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) <qyousef@xxxxxxxxxxx>
---
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
