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

[Paul E. McKenney]

On Wed, Nov 17, 2021 at 04:56:36PM +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.
> 
> 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>
> ---
>  kernel/rcu/tree_nocb.h | 10 ++++++----

Could you please also update kernel-parameters.txt?

>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 1871f15b8472..3845f1885ffc 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);
> +		}

Wouldn't "*str == '='" indicate that the parameter passed in was of
the form "rcu_nocbs==8"?

Or am I misreading the next_arg() function in lib/cmdline.c?

If I am reading it correctly, doesn't the test instead want to be
something of the form "if (str && *str)"?

							Thanx, Paul

>  	}
>  	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
>