rcu: Add a module parameter to force use of expedited RCU primitives

There have been some embedded applications that would benefit from
use of expedited grace-period primitives.  In some ways, this is
similar to synchronize_net() doing either a normal or an expedited
grace period depending on lock state, but with control outside of
the kernel.

This commit therefore adds rcu_expedited boot and sysfs parameters
that cause the kernel to substitute expedited primitives for the
normal grace-period primitives.

[ paulmck: Add trace/event/rcu.h to kernel/srcu.c to avoid build error.
	   Get rid of infinite loop through contention path.]

Signed-off-by: Antti P Miettinen <amiettinen@nvidia.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

7 files changed, 48 insertions, 7 deletions

diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c
index 4e316e1acf58..8715a798aa7c 100644
--- a/kernel/ksysfs.c
+++ b/kernel/ksysfs.c
@@ -141,6 +141,23 @@ static ssize_t fscaps_show(struct kobject *kobj,
 }
 KERNEL_ATTR_RO(fscaps);
 
+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);
+}
+static ssize_t rcu_expedited_store(struct kobject *kobj,
+                                   struct kobj_attribute *attr,
+                                   const char *buf, size_t count)
+{
+        if (kstrtoint(buf, 0, &rcu_expedited))
+                return -EINVAL;
+
+        return count;
+}
+KERNEL_ATTR_RW(rcu_expedited);
+
 /*
  * Make /sys/kernel/notes give the raw contents of our kernel .notes section.
  */
@@ -182,6 +199,7 @@ static struct attribute * kernel_attrs[] = {
         &kexec_crash_size_attr.attr,
         &vmcoreinfo_attr.attr,
 #endif
+        &rcu_expedited_attr.attr,
         NULL
 };
 
diff --git a/kernel/rcu.h b/kernel/rcu.h
index 8ba99cdc6515..20dfba576c2b 100644
--- a/kernel/rcu.h
+++ b/kernel/rcu.h
@@ -109,4 +109,6 @@ static inline bool __rcu_reclaim(char *rn, struct rcu_head *head)
         }
 }
 
+extern int rcu_expedited;
+
 #endif /* __LINUX_RCU_H */
diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
index 29ca1c6da594..a2cf76177b44 100644
--- a/kernel/rcupdate.c
+++ b/kernel/rcupdate.c
@@ -46,12 +46,15 @@
 #include <linux/export.h>
 #include <linux/hardirq.h>
 #include <linux/delay.h>
+#include <linux/module.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/rcu.h>
 
 #include "rcu.h"
 
+module_param(rcu_expedited, int, 0);
+
 #ifdef CONFIG_PREEMPT_RCU
 
 /*
diff --git a/kernel/rcutiny_plugin.h b/kernel/rcutiny_plugin.h
index 3d0190282204..f85016a2309b 100644
--- a/kernel/rcutiny_plugin.h
+++ b/kernel/rcutiny_plugin.h
@@ -706,7 +706,10 @@ void synchronize_rcu(void)
                 return;
 
         /* Once we get past the fastpath checks, same code as rcu_barrier(). */
-        rcu_barrier();
+        if (rcu_expedited)
+                synchronize_rcu_expedited();
+        else
+                rcu_barrier();
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu);
 
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 74df86bd9204..f9c17c399538 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2224,7 +2224,10 @@ void synchronize_sched(void)
                            "Illegal synchronize_sched() in RCU-sched read-side critical section");
         if (rcu_blocking_is_gp())
                 return;
-        wait_rcu_gp(call_rcu_sched);
+        if (rcu_expedited)
+                synchronize_sched_expedited();
+        else
+                wait_rcu_gp(call_rcu_sched);
 }
 EXPORT_SYMBOL_GPL(synchronize_sched);
 
@@ -2245,7 +2248,10 @@ void synchronize_rcu_bh(void)
                            "Illegal synchronize_rcu_bh() in RCU-bh read-side critical section");
         if (rcu_blocking_is_gp())
                 return;
-        wait_rcu_gp(call_rcu_bh);
+        if (rcu_expedited)
+                synchronize_rcu_bh_expedited();
+        else
+                wait_rcu_gp(call_rcu_bh);
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
 
@@ -2328,7 +2334,7 @@ void synchronize_sched_expedited(void)
                 if (trycount++ < 10) {
                         udelay(trycount * num_online_cpus());
                 } else {
-                        synchronize_sched();
+                        wait_rcu_gp(call_rcu_sched);
                         return;
                 }
 
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index f92115488187..c177ba0cce9a 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -679,7 +679,10 @@ void synchronize_rcu(void)
                            "Illegal synchronize_rcu() in RCU read-side critical section");
         if (!rcu_scheduler_active)
                 return;
-        wait_rcu_gp(call_rcu);
+        if (rcu_expedited)
+                synchronize_rcu_expedited();
+        else
+                wait_rcu_gp(call_rcu);
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu);
 
@@ -831,7 +834,7 @@ void synchronize_rcu_expedited(void)
                         udelay(trycount * num_online_cpus());
                 } else {
                         put_online_cpus();
-                        synchronize_rcu();
+                        wait_rcu_gp(call_rcu);
                         return;
                 }
         }
diff --git a/kernel/srcu.c b/kernel/srcu.c
index 97c465ebd844..de9074047c92 100644
--- a/kernel/srcu.c
+++ b/kernel/srcu.c
@@ -34,6 +34,10 @@
 #include <linux/delay.h>
 #include <linux/srcu.h>
 
+#include <trace/events/rcu.h>
+
+#include "rcu.h"
+
 /*
  * Initialize an rcu_batch structure to empty.
  */
@@ -464,7 +468,9 @@ static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
  */
 void synchronize_srcu(struct srcu_struct *sp)
 {
-        __synchronize_srcu(sp, SYNCHRONIZE_SRCU_TRYCOUNT);
+        __synchronize_srcu(sp, rcu_expedited
+                           ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
+                           : SYNCHRONIZE_SRCU_TRYCOUNT);
 }
 EXPORT_SYMBOL_GPL(synchronize_srcu);