[PATCH RFC v2] rcu: Add a minimum time for marking boot as completed

["Joel Fernandes (Google)" <joel@joelfernandes.org>]

On many systems, a great deal of boot happens after the kernel thinks the boot
has completed. It is difficult to determine if the system has really booted
from the kernel side. Some features like lazy-RCU can risk slowing down boot
time if, say, a callback has been added that the boot synchronously depends on.

Further, it is better to boot systems which pass 'rcu_normal_after_boot' to
stay expedited for as long as the system is still booting.

For these reasons, this commit adds a config option
'CONFIG_RCU_BOOT_END_DELAY' and a boot parameter rcupdate.boot_end_delay.

By default, this value is 20s. A system designer can choose to specify a value
here to keep RCU from marking boot completion.  The boot sequence will not be
marked ended until at least boot_end_delay milliseconds have passed.

Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
---
v1->v2:
	Update some comments and description.

 .../admin-guide/kernel-parameters.txt         |  4 +++
 cc_list                                       |  7 ++++++
 kernel/rcu/Kconfig                            | 12 +++++++++
 kernel/rcu/update.c                           | 25 +++++++++++++++++--
 4 files changed, 46 insertions(+), 2 deletions(-)
 create mode 100644 cc_list

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2429b5e3184b..0943139fdf01 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5085,6 +5085,10 @@
 	rcutorture.verbose= [KNL]
 			Enable additional printk() statements.
 
+	rcupdate.boot_end_delay= [KNL]
+			Minimum time that must elapse before the boot
+			sequence can be marked as completed.
+
 	rcupdate.rcu_cpu_stall_ftrace_dump= [KNL]
 			Dump ftrace buffer after reporting RCU CPU
 			stall warning.
diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
index 9071182b1284..0d77006a948f 100644
--- a/kernel/rcu/Kconfig
+++ b/kernel/rcu/Kconfig
@@ -217,6 +217,18 @@ config RCU_BOOST_DELAY
 
 	  Accept the default if unsure.
 
+config RCU_BOOT_END_DELAY
+	int "Minimum time before RCU may consider in-kernel boot as completed"
+	range 0 120000
+	default 20000
+	help
+	  This option specifies the minimum time in milliseconds from boot start
+	  before RCU believes boot has completed. The actual delay can be
+	  higher than this if the kernel takes a long time to initialize
+	  but it will never be smaller than this.
+
+	  Accept the default if unsure.
+
 config RCU_EXP_KTHREAD
 	bool "Perform RCU expedited work in a real-time kthread"
 	depends on RCU_BOOST && RCU_EXPERT
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 19bf6fa3ee6a..cbdad7b46841 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -62,6 +62,10 @@ module_param(rcu_normal_after_boot, int, 0444);
 #endif
 #endif /* #ifndef CONFIG_TINY_RCU */
 
+/* Minimum time in ms until RCU can consider in-kernel boot as completed. */
+static int boot_end_delay = CONFIG_RCU_BOOT_END_DELAY;
+module_param(boot_end_delay, int, 0444);
+
 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 /**
  * rcu_read_lock_held_common() - might we be in RCU-sched read-side critical section?
@@ -225,12 +229,29 @@ void rcu_unexpedite_gp(void)
 EXPORT_SYMBOL_GPL(rcu_unexpedite_gp);
 
 static bool rcu_boot_ended __read_mostly;
-
 /*
- * Inform RCU of the end of the in-kernel boot sequence.
+ * Inform RCU of the end of the in-kernel boot sequence. The boot sequence will
+ * not be marked ended until at least boot_end_delay milliseconds have passed.
  */
+void rcu_end_inkernel_boot(void);
+static void boot_rcu_work_fn(struct work_struct *work)
+{
+	rcu_end_inkernel_boot();
+}
+static DECLARE_DELAYED_WORK(boot_rcu_work, boot_rcu_work_fn);
+
 void rcu_end_inkernel_boot(void)
 {
+	if (boot_end_delay) {
+		u64 boot_ms = ktime_get_boot_fast_ns() / 1000000UL;
+
+		if (boot_ms < boot_end_delay) {
+			schedule_delayed_work(&boot_rcu_work,
+					boot_end_delay - boot_ms);
+			return;
+		}
+	}
+
 	rcu_unexpedite_gp();
 	rcu_async_relax();
 	if (rcu_normal_after_boot)
-- 
2.39.2.637.g21b0678d19-goog