linux-kernel - Re: [PATCH v4] rcu/nocb: Add an option to offload all CPUs on boot

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Date:   Mon, 18 Apr 2022 16:57:46 -0700
From:   Kalesh Singh <kaleshsingh@...gle.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        rushikesh.s.kadam@...el.com, vineethrp@...il.com, urezki@...il.com
Subject: Re: [PATCH v4] rcu/nocb: Add an option to offload all CPUs on boot

On Mon, Apr 18, 2022 at 05:54:03PM +0000, Joel Fernandes (Google) wrote:
> From: Joel Fernandes <joel@...lfernandes.org>
> 
> On systems with CONFIG_RCU_NOCB_CPU=y, there is no default mask provided
> which ends up not offloading any CPU. This patch removes a dependency
> from the bootloader having to know about RCU and about how to provide
> the mask.
> 
> With the new option enabled, all CPUs will be offloaded on boot unless
> rcu_nocbs= or rcu_nohz_full= kernel parameters provide a CPU list.
> 
> Signed-off-by: Joel Fernandes <joel@...lfernandes.org>

Hi Joel,

You can add Reviewed-by: Kalesh Singh <kaleshsingh@...gle.com>

Thanks,
Kalesh

> ---
> v4: mostly style related fixes.
> v3 is back to v1 but with a config option defaulting to 'n'.
> v2 was forcing the option to override no_cbs=
> 
>  Documentation/admin-guide/kernel-parameters.txt |  6 ++++++
>  kernel/rcu/Kconfig                              | 13 +++++++++++++
>  kernel/rcu/tree_nocb.h                          | 15 ++++++++++++++-
>  3 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index f5a27f067db9..4beb15ccac1a 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3506,6 +3506,9 @@
>  			just as if they had also been called out in the
>  			rcu_nocbs= boot parameter.
>  
> +			Note that this argument takes precedence over
> +			the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
> +
>  	noiotrap	[SH] Disables trapped I/O port accesses.
>  
>  	noirqdebug	[X86-32] Disables the code which attempts to detect and
> @@ -4398,6 +4401,9 @@
>  			no-callback mode from boot but the mode may be
>  			toggled at runtime via cpusets.
>  
> +			Note that this argument takes precedence over
> +			the CONFIG_RCU_NOCB_CPU_DEFAULT_ALL option.
> +
>  	rcu_nocb_poll	[KNL]
>  			Rather than requiring that offloaded CPUs
>  			(specified by rcu_nocbs= above) explicitly
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index bf8e341e75b4..746a668bf81d 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -223,6 +223,19 @@ config RCU_NOCB_CPU
>  	  Say Y here if you need reduced OS jitter, despite added overhead.
>  	  Say N here if you are unsure.
>  
> +config RCU_NOCB_CPU_DEFAULT_ALL
> +	bool "Offload RCU callback processing from all CPUs by default"
> +	depends on RCU_NOCB_CPU
> +	default n
> +	help
> +	  Use this option to offload callback processing from all CPUs
> +	  by default, in the absence of the rcu_nocbs or nohz_full boot
> +	  parameter. This also avoids the need to use any boot parameters
> +	  to achieve the effect of offloading all CPUs on boot.
> +
> +	  Say Y here if you want offload all CPUs by default on boot.
> +	  Say N here if you are unsure.
> +
>  config TASKS_TRACE_RCU_READ_MB
>  	bool "Tasks Trace RCU readers use memory barriers in user and idle"
>  	depends on RCU_EXPERT
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index eeafb546a7a0..f648f773600a 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1166,11 +1166,21 @@ void __init rcu_init_nohz(void)
>  {
>  	int cpu;
>  	bool need_rcu_nocb_mask = false;
> +	bool offload_all = false;
>  	struct rcu_data *rdp;
>  
> +#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> +	if (!rcu_nocb_is_setup) {
> +		need_rcu_nocb_mask = true;
> +		offload_all = true;
> +	}
> +#endif
> +
>  #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;
> +		offload_all = false; /* NO_HZ_FULL has its own mask. */
> +	}
>  #endif /* #if defined(CONFIG_NO_HZ_FULL) */
>  
>  	if (need_rcu_nocb_mask) {
> @@ -1191,6 +1201,9 @@ 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 (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,
> -- 
> 2.36.0.rc0.470.gd361397f0d-goog
> 
>