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

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Date:   Wed, 16 Feb 2022 17:53:32 -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 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.

Thoughts?

							Thanx, Paul

> ------------------------------------------------------------------------
> 
> commit 1a4f308b3b3841ef10043fe6c3dd12fc872b0400
> Author: Paul E. McKenney <paulmck@...nel.org>
> Date:   Wed Feb 16 13:27:42 2022 -0800
> 
>     squash! rcu/nocb: Move rcu_nocb_is_setup to rcu_state
>     
>     [ paulmck: Remove unused need_rcu_nocb_mask local variable. ]
>     
>     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index 9d6d4786bc703..9d97aa1f4d6ce 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -1154,14 +1154,8 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
>  void __init rcu_init_nohz(void)
>  {
>  	int cpu;
> -	bool need_rcu_nocb_mask = false;
>  	struct rcu_data *rdp;
>  
> -#if defined(CONFIG_NO_HZ_FULL)
> -	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> -		need_rcu_nocb_mask = true;

		rcu_state.nocb_is_setup = true;

> -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> -
>  	if (rcu_state.nocb_is_setup) {
>  		if (!cpumask_available(rcu_nocb_mask)) {
>  			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {