[PATCH v2 tip/core/rcu 0/11] Expedited-grace-period changes for 4.5

[PATCH v2 tip/core/rcu 0/11] Expedited-grace-period changes for 4.5

[Paul E. McKenney]

Hello!

This series contains follow-on cleanup changes to RCU's expedited
grace-period functionality.  The patches in this series are as follows:

1.	Short-circuit synchronize_sched_expedited() if there is only one CPU.

2.	Update comment to clarify the role of ->expmaskinitnext.

3.	Separate concerns by moving smp_mb() from rcu_seq_snap() to
	rcu_exp_gp_seq_snap().

4.	Safe a few lines of code by inverting the sense of
	sync_rcu_exp_select_cpus() "if" statement.

5.	Reduce expedited GP memory contention via per-CPU variables.

6.	Get rid of (some) empty stall-warning messages by resolving
	stall-warning ties.

7.	Add more diagnostics to expedited stall warning messages.

8.	Add rcu_normal kernel parameter to suppress expediting for
	the benefit of aggressive real-time systems.

9-10.	Allow expedited grace periods to be disabled just before init
	is spawned, allowing them to speed up boot without interfering
	with run-time real-time workloads.

11.	Remove TINY_RCU bloat from pointless (to TINY_RCU) boot parameters.

Changes since v1:

o	Add patch 11 removing TINY_RCU bloat.

o	Get cover letter and patches correctly associated.  :-/

							Thanx, Paul

------------------------------------------------------------------------

 Documentation/kernel-parameters.txt |   37 ++++++++++++----
 include/linux/rcupdate.h            |   15 ++++++
 init/main.c                         |    2 
 kernel/ksysfs.c                     |   26 ++++++++++-
 kernel/rcu/srcu.c                   |    2 
 kernel/rcu/tree.c                   |   79 +++++++++++++++++++++++-------------
 kernel/rcu/tree.h                   |   10 ++--
 kernel/rcu/tree_plugin.h            |    6 ++
 kernel/rcu/tree_trace.c             |   18 +++++---
 kernel/rcu/update.c                 |   24 +++++++++-
 10 files changed, 165 insertions(+), 54 deletions(-)

[PATCH v2 tip/core/rcu 01/11] rcu: Short-circuit synchronize_sched_expedited() if only one CPU

[Paul E. McKenney]

If there is only one CPU, then invoking synchronize_sched_expedited()
is by definition a grace period.  This commit checks for this condition
and does a short-circuit return in that case.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 81aa1cdc6bc9..bd2605c144cc 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3822,6 +3822,10 @@ void synchronize_sched_expedited(void)
 	struct rcu_node *rnp;
 	struct rcu_state *rsp = &rcu_sched_state;
 
+	/* If only one CPU, this is automatically a grace period. */
+	if (rcu_blocking_is_gp())
+		return;
+
 	/* Take a snapshot of the sequence number.  */
 	s = rcu_exp_gp_seq_snap(rsp);
 
-- 
2.5.2

[PATCH v2 tip/core/rcu 02/11] rcu: Clarify role of ->expmaskinitnext

[Paul E. McKenney]

Analogy with the ->qsmaskinitnext field might lead one to believe that
->expmaskinitnext tracks online CPUs.  This belief is incorrect: Any CPU
that has ever been online will have its bit set in the ->expmaskinitnext
field.  This commit therefore adds a comment to make this clear, at
least to people who read comments.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index f32bebb6bc90..8151971a8978 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -178,6 +178,8 @@ struct rcu_node {
 				/*  beginning of each expedited GP. */
 	unsigned long expmaskinitnext;
 				/* Online CPUs for next expedited GP. */
+				/*  Any CPU that has ever been online will */
+				/*  have its bit set. */
 	unsigned long grpmask;	/* Mask to apply to parent qsmask. */
 				/*  Only one bit will be set in this mask. */
 	int	grplo;		/* lowest-numbered CPU or group here. */
-- 
2.5.2

[PATCH v2 tip/core/rcu 03/11] rcu: Move smp_mb() from rcu_seq_snap() to rcu_exp_gp_seq_snap()

[Paul E. McKenney]

The memory barrier in rcu_seq_snap() is needed only for grace periods,
so this commit moves it to the grace-period-oriented wrapper
rcu_exp_gp_seq_snap().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index bd2605c144cc..a4a0475aede9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3347,7 +3347,6 @@ static unsigned long rcu_seq_snap(unsigned long *sp)
 {
 	unsigned long s;
 
-	smp_mb(); /* Caller's modifications seen first by other CPUs. */
 	s = (READ_ONCE(*sp) + 3) & ~0x1;
 	smp_mb(); /* Above access must not bleed into critical section. */
 	return s;
@@ -3374,6 +3373,7 @@ static void rcu_exp_gp_seq_end(struct rcu_state *rsp)
 }
 static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp)
 {
+	smp_mb(); /* Caller's modifications seen first by other CPUs. */
 	return rcu_seq_snap(&rsp->expedited_sequence);
 }
 static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s)
-- 
2.5.2

[PATCH v2 tip/core/rcu 04/11] rcu: Invert sync_rcu_exp_select_cpus() "if" statement

[Paul E. McKenney]

This commit saves a couple lines of code and reduces indentation
by inverting the sense of an "if" statement in the function
sync_rcu_exp_select_cpus().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 30 ++++++++++++++----------------
 1 file changed, 14 insertions(+), 16 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a4a0475aede9..00f07d6436ce 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3716,24 +3716,22 @@ retry_ipi:
 			ret = smp_call_function_single(cpu, func, rsp, 0);
 			if (!ret) {
 				mask_ofl_ipi &= ~mask;
-			} else {
-				/* Failed, raced with offline. */
-				raw_spin_lock_irqsave_rcu_node(rnp, flags);
-				if (cpu_online(cpu) &&
-				    (rnp->expmask & mask)) {
-					raw_spin_unlock_irqrestore(&rnp->lock,
-								   flags);
-					schedule_timeout_uninterruptible(1);
-					if (cpu_online(cpu) &&
-					    (rnp->expmask & mask))
-						goto retry_ipi;
-					raw_spin_lock_irqsave_rcu_node(rnp,
-								       flags);
-				}
-				if (!(rnp->expmask & mask))
-					mask_ofl_ipi &= ~mask;
+				continue;
+			}
+			/* Failed, raced with offline. */
+			raw_spin_lock_irqsave_rcu_node(rnp, flags);
+			if (cpu_online(cpu) &&
+			    (rnp->expmask & mask)) {
 				raw_spin_unlock_irqrestore(&rnp->lock, flags);
+				schedule_timeout_uninterruptible(1);
+				if (cpu_online(cpu) &&
+				    (rnp->expmask & mask))
+					goto retry_ipi;
+				raw_spin_lock_irqsave_rcu_node(rnp, flags);
 			}
+			if (!(rnp->expmask & mask))
+				mask_ofl_ipi &= ~mask;
+			raw_spin_unlock_irqrestore(&rnp->lock, flags);
 		}
 		/* Report quiescent states for those that went offline. */
 		mask_ofl_test |= mask_ofl_ipi;
-- 
2.5.2

[PATCH v2 tip/core/rcu 05/11] rcu: Reduce expedited GP memory contention via per-CPU variables

[Paul E. McKenney]

Currently, the piggybacked-work checks carried out by sync_exp_work_done()
atomically increment a small set of variables (the ->expedited_workdone0,
->expedited_workdone1, ->expedited_workdone2, ->expedited_workdone3
fields in the rcu_state structure), which will form a memory-contention
bottleneck given a sufficiently large number of CPUs concurrently invoking
either synchronize_rcu_expedited() or synchronize_sched_expedited().

This commit therefore moves these for fields to the per-CPU rcu_data
structure, eliminating the memory contention.  The show_rcuexp() function
also changes to sum up each field in the rcu_data structures.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c       | 11 +++++------
 kernel/rcu/tree.h       |  8 ++++----
 kernel/rcu/tree_trace.c | 18 ++++++++++++------
 3 files changed, 21 insertions(+), 16 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 00f07d6436ce..33d7e2551165 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3585,7 +3585,7 @@ static bool sync_exp_work_done(struct rcu_state *rsp, struct rcu_node *rnp,
  */
 static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
 {
-	struct rcu_data *rdp;
+	struct rcu_data *rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
 	struct rcu_node *rnp0;
 	struct rcu_node *rnp1 = NULL;
 
@@ -3599,7 +3599,7 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
 	if (!mutex_is_locked(&rnp0->exp_funnel_mutex)) {
 		if (mutex_trylock(&rnp0->exp_funnel_mutex)) {
 			if (sync_exp_work_done(rsp, rnp0, NULL,
-					       &rsp->expedited_workdone0, s))
+					       &rdp->expedited_workdone0, s))
 				return NULL;
 			return rnp0;
 		}
@@ -3613,14 +3613,13 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
 	 * can be inexact, as it is just promoting locality and is not
 	 * strictly needed for correctness.
 	 */
-	rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
-	if (sync_exp_work_done(rsp, NULL, NULL, &rsp->expedited_workdone1, s))
+	if (sync_exp_work_done(rsp, NULL, NULL, &rdp->expedited_workdone1, s))
 		return NULL;
 	mutex_lock(&rdp->exp_funnel_mutex);
 	rnp0 = rdp->mynode;
 	for (; rnp0 != NULL; rnp0 = rnp0->parent) {
 		if (sync_exp_work_done(rsp, rnp1, rdp,
-				       &rsp->expedited_workdone2, s))
+				       &rdp->expedited_workdone2, s))
 			return NULL;
 		mutex_lock(&rnp0->exp_funnel_mutex);
 		if (rnp1)
@@ -3630,7 +3629,7 @@ static struct rcu_node *exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
 		rnp1 = rnp0;
 	}
 	if (sync_exp_work_done(rsp, rnp1, rdp,
-			       &rsp->expedited_workdone3, s))
+			       &rdp->expedited_workdone3, s))
 		return NULL;
 	return rnp1;
 }
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 8151971a8978..6cbec31f99d6 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -386,6 +386,10 @@ struct rcu_data {
 	struct rcu_head oom_head;
 #endif /* #ifdef CONFIG_RCU_FAST_NO_HZ */
 	struct mutex exp_funnel_mutex;
+	atomic_long_t expedited_workdone0;	/* # done by others #0. */
+	atomic_long_t expedited_workdone1;	/* # done by others #1. */
+	atomic_long_t expedited_workdone2;	/* # done by others #2. */
+	atomic_long_t expedited_workdone3;	/* # done by others #3. */
 
 	/* 7) Callback offloading. */
 #ifdef CONFIG_RCU_NOCB_CPU
@@ -500,10 +504,6 @@ struct rcu_state {
 	/* End of fields guarded by barrier_mutex. */
 
 	unsigned long expedited_sequence;	/* Take a ticket. */
-	atomic_long_t expedited_workdone0;	/* # done by others #0. */
-	atomic_long_t expedited_workdone1;	/* # done by others #1. */
-	atomic_long_t expedited_workdone2;	/* # done by others #2. */
-	atomic_long_t expedited_workdone3;	/* # done by others #3. */
 	atomic_long_t expedited_normal;		/* # fallbacks to normal. */
 	atomic_t expedited_need_qs;		/* # CPUs left to check in. */
 	wait_queue_head_t expedited_wq;		/* Wait for check-ins. */
diff --git a/kernel/rcu/tree_trace.c b/kernel/rcu/tree_trace.c
index 8efaba870d96..d43649450ea4 100644
--- a/kernel/rcu/tree_trace.c
+++ b/kernel/rcu/tree_trace.c
@@ -183,14 +183,20 @@ static const struct file_operations rcudata_fops = {
 
 static int show_rcuexp(struct seq_file *m, void *v)
 {
+	int cpu;
 	struct rcu_state *rsp = (struct rcu_state *)m->private;
-
+	struct rcu_data *rdp;
+	unsigned long s0 = 0, s1 = 0, s2 = 0, s3 = 0;
+
+	for_each_possible_cpu(cpu) {
+		rdp = per_cpu_ptr(rsp->rda, cpu);
+		s0 += atomic_long_read(&rdp->expedited_workdone0);
+		s1 += atomic_long_read(&rdp->expedited_workdone1);
+		s2 += atomic_long_read(&rdp->expedited_workdone2);
+		s3 += atomic_long_read(&rdp->expedited_workdone3);
+	}
 	seq_printf(m, "s=%lu wd0=%lu wd1=%lu wd2=%lu wd3=%lu n=%lu enq=%d sc=%lu\n",
-		   rsp->expedited_sequence,
-		   atomic_long_read(&rsp->expedited_workdone0),
-		   atomic_long_read(&rsp->expedited_workdone1),
-		   atomic_long_read(&rsp->expedited_workdone2),
-		   atomic_long_read(&rsp->expedited_workdone3),
+		   rsp->expedited_sequence, s0, s1, s2, s3,
 		   atomic_long_read(&rsp->expedited_normal),
 		   atomic_read(&rsp->expedited_need_qs),
 		   rsp->expedited_sequence / 2);
-- 
2.5.2

[PATCH v2 tip/core/rcu 06/11] rcu: Make expedited grace periods resolve stall-warning ties

[Paul E. McKenney]

Currently, if a grace period ends just as the stall-warning timeout
fires, an empty stall warning will be printed.  This is not helpful,
so this commit avoids these useless warnings by rechecking completion
after awakening in synchronize_sched_expedited_wait().

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 33d7e2551165..bc6b79716a86 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3757,7 +3757,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 				rsp->expedited_wq,
 				sync_rcu_preempt_exp_done(rnp_root),
 				jiffies_stall);
-		if (ret > 0)
+		if (ret > 0 || sync_rcu_preempt_exp_done(rnp_root))
 			return;
 		if (ret < 0) {
 			/* Hit a signal, disable CPU stall warnings. */
-- 
2.5.2

[PATCH v2 tip/core/rcu 07/11] rcu: Add more diagnostics to expedited stall warning messages.

[Paul E. McKenney]

This commit adds print statements that check the rcu_node structure to
find which ->expmask bits and which ->exp_tasks structures are blocking
the current expedited grace period.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 kernel/rcu/tree.c | 24 +++++++++++++++++++++---
 1 file changed, 21 insertions(+), 3 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index bc6b79716a86..6a652d1f3d7f 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3745,6 +3745,7 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 	unsigned long jiffies_stall;
 	unsigned long jiffies_start;
 	unsigned long mask;
+	int ndetected;
 	struct rcu_node *rnp;
 	struct rcu_node *rnp_root = rcu_get_root(rsp);
 	int ret;
@@ -3767,14 +3768,16 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 		}
 		pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
 		       rsp->name);
+		ndetected = 0;
 		rcu_for_each_leaf_node(rsp, rnp) {
-			(void)rcu_print_task_exp_stall(rnp);
+			ndetected = rcu_print_task_exp_stall(rnp);
 			mask = 1;
 			for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
 				struct rcu_data *rdp;
 
 				if (!(rnp->expmask & mask))
 					continue;
+				ndetected++;
 				rdp = per_cpu_ptr(rsp->rda, cpu);
 				pr_cont(" %d-%c%c%c", cpu,
 					"O."[cpu_online(cpu)],
@@ -3783,8 +3786,23 @@ static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
 			}
 			mask <<= 1;
 		}
-		pr_cont(" } %lu jiffies s: %lu\n",
-			jiffies - jiffies_start, rsp->expedited_sequence);
+		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
+			jiffies - jiffies_start, rsp->expedited_sequence,
+			rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
+		if (!ndetected) {
+			pr_err("blocking rcu_node structures:");
+			rcu_for_each_node_breadth_first(rsp, rnp) {
+				if (rnp == rnp_root)
+					continue; /* printed unconditionally */
+				if (sync_rcu_preempt_exp_done(rnp))
+					continue;
+				pr_cont(" l=%u:%d-%d:%#lx/%c",
+					rnp->level, rnp->grplo, rnp->grphi,
+					rnp->expmask,
+					".T"[!!rnp->exp_tasks]);
+			}
+			pr_cont("\n");
+		}
 		rcu_for_each_leaf_node(rsp, rnp) {
 			mask = 1;
 			for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++, mask <<= 1) {
-- 
2.5.2

[PATCH v2 tip/core/rcu 08/11] rcu: Add rcu_normal kernel parameter to suppress expediting

[Paul E. McKenney]

Although expedited grace periods can be quite useful, and although their
OS jitter has been greatly reduced, they can still pose problems for
extreme real-time workloads.  This commit therefore adds a rcu_normal
kernel boot parameter (which can also be manipulated via sysfs)
to suppress expedited grace periods, that is, to treat requests for
expedited grace periods as if they were requests for normal grace periods.
If both rcu_expedited and rcu_normal are specified, rcu_normal wins.
This means that if you are relying on expedited grace periods to speed up
boot, you will want to specify rcu_expedited on the kernel command line,
and then specify rcu_normal via sysfs once boot completes.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 Documentation/kernel-parameters.txt | 19 ++++++++++++++-----
 include/linux/rcupdate.h            |  6 ++++++
 kernel/ksysfs.c                     | 22 ++++++++++++++++++++--
 kernel/rcu/srcu.c                   |  2 +-
 kernel/rcu/tree.c                   |  6 ++++++
 kernel/rcu/tree_plugin.h            |  6 ++++++
 kernel/rcu/update.c                 | 12 ++++++++++++
 7 files changed, 65 insertions(+), 8 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 742f69d18fc8..7673943d3085 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3296,6 +3296,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 	rcutorture.verbose= [KNL]
 			Enable additional printk() statements.
 
+	rcupdate.rcu_cpu_stall_suppress= [KNL]
+			Suppress RCU CPU stall warning messages.
+
+	rcupdate.rcu_cpu_stall_timeout= [KNL]
+			Set timeout for RCU CPU stall warning messages.
+
 	rcupdate.rcu_expedited= [KNL]
 			Use expedited grace-period primitives, for
 			example, synchronize_rcu_expedited() instead
@@ -3303,11 +3309,14 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			but can increase CPU utilization, degrade
 			real-time latency, and degrade energy efficiency.
 
-	rcupdate.rcu_cpu_stall_suppress= [KNL]
-			Suppress RCU CPU stall warning messages.
-
-	rcupdate.rcu_cpu_stall_timeout= [KNL]
-			Set timeout for RCU CPU stall warning messages.
+	rcupdate.rcu_normal= [KNL]
+			Use only normal grace-period primitives,
+			for example, synchronize_rcu() instead of
+			synchronize_rcu_expedited().  This improves
+			real-time latency, CPU utilization, and energy
+			efficiency, but can expose users to increased
+			grace-period latency.  This parameter overrides
+			rcupdate.rcu_expedited.
 
 	rcupdate.rcu_task_stall_timeout= [KNL]
 			Set timeout in jiffies for RCU task stall warning
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index a0189ba67fde..98d9f30c02d4 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -49,9 +49,14 @@
 #include <asm/barrier.h>
 
 extern int rcu_expedited; /* for sysctl */
+extern int rcu_normal;    /* also for sysctl */
 
 #ifdef CONFIG_TINY_RCU
 /* Tiny RCU doesn't expedite, as its purpose in life is instead to be tiny. */
+static inline bool rcu_gp_is_normal(void)  /* Internal RCU use. */
+{
+	return true;
+}
 static inline bool rcu_gp_is_expedited(void)  /* Internal RCU use. */
 {
 	return false;
@@ -65,6 +70,7 @@ static inline void rcu_unexpedite_gp(void)
 {
 }
 #else /* #ifdef CONFIG_TINY_RCU */
+bool rcu_gp_is_normal(void);     /* Internal RCU use. */
 bool rcu_gp_is_expedited(void);  /* Internal RCU use. */
 void rcu_expedite_gp(void);
 void rcu_unexpedite_gp(void);
diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index e83b26464061..b4e2fa52d8bc 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -20,7 +20,7 @@
 #include <linux/capability.h>
 #include <linux/compiler.h>
 
-#include <linux/rcupdate.h>	/* rcu_expedited */
+#include <linux/rcupdate.h>	/* rcu_expedited and rcu_normal */
 
 #define KERNEL_ATTR_RO(_name) \
 static struct kobj_attribute _name##_attr = __ATTR_RO(_name)
@@ -148,7 +148,7 @@ int rcu_expedited;
 static ssize_t rcu_expedited_show(struct kobject *kobj,
 				  struct kobj_attribute *attr, char *buf)
 {
-	return sprintf(buf, "%d\n", rcu_expedited);
+	return sprintf(buf, "%d\n", READ_ONCE(rcu_expedited));
 }
 static ssize_t rcu_expedited_store(struct kobject *kobj,
 				   struct kobj_attribute *attr,
@@ -161,6 +161,23 @@ static ssize_t rcu_expedited_store(struct kobject *kobj,
 }
 KERNEL_ATTR_RW(rcu_expedited);
 
+int rcu_normal;
+static ssize_t rcu_normal_show(struct kobject *kobj,
+			       struct kobj_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%d\n", READ_ONCE(rcu_normal));
+}
+static ssize_t rcu_normal_store(struct kobject *kobj,
+				struct kobj_attribute *attr,
+				const char *buf, size_t count)
+{
+	if (kstrtoint(buf, 0, &rcu_normal))
+		return -EINVAL;
+
+	return count;
+}
+KERNEL_ATTR_RW(rcu_normal);
+
 /*
  * Make /sys/kernel/notes give the raw contents of our kernel .notes section.
  */
@@ -203,6 +220,7 @@ static struct attribute * kernel_attrs[] = {
 	&vmcoreinfo_attr.attr,
 #endif
 	&rcu_expedited_attr.attr,
+	&rcu_normal_attr.attr,
 	NULL
 };
 
diff --git a/kernel/rcu/srcu.c b/kernel/rcu/srcu.c
index a63a1ea5a41b..9b9cdd549caa 100644
--- a/kernel/rcu/srcu.c
+++ b/kernel/rcu/srcu.c
@@ -489,7 +489,7 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
  */
 void synchronize_srcu(struct srcu_struct *sp)
 {
-	__synchronize_srcu(sp, rcu_gp_is_expedited()
+	__synchronize_srcu(sp, (rcu_gp_is_expedited() && !rcu_gp_is_normal())
 			   ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
 			   : SYNCHRONIZE_SRCU_TRYCOUNT);
 }
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 6a652d1f3d7f..489992997c06 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3841,6 +3841,12 @@ void synchronize_sched_expedited(void)
 	if (rcu_blocking_is_gp())
 		return;
 
+	/* If expedited grace periods are prohibited, fall back to normal. */
+	if (rcu_gp_is_normal()) {
+		wait_rcu_gp(call_rcu_sched);
+		return;
+	}
+
 	/* Take a snapshot of the sequence number.  */
 	s = rcu_exp_gp_seq_snap(rsp);
 
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 57ba873d2f18..d45df3781551 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -746,6 +746,12 @@ void synchronize_rcu_expedited(void)
 	struct rcu_state *rsp = rcu_state_p;
 	unsigned long s;
 
+	/* If expedited grace periods are prohibited, fall back to normal. */
+	if (rcu_gp_is_normal()) {
+		wait_rcu_gp(call_rcu);
+		return;
+	}
+
 	s = rcu_exp_gp_seq_snap(rsp);
 
 	rnp_unlock = exp_funnel_lock(rsp, s);
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 5f748c5a40f0..8fccda3a794d 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -61,6 +61,7 @@ MODULE_ALIAS("rcupdate");
 #define MODULE_PARAM_PREFIX "rcupdate."
 
 module_param(rcu_expedited, int, 0);
+module_param(rcu_normal, int, 0);
 
 #if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
 /**
@@ -113,6 +114,17 @@ EXPORT_SYMBOL(rcu_read_lock_sched_held);
 
 #ifndef CONFIG_TINY_RCU
 
+/*
+ * Should expedited grace-period primitives always fall back to their
+ * non-expedited counterparts?  Intended for use within RCU.  Note
+ * that if the user specifies both rcu_expedited and rcu_normal, then
+ * rcu_normal wins.
+ */
+bool rcu_gp_is_normal(void)
+{
+	return READ_ONCE(rcu_normal);
+}
+
 static atomic_t rcu_expedited_nesting =
 	ATOMIC_INIT(IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT) ? 1 : 0);
 
-- 
2.5.2

[PATCH v2 tip/core/rcu 09/11] rcu: Wire up rcu_end_inkernel_boot()

[Paul E. McKenney]

This commit adds the invocation of rcu_end_inkernel_boot() just before
init is invoked.  This allows the CONFIG_RCU_EXPEDITE_BOOT Kconfig
option to do something useful and prepares for the upcoming
rcupdate.rcu_normal_after_boot kernel parameter.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 init/main.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/init/main.c b/init/main.c
index 9e64d7097f1a..c6ebefafa496 100644
--- a/init/main.c
+++ b/init/main.c
@@ -943,6 +943,8 @@ static int __ref kernel_init(void *unused)
 
 	flush_delayed_fput();
 
+	rcu_end_inkernel_boot();
+
 	if (ramdisk_execute_command) {
 		ret = run_init_process(ramdisk_execute_command);
 		if (!ret)
-- 
2.5.2

[PATCH v2 tip/core/rcu 10/11] rcu: Allow expedited grace periods to be disabled at init

[Paul E. McKenney]

Expedited grace periods can speed up boot, but are undesirable in
aggressive real-time systems.  This commit therefore introduces a
kernel parameter rcupdate.rcu_normal_after_boot that disables
expedited grace periods just before init is spawned.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
 Documentation/kernel-parameters.txt | 5 +++++
 kernel/rcu/update.c                 | 5 +++++
 2 files changed, 10 insertions(+)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 7673943d3085..197305bbb9b7 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3318,6 +3318,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			grace-period latency.  This parameter overrides
 			rcupdate.rcu_expedited.
 
+	rcupdate.rcu_normal_after_boot= [KNL]
+			Once boot has completed (that is, after
+			rcu_end_inkernel_boot() has been invoked), use
+			only normal grace-period primitives.
+
 	rcupdate.rcu_task_stall_timeout= [KNL]
 			Set timeout in jiffies for RCU task stall warning
 			messages.  Disable with a value less than or equal
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 8fccda3a794d..12b91f5a60a6 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -63,6 +63,9 @@ MODULE_ALIAS("rcupdate");
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
 
+static int rcu_normal_after_boot;
+module_param(rcu_normal_after_boot, int, 0);
+
 #if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
 /**
  * rcu_read_lock_sched_held() - might we be in RCU-sched read-side critical section?
@@ -178,6 +181,8 @@ void rcu_end_inkernel_boot(void)
 {
 	if (IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT))
 		rcu_unexpedite_gp();
+	if (rcu_normal_after_boot)
+		WRITE_ONCE(rcu_normal, 1);
 }
 
 #ifdef CONFIG_PREEMPT_RCU
-- 
2.5.2

[PATCH v2 tip/core/rcu 11/11] rcu: Remove TINY_RCU bloat from pointless boot parameters

[Paul E. McKenney]

The rcu_expedited, rcu_normal, and rcu_normal_after_boot kernel boot
parameters are pointless in the case of TINY_RCU because in that case
synchronous grace periods, both expedited and normal, are no-ops.
However, these three symbols contribute several hundred bytes of bloat.
This commit therefore uses CPP directives to avoid compiling this code
in TINY_RCU kernels.

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
---
 Documentation/kernel-parameters.txt | 13 ++++++++-----
 include/linux/rcupdate.h            |  9 ++++++++-
 kernel/ksysfs.c                     |  4 ++++
 kernel/rcu/update.c                 |  7 ++++---
 4 files changed, 24 insertions(+), 9 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 197305bbb9b7..d8186da15ca1 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -3308,20 +3308,23 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			of synchronize_rcu().  This reduces latency,
 			but can increase CPU utilization, degrade
 			real-time latency, and degrade energy efficiency.
+			No effect on CONFIG_TINY_RCU kernels.
 
 	rcupdate.rcu_normal= [KNL]
 			Use only normal grace-period primitives,
 			for example, synchronize_rcu() instead of
 			synchronize_rcu_expedited().  This improves
-			real-time latency, CPU utilization, and energy
-			efficiency, but can expose users to increased
-			grace-period latency.  This parameter overrides
-			rcupdate.rcu_expedited.
+			real-time latency, CPU utilization, and
+			energy efficiency, but can expose users to
+			increased grace-period latency.  This parameter
+			overrides rcupdate.rcu_expedited.  No effect on
+			CONFIG_TINY_RCU kernels.
 
 	rcupdate.rcu_normal_after_boot= [KNL]
 			Once boot has completed (that is, after
 			rcu_end_inkernel_boot() has been invoked), use
-			only normal grace-period primitives.
+			only normal grace-period primitives.  No effect
+			on CONFIG_TINY_RCU kernels.
 
 	rcupdate.rcu_task_stall_timeout= [KNL]
 			Set timeout in jiffies for RCU task stall warning
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 98d9f30c02d4..47e95b80bebd 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -48,8 +48,10 @@
 
 #include <asm/barrier.h>
 
+#ifndef CONFIG_TINY_RCU
 extern int rcu_expedited; /* for sysctl */
 extern int rcu_normal;    /* also for sysctl */
+#endif /* #ifndef CONFIG_TINY_RCU */
 
 #ifdef CONFIG_TINY_RCU
 /* Tiny RCU doesn't expedite, as its purpose in life is instead to be tiny. */
@@ -327,7 +329,6 @@ static inline int rcu_preempt_depth(void)
 
 /* Internal to kernel */
 void rcu_init(void);
-void rcu_end_inkernel_boot(void);
 void rcu_sched_qs(void);
 void rcu_bh_qs(void);
 void rcu_check_callbacks(int user);
@@ -335,6 +336,12 @@ struct notifier_block;
 int rcu_cpu_notify(struct notifier_block *self,
 		   unsigned long action, void *hcpu);
 
+#ifndef CONFIG_TINY_RCU
+void rcu_end_inkernel_boot(void);
+#else /* #ifndef CONFIG_TINY_RCU */
+static inline void rcu_end_inkernel_boot(void) { }
+#endif /* #ifndef CONFIG_TINY_RCU */
+
 #ifdef CONFIG_RCU_STALL_COMMON
 void rcu_sysrq_start(void);
 void rcu_sysrq_end(void);
diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index b4e2fa52d8bc..152da4a48867 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -144,6 +144,7 @@ static ssize_t fscaps_show(struct kobject *kobj,
 }
 KERNEL_ATTR_RO(fscaps);
 
+#ifndef CONFIG_TINY_RCU
 int rcu_expedited;
 static ssize_t rcu_expedited_show(struct kobject *kobj,
 				  struct kobj_attribute *attr, char *buf)
@@ -177,6 +178,7 @@ static ssize_t rcu_normal_store(struct kobject *kobj,
 	return count;
 }
 KERNEL_ATTR_RW(rcu_normal);
+#endif /* #ifndef CONFIG_TINY_RCU */
 
 /*
  * Make /sys/kernel/notes give the raw contents of our kernel .notes section.
@@ -219,8 +221,10 @@ static struct attribute * kernel_attrs[] = {
 	&kexec_crash_size_attr.attr,
 	&vmcoreinfo_attr.attr,
 #endif
+#ifndef CONFIG_TINY_RCU
 	&rcu_expedited_attr.attr,
 	&rcu_normal_attr.attr,
+#endif
 	NULL
 };
 
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 12b91f5a60a6..76b94e19430b 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -60,11 +60,12 @@ MODULE_ALIAS("rcupdate");
 #endif
 #define MODULE_PARAM_PREFIX "rcupdate."
 
+#ifndef CONFIG_TINY_RCU
 module_param(rcu_expedited, int, 0);
 module_param(rcu_normal, int, 0);
-
 static int rcu_normal_after_boot;
 module_param(rcu_normal_after_boot, int, 0);
+#endif /* #ifndef CONFIG_TINY_RCU */
 
 #if defined(CONFIG_DEBUG_LOCK_ALLOC) && defined(CONFIG_PREEMPT_COUNT)
 /**
@@ -172,8 +173,6 @@ void rcu_unexpedite_gp(void)
 }
 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
 
-#endif /* #ifndef CONFIG_TINY_RCU */
-
 /*
  * Inform RCU of the end of the in-kernel boot sequence.
  */
@@ -185,6 +184,8 @@ void rcu_end_inkernel_boot(void)
 		WRITE_ONCE(rcu_normal, 1);
 }
 
+#endif /* #ifndef CONFIG_TINY_RCU */
+
 #ifdef CONFIG_PREEMPT_RCU
 
 /*
-- 
2.5.2

Re: [PATCH v2 tip/core/rcu 03/11] rcu: Move smp_mb() from rcu_seq_snap() to rcu_exp_gp_seq_snap()

[Peter Zijlstra]

On Wed, Dec 09, 2015 at 03:03:00PM -0800, Paul E. McKenney wrote:
> The memory barrier in rcu_seq_snap() is needed only for grace periods,
> so this commit moves it to the grace-period-oriented wrapper
> rcu_exp_gp_seq_snap().

This is a bit short for a memory barrier changelog. I think I remember
enough of the code to see this is indeed so, but who knows who will
remember what in another few weeks :-)

Re: [PATCH v2 tip/core/rcu 03/11] rcu: Move smp_mb() from rcu_seq_snap() to rcu_exp_gp_seq_snap()

[Paul E. McKenney]

On Thu, Dec 10, 2015 at 09:29:39PM +0100, Peter Zijlstra wrote:
> On Wed, Dec 09, 2015 at 03:03:00PM -0800, Paul E. McKenney wrote:
> > The memory barrier in rcu_seq_snap() is needed only for grace periods,
> > so this commit moves it to the grace-period-oriented wrapper
> > rcu_exp_gp_seq_snap().
> 
> This is a bit short for a memory barrier changelog. I think I remember
> enough of the code to see this is indeed so, but who knows who will
> remember what in another few weeks :-)

;-)

Would it help to add something like the following?

	This change does not add or remove a memory barrier, but instead
	associates it with the right level of abstraction.

							Thanx, Paul