On Thu, Oct 23, 2025 at 11:27:42AM +0800, Kunwu Chan wrote:
> From: Kunwu Chan <[email protected]>
>
> The original comments introduced in commit 05c5df31afd1
> ("rcu: Make RCU able to tolerate undefined CONFIG_RCU_FANOUT"),
> contained confusing annotations.
>
> Specifically, the #else and #endif comments did not clearly reflect
> their corresponding condition blocks, hampering readability.
>
> Fixes condition branch comments. And adds explicit explanations of
> the overall purpose:
> defining middle/leaf fan-out parameters, their relation to Kconfig,
> and how they shape the RCU hierarchy based on CPU count.
>
> Make the hierarchical configuration logic of the RCU easier to understand.
>
> Signed-off-by: Kunwu Chan <[email protected]>
Thank you for posting this! Please see below for some comments.
Thanx, Paul
> ---
> include/linux/rcu_node_tree.h | 16 ++++++++++++----
> 1 file changed, 12 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/rcu_node_tree.h b/include/linux/rcu_node_tree.h
> index 78feb8ba7358..b03c0ce91dec 100644
> --- a/include/linux/rcu_node_tree.h
> +++ b/include/linux/rcu_node_tree.h
> @@ -25,26 +25,34 @@
> /*
> * Define shape of hierarchy based on NR_CPUS, CONFIG_RCU_FANOUT, and
> * CONFIG_RCU_FANOUT_LEAF.
> + * - RCU_FANOUT: Controls fan-out of middle levels in the RCU hierarchy.
> + * - RCU_FANOUT_LEAF: Controls fan-out of the leaf level (directly managing
> CPUs).
> + *
> + * These parameters are determined by Kconfig options if configured;
> otherwise,
> + * they use sensible defaults based on system architecture (for RCU_FANOUT)
> + * or a fixed default (for RCU_FANOUT_LEAF).
I have no objections to this change if at least one of my fellow
maintainers is willing to speak up for it and none of the others object
to it.
> * In theory, it should be possible to add more levels straightforwardly.
> * In practice, this did work well going from three levels to four.
> * Of course, your mileage may vary.
> */
>
> +/* Define RCU_FANOUT: middle-level fan-out parameter */
> #ifdef CONFIG_RCU_FANOUT
> #define RCU_FANOUT CONFIG_RCU_FANOUT
> -#else /* #ifdef CONFIG_RCU_FANOUT */
> +#else /* #ifndef CONFIG_RCU_FANOUT */
> # ifdef CONFIG_64BIT
> # define RCU_FANOUT 64
> # else
> # define RCU_FANOUT 32
> # endif
> -#endif /* #else #ifdef CONFIG_RCU_FANOUT */
> +#endif
>
> +/* Define RCU_FANOUT_LEAF: leaf-level fan-out parameter (manages CPUs
> directly) */
> #ifdef CONFIG_RCU_FANOUT_LEAF
> #define RCU_FANOUT_LEAF CONFIG_RCU_FANOUT_LEAF
> -#else /* #ifdef CONFIG_RCU_FANOUT_LEAF */
> +#else /* #ifndef CONFIG_RCU_FANOUT_LEAF */
> #define RCU_FANOUT_LEAF 16
> -#endif /* #else #ifdef CONFIG_RCU_FANOUT_LEAF */
> +#endif
But these much stay as they are. The #else echos the "#if" condition, and
the #endif contains "#else" followed by the "#if" condition. This means
that you can tell where you are without having to find the matching "#if"
and without having to figure out whether there is an intervening "#else".
> #define RCU_FANOUT_1 (RCU_FANOUT_LEAF)
> #define RCU_FANOUT_2 (RCU_FANOUT_1 * RCU_FANOUT)
> --
> 2.25.1
>
