Re: [PATCH RFC] rcu/nocb: Provide default all-CPUs mask for RCU_NOCB_CPU=y

[Paul E. McKenney]

On Thu, Apr 07, 2022 at 09:07:33PM +0000, Joel Fernandes wrote:
> On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
> which ends up not offloading any CPU. This patch removes yet another
> dependency from the bootloader having to know about RCU, about how many
> CPUs the system has, and about how to provide the mask. Basically, I
> think we should stop pretending that the user knows what they are doing :).
> In other words, if NO_CB_CPU is enabled, lets make use of it.
> 
> My goal is to make RCU as zero-config as possible with sane defaults. If
> user wants to provide rcu_nocbs= or nohz_full= options, then those will
> take precedence and this patch will have no effect.
> 
> I tested providing rcu_nocbs= option, ensuring that is preferred over this.

Unless something has changed, this would change behavior relied upon
the enterprise distros.  Last I checked, they want to supply a single
binary, as evidenced by the recent CONFIG_PREEMPT_DYNAMIC Kconfig option,
and they also want the default to be non-offloaded.  That is, given a
kernel built with CONFIG_RCU_NOCB_CPU=y and without either a nohz_full
or a nocbs_cpu boot parameter, all of the CPUs must be non-offloaded.

So for me to push this to mainline, I need an ack from someone from each
of the enterprise distros, and each of those someones needs to understand
the single-binary strategy used by the corresponding distro.

And is it really all -that- hard to specify an additional boot parameter
across ChromeOS devices?  Android seems to manage it.  ;-)

							Thanx, Paul

> Signed-off-by: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> ---
>  kernel/rcu/tree_nocb.h | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index eeafb546a7a0..607fbf843467 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1165,12 +1165,25 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
>  void __init rcu_init_nohz(void)
>  {
>  	int cpu;
> -	bool need_rcu_nocb_mask = false;
> +	bool need_rcu_nocb_mask = false, set_nocb_mask_all = false;
>  	struct rcu_data *rdp;
>  
> +	/*
> +	 * In case rcu_nocbs= was not passed on the kernel command line,
> +	 * provide a sane default by offloading all CPUs. This provides a
> +	 * sane default for rcu_nocbs and prevents users overlooking these
> +	 * details.
> +	 */
> +	if (!rcu_nocb_is_setup) {
> +		need_rcu_nocb_mask = true;
> +		set_nocb_mask_all = true;
> +	}
> +
>  #if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
> +	if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask)) {
>  		need_rcu_nocb_mask = true;
> +		set_nocb_mask_all = false; /* NO_HZ_FULL provides its own mask. */
> +	}
>  #endif /* #if defined(CONFIG_NO_HZ_FULL) */
>  
>  	if (need_rcu_nocb_mask) {
> @@ -1191,6 +1204,9 @@ void __init rcu_init_nohz(void)
>  		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
>  #endif /* #if defined(CONFIG_NO_HZ_FULL) */
>  
> +	if (set_nocb_mask_all)
> +		cpumask_setall(rcu_nocb_mask);
> +
>  	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
>  		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
>  		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> -- 
> 2.35.1.1178.g4f1659d476-goog
>