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);