rcu: Add Kconfig option to expedite grace periods during boot

This commit adds a CONFIG_RCU_EXPEDITE_BOOT Kconfig parameter
that emulates a very early boot rcu_expedite_gp().  A late-boot
call to rcu_end_inkernel_boot() will provide the corresponding
rcu_unexpedite_gp().  The late-boot call to rcu_end_inkernel_boot()
should be made just before init is spawned.

According to Arjan:

> To show the boot time, I'm using the timestamp of the "Write protecting"
> line, that's pretty much the last thing we print prior to ring 3 execution.
>
> A kernel with default RCU behavior (inside KVM, only virtual devices)
> looks like this:
>
> [    0.038724] Write protecting the kernel read-only data: 10240k
>
> a kernel with expedited RCU (using the command line option, so that I
> don't have to recompile between measurements and thus am completely
> oranges-to-oranges)
>
> [    0.031768] Write protecting the kernel read-only data: 10240k
>
> which, in percentage, is an 18% improvement.

Reported-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Arjan van de Ven <arjan@linux.intel.com>

3 files changed, 24 insertions, 1 deletions

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 57a4d1f73a00..b9f039b11d31 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -278,6 +278,7 @@ 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);
diff --git a/init/Kconfig b/init/Kconfig
index f5dbc6d4261b..9a0592516f48 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -791,6 +791,19 @@ config RCU_NOCB_CPU_ALL
 
 endchoice
 
+config RCU_EXPEDITE_BOOT
+        bool
+        default n
+        help
+          This option enables expedited grace periods at boot time,
+          as if rcu_expedite_gp() had been invoked early in boot.
+          The corresponding rcu_unexpedite_gp() is invoked from
+          rcu_end_inkernel_boot(), which is intended to be invoked
+          at the end of the kernel-only boot sequence, just before
+          init is exec'ed.
+
+          Accept the default if unsure.
+
 endmenu # "RCU Subsystem"
 
 config BUILD_BIN2C
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 5f850823c187..7b12466f90bc 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -64,7 +64,8 @@ module_param(rcu_expedited, int, 0);
 
 #ifndef CONFIG_TINY_RCU
 
-static atomic_t rcu_expedited_nesting;
+static atomic_t rcu_expedited_nesting =
+        ATOMIC_INIT(IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT) ? 1 : 0);
 
 /*
  * Should normal grace-period primitives be expedited?  Intended for
@@ -109,6 +110,14 @@ EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
 
 #endif /* #ifndef CONFIG_TINY_RCU */
 
+/*
+ * Inform RCU of the end of the in-kernel boot sequence.
+ */
+void rcu_end_inkernel_boot(void)
+{
+        if (IS_ENABLED(CONFIG_RCU_EXPEDITE_BOOT))
+                rcu_unexpedite_gp();
+}
 
 #ifdef CONFIG_PREEMPT_RCU