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>
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 57a4d1f..b9f039b 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -278,6 +278,7 @@
/* 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 f5dbc6d..9a05925 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -791,6 +791,19 @@
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 5f85082..7b12466 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -64,7 +64,8 @@
#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 @@
#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
