APPLIED: [N/U][PATCH 1/1] UBUNTU: SAUCE: rcu: Provide a boot time parameter to control lazy RCU

[Dimitri John Ledkov]

On Sun, 3 Dec 2023 at 08:59, Andrea Righi <andrea.righi at canonical.com> wrote:
>
> From: Qais Yousef <qyousef at layalina.io>
>
> BugLink: https://bugs.launchpad.net/bugs/2045492
>
> 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 at layalina.io>
> (cherry picked from https://lore.kernel.org/lkml/20231203011252.233748-1-qyousef@layalina.io/T/#u)
> Signed-off-by: Andrea Righi <andrea.righi at canonical.com>
> ---
>  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 b9c7a580f7ec..189020bf0ae9 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -5031,6 +5031,11 @@
>                         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);
>
> --
> 2.40.1
>
>
> --
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-- 
Dimitri

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