linux-kernel - Re: [PATCH 2/5] rcu/nocb: Move rcu_nocb_is_setup to rcu

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Date:   Thu, 17 Feb 2022 09:50:47 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Frederic Weisbecker <frederic@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 2/5] rcu/nocb: Move rcu_nocb_is_setup to rcu_state

On Thu, Feb 17, 2022 at 07:11:43AM -0800, Paul E. McKenney wrote:
> On Thu, Feb 17, 2022 at 12:30:12PM +0100, Frederic Weisbecker wrote:
> > On Wed, Feb 16, 2022 at 09:28:04PM -0800, Paul E. McKenney wrote:
> > > On Wed, Feb 16, 2022 at 05:53:32PM -0800, Paul E. McKenney wrote:
> > > > On Wed, Feb 16, 2022 at 01:30:35PM -0800, Paul E. McKenney wrote:
> > > > > On Wed, Feb 16, 2022 at 04:42:05PM +0100, Frederic Weisbecker wrote:
> > > > > > In order to avoid scattering the global RCU states, move the RCU nocb
> > > > > > initialization witness within rcu_state.
> > > > > > 
> > > > > > Reported-by: Paul E. McKenney <paulmck@...nel.org>
> > > > > > Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> > > > > > Cc: Uladzislau Rezki <uladzislau.rezki@...y.com>
> > > > > > Cc: Joel Fernandes <joel@...lfernandes.org>
> > > > > > Cc: Boqun Feng <boqun.feng@...il.com>
> > > > > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > > > > 
> > > > > Build testing for CONFIG_NO_HZ_FULL=n kernels suggested the following
> > > > > patch be merged into this one.  Or does this variable need to be used
> > > > > somewhere?  Either way, please let me know!
> > > > 
> > > > TASKS03, TREE04, and TREE07 don't like this much, according to git
> > > > bisect.  They are the ones with nohz_full CPUs, in case that is crucial.
> > > > Trying again after reverting this commit locally.
> > > > 
> > > > If that works, I would be tempted to try the modification shown below.
> > > 
> > > And that does pass significant rcutorture testing, but over to you.  ;-)
> > 
> > Ah no we still need that variable, but I screwed up and wrongly converted
> > need_rcu_nocb_mask to rcu_state.nocb_is_setup somewhere.
> > 
> > Here is the v2 of the culprit patch (sorry):
> 
> Been there, done that!  ;-)
> 
> I pulled this one in and I will let you know how it goes.

And it passed moderate rcutorture testing with flying colors!  ;-)

So let's see what -next makes of it...

							Thanx, Paul

> > ---
> > From: Frederic Weisbecker <frederic@...nel.org>
> > Date: Mon, 14 Feb 2022 14:23:39 +0100
> > Subject: [PATCH] rcu/nocb: Move rcu_nocb_is_setup to rcu_state
> > 
> > In order to avoid scattering the global RCU states, move the RCU nocb
> > initialization witness within rcu_state.
> > 
> > Reported-by: Paul E. McKenney <paulmck@...nel.org>
> > Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> > Cc: Uladzislau Rezki <uladzislau.rezki@...y.com>
> > Cc: Joel Fernandes <joel@...lfernandes.org>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > ---
> >  kernel/rcu/tree.h      |  1 +
> >  kernel/rcu/tree_nocb.h | 13 +++++--------
> >  2 files changed, 6 insertions(+), 8 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> > index 19fc9acce3ce..735fc410d76a 100644
> > --- a/kernel/rcu/tree.h
> > +++ b/kernel/rcu/tree.h
> > @@ -368,6 +368,7 @@ struct rcu_state {
> >  	arch_spinlock_t ofl_lock ____cacheline_internodealigned_in_smp;
> >  						/* Synchronize offline with */
> >  						/*  GP pre-initialization. */
> > +	int nocb_is_setup;			/* nocb is setup from boot */
> >  };
> >  
> >  /* Values for rcu_state structure's gp_flags field. */
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 02e1d05a11fc..3c00240833d6 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -60,9 +60,6 @@ static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
> >   * Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.
> >   * If the list is invalid, a warning is emitted and all CPUs are offloaded.
> >   */
> > -
> > -static bool rcu_nocb_is_setup;
> > -
> >  static int __init rcu_nocb_setup(char *str)
> >  {
> >  	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
> > @@ -72,7 +69,7 @@ static int __init rcu_nocb_setup(char *str)
> >  			cpumask_setall(rcu_nocb_mask);
> >  		}
> >  	}
> > -	rcu_nocb_is_setup = true;
> > +	rcu_state.nocb_is_setup = true;
> >  	return 1;
> >  }
> >  __setup("rcu_nocbs", rcu_nocb_setup);
> > @@ -1172,10 +1169,10 @@ void __init rcu_init_nohz(void)
> >  				return;
> >  			}
> >  		}
> > -		rcu_nocb_is_setup = true;
> > +		rcu_state.nocb_is_setup = true;
> >  	}
> >  
> > -	if (!rcu_nocb_is_setup)
> > +	if (!rcu_state.nocb_is_setup)
> >  		return;
> >  
> >  #if defined(CONFIG_NO_HZ_FULL)
> > @@ -1233,7 +1230,7 @@ static void rcu_spawn_cpu_nocb_kthread(int cpu)
> >  	struct task_struct *t;
> >  	struct sched_param sp;
> >  
> > -	if (!rcu_scheduler_fully_active || !rcu_nocb_is_setup)
> > +	if (!rcu_scheduler_fully_active || !rcu_state.nocb_is_setup)
> >  		return;
> >  
> >  	/* If there already is an rcuo kthread, then nothing to do. */
> > @@ -1279,7 +1276,7 @@ static void __init rcu_spawn_nocb_kthreads(void)
> >  {
> >  	int cpu;
> >  
> > -	if (rcu_nocb_is_setup) {
> > +	if (rcu_state.nocb_is_setup) {
> >  		for_each_online_cpu(cpu)
> >  			rcu_spawn_cpu_nocb_kthread(cpu);
> >  	}
> > -- 
> > 2.25.1