Re: [PATCH 5/6] rcu/nocb: Allow empty "rcu_nocbs" kernel parameter

[Paul E. McKenney]

On Tue, Nov 23, 2021 at 01:37:07AM +0100, Frederic Weisbecker wrote:
> If a user wants to boot without any CPU in offloaded mode initially but
> with the possibility to offload them later using cpusets, provide a way
> to simply pass an empty "rcu_nocbs" kernel parameter which will enforce
> the creation of dormant nocb kthreads.

Huh.  This would have been a use for Yury Norov's "none" bitmask
specifier.  ;-)

I pulled this one in with the usual wordsmithing.

							Thanx, Paul

> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> ---
>  .../admin-guide/kernel-parameters.txt         | 26 ++++++++++++-------
>  kernel/rcu/tree_nocb.h                        | 10 ++++---
>  2 files changed, 23 insertions(+), 13 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index cd860dc7c60b..6ff1a5f06383 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -4351,20 +4351,28 @@
>  				Disable the Correctable Errors Collector,
>  				see CONFIG_RAS_CEC help text.
>  
> -	rcu_nocbs=	[KNL]
> -			The argument is a cpu list, as described above.
> +	rcu_nocbs[=cpu-list]
> +			[KNL] The optional argument is a cpu list,
> +			as described above.
>  
> -			In kernels built with CONFIG_RCU_NOCB_CPU=y, set
> -			the specified list of CPUs to be no-callback CPUs.
> -			Invocation of these CPUs' RCU callbacks will be
> -			offloaded to "rcuox/N" kthreads created for that
> -			purpose, where "x" is "p" for RCU-preempt, and
> -			"s" for RCU-sched, and "N" is the CPU number.
> -			This reduces OS jitter on the offloaded CPUs,
> +			In kernels built with CONFIG_RCU_NOCB_CPU=y, enable the
> +			no-callback CPU mode. Invocation of such CPUs' RCU
> +			callbacks will be offloaded to "rcuox/N" kthreads
> +			created	for that purpose, where "x" is "p" for
> +			RCU-preempt, and "s" for RCU-sched, and "N" is the CPU
> +			number.	This reduces OS jitter on the offloaded CPUs,
>  			which can be useful for HPC and real-time
>  			workloads.  It can also improve energy efficiency
>  			for asymmetric multiprocessors.
>  
> +			If a cpulist is passed as an argument, the specified
> +			list of	CPUs is set to no-callback mode from boot.
> +
> +			If otherwise the '=' sign and the cpulist arguments are
> +			omitted, no CPU will be set to no-callback mode from
> +			boot but cpuset will allow for toggling that mode at
> +			runtime.
> +
>  	rcu_nocb_poll	[KNL]
>  			Rather than requiring that offloaded CPUs
>  			(specified by rcu_nocbs= above) explicitly
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 9d37916278d4..d915780d40c8 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -66,14 +66,16 @@ static bool rcu_nocb_is_setup;
>  static int __init rcu_nocb_setup(char *str)
>  {
>  	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
> -	if (cpulist_parse(str, rcu_nocb_mask)) {
> -		pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
> -		cpumask_setall(rcu_nocb_mask);
> +	if (*str == '=') {
> +		if (cpulist_parse(++str, rcu_nocb_mask)) {
> +			pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
> +			cpumask_setall(rcu_nocb_mask);
> +		}
>  	}
>  	rcu_nocb_is_setup = true;
>  	return 1;
>  }
> -__setup("rcu_nocbs=", rcu_nocb_setup);
> +__setup("rcu_nocbs", rcu_nocb_setup);
>  
>  static int __init parse_rcu_nocb_poll(char *arg)
>  {
> -- 
> 2.25.1
>