss = rss;
+			rss = ACCESS_ONCE(full_sysidle_state);
+		}
+	}
+
+	/* If this is the first observation of an idle period, record it. */
+	if (rss == RCU_SYSIDLE_FULL) {
+		rss = cmpxchg(&full_sysidle_state,
+			      RCU_SYSIDLE_FULL, RCU_SYSIDLE_FULL_NOTED);
+		return rss == RCU_SYSIDLE_FULL;
+	}
+
+	smp_mb(); /* ensure rss load happens before later caller actions. */
+
+	/* If already fully idle, tell the caller (in case of races). */
+	if (rss == RCU_SYSIDLE_FULL_NOTED)
+		return true;
+
+	/*
+	 * If we aren't there yet, and a grace period is not in flight,
+	 * initiate a grace period.  Either way, tell the caller that
+	 * we are not there yet.
+	 */
+	if (nr_cpu_ids > RCU_SYSIDLE_SMALL &&
+	    !rcu_gp_in_progress(rcu_sysidle_state) &&
+	    !rsh.inuse && xchg(&rsh.inuse, 1) == 0)
+		call_rcu(&rsh.rh, rcu_sysidle_cb);
+	return false;
 }
 
 /*
@@ -2494,6 +2755,21 @@ static void rcu_sysidle_exit(struct rcu_dynticks *rdtp, int irq)
 {
 }
 
+static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
+				  unsigned long *maxj)
+{
+}
+
+static bool is_sysidle_rcu_state(struct rcu_state *rsp)
+{
+	return false;
+}
+
+static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
+				  unsigned long maxj)
+{
+}
+
 static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp)
 {
 }
-- 
1.8.1.5

[PATCH tip/core/rcu 9/9] nohz_full: Force RCU's grace-period kthreads onto timekeeping CPU

[Paul E. McKenney]

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

Because RCU's quiescent-state-forcing mechanism is used to drive the
full-system-idle state machine, and because this mechanism is executed
by RCU's grace-period kthreads, this commit forces these kthreads to
run on the timekeeping CPU (tick_do_timer_cpu).  To do otherwise would
mean that the RCU grace-period kthreads would force the system into
non-idle state every time they drove the state machine, which would
be just a bit on the futile side.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
---
 kernel/rcutree.c        |  1 +
 kernel/rcutree.h        |  1 +
 kernel/rcutree_plugin.h | 21 ++++++++++++++++++++-
 3 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 3d898e5..1c88e26 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -1302,6 +1302,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
 	struct rcu_data *rdp;
 	struct rcu_node *rnp = rcu_get_root(rsp);
 
+	rcu_bind_gp_kthread();
 	raw_spin_lock_irq(&rnp->lock);
 	rsp->gp_flags = 0; /* Clear all flags: New grace period. */
 
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index 6fd3659..5f97eab 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -560,6 +560,7 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
 static bool is_sysidle_rcu_state(struct rcu_state *rsp);
 static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
 				  unsigned long maxj);
+static void rcu_bind_gp_kthread(void);
 static void rcu_sysidle_init_percpu_data(struct rcu_dynticks *rdtp);
 
 #endif /* #ifndef RCU_TREE_NONCORE */
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 58573c8..50a90bb 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -2542,7 +2542,8 @@ static void rcu_sysidle_check_cpu(struct rcu_data *rdp, bool *isidle,
 	if (!*isidle || rdp->rsp != rcu_sysidle_state ||
 	    cpu_is_offline(rdp->cpu) || rdp->cpu == tick_do_timer_cpu)
 		return;
-	/* WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu); */
+	if (rcu_gp_in_progress(rdp->rsp))
+		WARN_ON_ONCE(smp_processor_id() != tick_do_timer_cpu);
 
 	/* Pick up current idle and NMI-nesting counter and check. */
 	cur = atomic_read(&rdtp->dynticks_idle);
@@ -2568,6 +2569,20 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp)
 }
 
 /*
+ * Bind the grace-period kthread for the sysidle flavor of RCU to the
+ * timekeeping CPU.
+ */
+static void rcu_bind_gp_kthread(void)
+{
+	int cpu = ACCESS_ONCE(tick_do_timer_cpu);
+
+	if (cpu < 0 || cpu >= nr_cpu_ids)
+		return;
+	if (raw_smp_processor_id() != cpu)
+		set_cpus_allowed_ptr(current, cpumask_of(cpu));
+}
+
+/*
  * Return a delay in jiffies based on the number of CPUs, rcu_node
  * leaf fanout, and jiffies tick rate.  The idea is to allow larger
  * systems more time to transition to full-idle state in order to
@@ -2765,6 +2780,10 @@ static bool is_sysidle_rcu_state(struct rcu_state *rsp)
 	return false;
 }
 
+static void rcu_bind_gp_kthread(void)
+{
+}
+
 static void rcu_sysidle_report_gp(struct rcu_state *rsp, int isidle,
 				  unsigned long maxj)
 {
-- 
1.8.1.5

Re: [PATCH tip/core/rcu 0/9] sysidle changes for v3.12

[Josh Triplett]

On Sat, Aug 17, 2013 at 06:49:18PM -0700, Paul E. McKenney wrote:
> Hello!
> 
> Whenever there is at least one non-idle CPU, it is necessary to
> periodically update timekeeping information.  Before NO_HZ_FULL, this
> updating was carried out by the scheduling-clock tick, which ran on
> every non-idle CPU.  With the advent of NO_HZ_FULL, it is possible
> to have non-idle CPUs that are not receiving scheduling-clock ticks.
> This possibility is handled by assigning a timekeeping CPU that continues
> taking scheduling-clock ticks.
> 
> Unfortunately, timekeeping CPU continues taking scheduling-clock
> interrupts even when all other CPUs are completely idle, which is
> not so good for energy efficiency and battery lifetime.  Clearly, it
> would be good to turn off the timekeeping CPU's scheduling-clock tick
> when all CPUs are completely idle.  This is conceptually simple, but
> we also need good performance and scalability on large systems, which
> rules out implementations based on frequently updated global counts of
> non-idle CPUs as well as implementations that frequently scan all CPUs.
> Nevertheless, we need a single global indicator in order to keep the
> overhead of checking acceptably low.
> 
> The chosen approach is to enforce hysteresis on the non-idle to
> full-system-idle transition, with the amount of hysteresis increasing
> linearly with the number of CPUs, thus keeping contention acceptably low.
> This approach piggybacks on RCU's existing force-quiescent-state scanning
> of idle CPUs, which has the advantage of avoiding the scan entirely on
> busy systems that have high levels of multiprogramming.  This scan
> takes per-CPU idleness information and feeds it into a state machine
> that applies the level of hysteresis required to arrive at a single
> full-system-idle indicator.
> 
> The individual patches are as follows:
> 
> 1.	Eliminate unused APIs that were intended for adaptive ticks.
> 
> 2.	Add documentation covering the testing of nohz_full.
> 
> 3.	Add a CONFIG_NO_HZ_FULL_SYSIDLE Kconfig parameter to enable
> 	this feature.  Kernels built with CONFIG_NO_HZ_FULL_SYSIDLE=n
> 	act exactly as they do today.
> 
> 4.	Add new fields to the rcu_dynticks structure that track CPU-idle
> 	information.  These fields consider CPUs running usermode to be
> 	non-idle, in contrast with the existing fields in that structure.
> 
> 5.	Track per-CPU idle states.
> 
> 6.	Add full-system idle states and state variables.
> 
> 7.	Expand force_qs_rnp(), dyntick_save_progress_counter(), and
> 	rcu_implicit_dynticks_qs() APIs to enable passing full-system
> 	idle state information.
> 
> 8.	Add full-system-idle state machine.
> 
> 9.	Force RCU's grace-period kthreads onto the timekeeping CPU.

Comments on 4, 5, and 6; for 1-3 and 7-9,
Reviewed-by: Josh Triplett <josh@joshtriplett.org>

[PATCH tip/core/rcu 0/9] v2 sysidle changes for 3.12

[Paul E. McKenney]

Hello!

Whenever there is at least one non-idle CPU, it is necessary to
periodically update timekeeping information.  Before NO_HZ_FULL, this
updating was carried out by the scheduling-clock tick, which ran on
every non-idle CPU.  With the advent of NO_HZ_FULL, it is possible
to have non-idle CPUs that are not receiving scheduling-clock ticks.
This possibility is handled by assigning a timekeeping CPU that continues
taking scheduling-clock ticks.

Unfortunately, timekeeping CPU continues taking scheduling-clock
interrupts even when all other CPUs are completely idle, which is
not so good for energy efficiency and battery lifetime.  Clearly, it
would be good to turn off the timekeeping CPU's scheduling-clock tick
when all CPUs are completely idle.  This is conceptually simple, but
we also need good performance and scalability on large systems, which
rules out implementations based on frequently updated global counts of
non-idle CPUs as well as implementations that frequently scan all CPUs.
Nevertheless, we need a single global indicator in order to keep the
overhead of checking acceptably low.

The chosen approach is to enforce hysteresis on the non-idle to
full-system-idle transition, with the amount of hysteresis increasing
linearly with the number of CPUs, thus keeping contention acceptably low.
This approach piggybacks on RCU's existing force-quiescent-state scanning
of idle CPUs, which has the advantage of avoiding the scan entirely on
busy systems that have high levels of multiprogramming.  This scan
takes per-CPU idleness information and feeds it into a state machine
that applies the level of hysteresis required to arrive at a single
full-system-idle indicator.

The individual patches are as follows:

1.	Eliminate unused APIs that were intended for adaptive ticks.

2.	Add documentation covering the testing of nohz_full.

3.	Add a CONFIG_NO_HZ_FULL_SYSIDLE Kconfig parameter to enable
	this feature.  Kernels built with CONFIG_NO_HZ_FULL_SYSIDLE=n
	act exactly as they do today.

4.	Add new fields to the rcu_dynticks structure that track CPU-idle
	information.  These fields consider CPUs running usermode to be
	non-idle, in contrast with the existing fields in that structure.

5.	Track per-CPU idle states.

6.	Add full-system idle states and state variables.

7.	Expand force_qs_rnp(), dyntick_save_progress_counter(), and
	rcu_implicit_dynticks_qs() APIs to enable passing full-system
	idle state information.

8.	Add full-system-idle state machine.

9.	Force RCU's grace-period kthreads onto the timekeeping CPU.

Changes since v4 (https://lkml.org/lkml/2013/8/17/108):

o	Move small-system cutoff to Kconfig symbol as suggested
	by Josh Triplett.

o	Provide names (not just types) for function pointer argument
	as suggested by Josh Triplett.

Changes since v3 (https://lkml.org/lkml/2013/7/8/441):

o	Updated Kconfig help text as suggested by Frederic.

o	Added a few adaptive-ticks-related fixes (#1 and #2 above).

Changes since v2:

o	Completed removing NMI support (thanks to Frederic for spotting
	the remaining cruft).

o	Fix a state-machine bug, again spotted by Frederic.  See
	http://lists-archives.com/linux-kernel/27865835-nohz_full-add-full-system-idle-state-machine.html
	for the full details of the bug.

o	Updated commit log and comment as suggested by Josh Triplett.

Changes since v1:

o	Removed NMI support because NMI handlers cannot safely read
	the time anyway (thanks to Thomas Gleixner and Peter Zijlstra).

						Thanx, Paul


 b/Documentation/timers/NO_HZ.txt |   44 +++-
 b/include/linux/rcupdate.h       |   22 +-
 b/kernel/rcutree.c               |   94 +++-----
 b/kernel/rcutree.h               |   17 +
 b/kernel/rcutree_plugin.h        |  420 ++++++++++++++++++++++++++++++++++++++-
 b/kernel/time/Kconfig            |   50 ++++
 6 files changed, 575 insertions(+), 72 deletions(-)

[PATCH tip/core/rcu 1/9] rcu: Eliminate unused APIs intended for adaptive ticks

[Paul E. McKenney]

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

The rcu_user_enter_after_irq() and rcu_user_exit_after_irq()
functions were intended for use by adaptive ticks, but changes
in implementation have rendered them unnecessary.  This commit
therefore removes them.

Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
---
 include/linux/rcupdate.h |  4 ----
 kernel/rcutree.c         | 43 -------------------------------------------
 2 files changed, 47 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 0c38abb..30bea9c 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -229,13 +229,9 @@ extern void rcu_irq_exit(void);
 #ifdef CONFIG_RCU_USER_QS
 extern void rcu_user_enter(void);
 extern void rcu_user_exit(void);
-extern void rcu_user_enter_after_irq(void);
-extern void rcu_user_exit_after_irq(void);
 #else
 static inline void rcu_user_enter(void) { }
 static inline void rcu_user_exit(void) { }
-static inline void rcu_user_enter_after_irq(void) { }
-static inline void rcu_user_exit_after_irq(void) { }
 static inline void rcu_user_hooks_switch(struct task_struct *prev,
 					 struct task_struct *next) { }
 #endif /* CONFIG_RCU_USER_QS */
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 338f1d1..8807019 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -444,27 +444,6 @@ void rcu_user_enter(void)
 {
 	rcu_eqs_enter(1);
 }
-
-/**
- * rcu_user_enter_after_irq - inform RCU that we are going to resume userspace
- * after the current irq returns.
- *
- * This is similar to rcu_user_enter() but in the context of a non-nesting
- * irq. After this call, RCU enters into idle mode when the interrupt
- * returns.
- */
-void rcu_user_enter_after_irq(void)
-{
-	unsigned long flags;
-	struct rcu_dynticks *rdtp;
-
-	local_irq_save(flags);
-	rdtp = &__get_cpu_var(rcu_dynticks);
-	/* Ensure this irq is interrupting a non-idle RCU state.  */
-	WARN_ON_ONCE(!(rdtp->dynticks_nesting & DYNTICK_TASK_MASK));
-	rdtp->dynticks_nesting = 1;
-	local_irq_restore(flags);
-}
 #endif /* CONFIG_RCU_USER_QS */
 
 /**
@@ -581,28 +560,6 @@ void rcu_user_exit(void)
 {
 	rcu_eqs_exit(1);
 }
-
-/**
- * rcu_user_exit_after_irq - inform RCU that we won't resume to userspace
- * idle mode after the current non-nesting irq returns.
- *
- * This is similar to rcu_user_exit() but in the context of an irq.
- * This is called when the irq has interrupted a userspace RCU idle mode
- * context. When the current non-nesting interrupt returns after this call,
- * the CPU won't restore the RCU idle mode.
- */
-void rcu_user_exit_after_irq(void)
-{
-	unsigned long flags;
-	struct rcu_dynticks *rdtp;
-
-	local_irq_save(flags);
-	rdtp = &__get_cpu_var(rcu_dynticks);
-	/* Ensure we are interrupting an RCU idle mode. */
-	WARN_ON_ONCE(rdtp->dynticks_nesting & DYNTICK_TASK_NEST_MASK);
-	rdtp->dynticks_nesting += DYNTICK_TASK_EXIT_IDLE;
-	local_irq_restore(flags);
-}
 #endif /* CONFIG_RCU_USER_QS */
 
 /**
-- 
1.8.1.5

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

[Paul E. McKenney]

From: "Paul E. McKenney" <paulmck@linux.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@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>
---
 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