On Sun, Dec 03, 2023 at 01:12:52AM +0000, Qais Yousef wrote: > To allow more flexible arrangements while still provide a single kernel > for distros, provide a boot time parameter to enable/disable lazy RCU. > > Specify: > > rcutree.enable_rcu_lazy=[y|1|n|0] > > Which also requires > > rcu_nocbs=all > > at boot time to enable/disable lazy RCU. > > To disable it by default at build time when CONFIG_RCU_LAZY=y, the new > CONFIG_RCU_LAZY_DEFAULT_OFF can be used. > > Signed-off-by: Qais Yousef (Google) <[email protected]> > --- > > Changes since v1: > > * Use module_param() instead of module_param_cb() > * Add new CONFIG_RCU_LAZY_DEFAULT_OFF to force default off. > * Remove unnecessary READ_ONCE() > > Tested on qemu only this time with various config/boot configuration to ensure > expected values are in sysfs. > > Did a bunch of build tests against various configs/archs. > > Documentation/admin-guide/kernel-parameters.txt | 5 +++++ > kernel/rcu/Kconfig | 13 +++++++++++++ > kernel/rcu/tree.c | 7 ++++++- > 3 files changed, 24 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/Kconfig b/kernel/rcu/Kconfig > index bdd7eadb33d8..e7d2dd267593 100644 > --- a/kernel/rcu/Kconfig > +++ b/kernel/rcu/Kconfig > @@ -314,6 +314,19 @@ config RCU_LAZY > To save power, batch RCU callbacks and flush after delay, memory > pressure, or callback list growing too big. > > + Requires rcu_nocbs=all to be set. > + > + Use rcutree.enable_rcu_lazy=0 to turn it off at boot time. > + > +config RCU_LAZY_DEFAULT_OFF > + bool "Turn RCU lazy invocation off by default" > + depends on RCU_LAZY > + default n > + help > + Allows building the kernel with CONFIG_RCU_LAZY=y yet keep it default > + off. Boot time param rcutree.enable_rcu_lazy=1 can be used to switch > + it back on. > + > config RCU_DOUBLE_CHECK_CB_TIME > bool "RCU callback-batch backup time check" > depends on RCU_EXPERT > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 3ac3c846105f..8b7675624815 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2719,6 +2719,9 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t > func, bool lazy_in) > } > > #ifdef CONFIG_RCU_LAZY > +static bool enable_rcu_lazy __read_mostly = > !IS_ENABLED(CONFIG_RCU_LAZY_DEFAULT_OFF); > +module_param(enable_rcu_lazy, bool, 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 > @@ -2744,6 +2747,8 @@ void call_rcu_hurry(struct rcu_head *head, > rcu_callback_t func) > __call_rcu_common(head, func, false); > } > EXPORT_SYMBOL_GPL(call_rcu_hurry); > +#else > +#define enable_rcu_lazy false > #endif > > /** > @@ -2792,7 +2797,7 @@ 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, enable_rcu_lazy); > } > EXPORT_SYMBOL_GPL(call_rcu); > I think, it makes sense. Especially for devices/systems where it is hard to recompile the kernel and deploy it. For example, Google and GKI approach.
Reviewed-by: Uladzislau Rezki (Sony) <[email protected]> -- Uladzislau Rezki
