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

Paul E. McKenney
2021-11-17T22:33:20Z

On Wed, Nov 17, 2021 at 11:02:30PM +0100, Frederic Weisbecker wrote:
> 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...)

My next step would be to add a printk() and try booting with different
rcu_nocbs parameter settings. ;-)

Thanx, Paul

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