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) <qyousef@xxxxxxxxxxx>
Thanks Qais, I have a comment below:
> ---
>
> 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.
> +
I think a better approach is not do an anti-CONFIG option and instead do
a shorter parameter "rcutree.lazy=0". If CONFIG_RCU_LAZY is set, then we can
just default to keeping lazy on. I'd like to avoid proliferation of already
large number of RCU config options and more chances of errors.
I also want lazy to be ON for everybody configuring it into the kernel by
default (those who don't want it just set CONFIG_RCU_LAZY=n), this is what
tglx also suggested that's why we made changed of our initial prototypes of
call_rcu_lazy() and instead we made call_rcu() to put everyone on the lazy
train and not add more APIs (thus causing more confusion to kernel
developers). This was a bit painful, but it was worth it.
> 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);
And then this can just be = true;
thanks,
- Joel
> +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);
>
> --
> 2.34.1
>
