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

Frederic Weisbecker
2021-11-17T22:02:40Z

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@kernel.org>
> > Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> > Cc: Boqun Feng <boqun.feng@gmail.com>
> > Cc: Uladzislau Rezki <urezki@gmail.com>
> > Cc: Josh Triplett <josh@joshtriplett.org>
> > Cc: Joel Fernandes <joel@joelfernandes.org>
> > ---
> > 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
> >