[PATCH] rcu: Provide a boot time parameter to enable lazy RCU

[Qais Yousef]

To allow more flexible opt-in arrangements while still provide a single
kernel for distros, provide a boot time parameter to enable lazy RCU.

Specify:

        rcutree.enable_rcu_lazy

Which also requires

        rcu_nocbs=all

at boot time to enable lazy RCU assuming CONFIG_RCU_LAZY=y. The
parameter will be ignored if CONFIG_RCU_LAZY is not set.

With this change now lazy RCU is disabled by default if the boot
parameter is not set even when CONFIG_RCU_LAZY is enabled.

Signed-off-by: Qais Yousef (Google) <qyou...@layalina.io>
---

Makes sense to remove the CONFIG_RCU_LAZY now we have a boot time param?

We can make it a static key too if it *really* matters.

Thanks to Joel for helping initially in reviewing this patch which was intended
originally for Android.

I got some requests to make this a runtime modifiable for init scripts; but
Paul suggested there shall be dragons. So RO it is.


 .../admin-guide/kernel-parameters.txt         |  5 ++++
 kernel/rcu/tree.c                             | 26 ++++++++++++++++++-
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt 
b/Documentation/admin-guide/kernel-parameters.txt
index 65731b060e3f..2f0386a12aa7 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -5021,6 +5021,11 @@
                        this kernel boot parameter, forcibly setting it
                        to zero.
 
+       rcutree.enable_rcu_lazy= [KNL]
+                       To save power, batch RCU callbacks and flush after
+                       delay, memory pressure or callback list growing too
+                       big.
+
        rcuscale.gp_async= [KNL]
                        Measure performance of asynchronous
                        grace-period primitives such as call_rcu().
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3ac3c846105f..e0885905b3f6 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2718,7 +2718,30 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t 
func, bool lazy_in)
        }
 }
 
+static bool enable_rcu_lazy;
 #ifdef CONFIG_RCU_LAZY
+/* Enable lazy rcu at boot time */
+static int param_set_rcu_lazy(const char *val, const struct kernel_param *kp)
+{
+       int ret;
+
+       /*
+        * Make sure a grace period has passed before and after flipping the
+        * switch.
+        */
+       rcu_barrier();
+       ret = param_set_bool(val, kp);
+       rcu_barrier();
+
+       return ret;
+}
+static const struct kernel_param_ops rcu_lazy_ops = {
+       .flags = KERNEL_PARAM_OPS_FL_NOARG,
+       .set = param_set_rcu_lazy,
+       .get = param_get_bool,
+};
+module_param_cb(enable_rcu_lazy, &rcu_lazy_ops, &enable_rcu_lazy, 0444);
+
 /**
  * call_rcu_hurry() - Queue RCU callback for invocation after grace period, and
  * flush all lazy callbacks (including the new one) to the main ->cblist while
@@ -2792,7 +2815,8 @@ EXPORT_SYMBOL_GPL(call_rcu_hurry);
  */
 void call_rcu(struct rcu_head *head, rcu_callback_t func)
 {
-       __call_rcu_common(head, func, IS_ENABLED(CONFIG_RCU_LAZY));
+       __call_rcu_common(head, func, IS_ENABLED(CONFIG_RCU_LAZY) &&
+                         READ_ONCE(enable_rcu_lazy));
 }
 EXPORT_SYMBOL_GPL(call_rcu);
 
-- 
2.34.1