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

[Frederic Weisbecker]

On Wed, Nov 17, 2021 at 11:46:05AM -0800, Paul E. McKenney wrote:
> 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?

Ah right!

> 
> >  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);

Don't miss that line, that should probably answer your above question, if
I didn't miss something from my end (which is not unlikely...)

> >  
> >  static int __init parse_rcu_nocb_poll(char *arg)
> >  {
> > -- 
> > 2.25.1
> >