Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz()

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Aug 25, 2022 at 10:26:54AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 25, 2022 at 05:23:11PM +0800, Zhen Lei wrote:
> > When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
> > CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
> > available now, due to 'rcu_nocbs' is not specified. Check and initialize
> > 'rcu_nocb_mask' before using it. This code simplification strictly follows
> > this logic, compared with old implementations, unnecessary crossovers are
> > avoided and easy to understand.
> > 
> > Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
> 
> Much nicer, thank you!
> 
> As usual, I could not resist the urge to wordsmith.  Could you please
> check to make sure that I did not mess anything up?
> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit 4ac3b3d1a19943b1522c0b1d0895aefbb80ec294
> Author: Zhen Lei <thunder.leizhen@huawei.com>
> Date:   Thu Aug 25 17:23:11 2022 +0800
> 
>     rcu: Simplify rcu_init_nohz() cpumask handling
>     
>     In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
>     CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
>     Except that kernels booted without the rcu_nocb_mask= will not have
>     allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
>     its need_rcu_nocb_mask and offload_all local variables to track the
>     rcu_nocb and nohz_full state.
>     
>     But there is a much simpler approach, namely creating a cpumask pointer
>     to track the default and then using cpumask_available() to check the
>     rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
>     and shortening the rcu_init_nohz() function.
>     
>     Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>     Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

thanks,

 - Joel


> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 0a5f0ef414845..c8167be2288fa 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1210,45 +1210,31 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
>  void __init rcu_init_nohz(void)
>  {
>  	int cpu;
> -	bool need_rcu_nocb_mask = false;
> -	bool offload_all = false;
>  	struct rcu_data *rdp;
> +	const struct cpumask *cpumask = NULL;
>  
>  #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> -	if (!rcu_state.nocb_is_setup) {
> -		need_rcu_nocb_mask = true;
> -		offload_all = true;
> -	}
> -#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
> -
> -#if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) {
> -		need_rcu_nocb_mask = true;
> -		offload_all = false; /* NO_HZ_FULL has its own mask. */
> -	}
> -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> +	cpumask = cpu_possible_mask;
> +#elif defined(CONFIG_NO_HZ_FULL)
> +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> +		cpumask = tick_nohz_full_mask;
> +#endif
>  
> -	if (need_rcu_nocb_mask) {
> +	if (cpumask) {
>  		if (!cpumask_available(rcu_nocb_mask)) {
>  			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
>  				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
>  				return;
>  			}
>  		}
> +
> +		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
>  		rcu_state.nocb_is_setup = true;
>  	}
>  
>  	if (!rcu_state.nocb_is_setup)
>  		return;
>  
> -#if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running)
> -		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> -
> -	if (offload_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,