linux-kernel - Re: [PATCH v6 1/2] rcu: Simplify rcu_init

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Date:   Thu, 1 Sep 2022 09:54:55 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/2] rcu: Simplify rcu_init_nohz() cpumask handling

On Thu, Sep 01, 2022 at 09:25:06PM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/9/1 21:14, Zhen Lei wrote:
> > In kernels built with either CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or
> > CONFIG_NO_HZ_FULL=y, additional CPUs must be added to rcu_nocb_mask.
> > Except that kernels booted without the rcu_nocbs= will not have
> > allocated rcu_nocb_mask.  And the current rcu_init_nohz() function uses
> > its need_rcu_nocb_mask and offload_all local variables to track the
> > rcu_nocb and nohz_full state.
> > 
> > But there is a much simpler approach, namely creating a cpumask pointer
> > to track the default and then using cpumask_available() to check the
> > rcu_nocb_mask state.  This commit takes this approach, thereby simplifying
> > and shortening the rcu_init_nohz() function.
> > 
> > Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> Sorry, I forgot remove this "Signed-off-by".

Not a problem, especially given that in view of my oversight on the
previous version, I am going to let Frederic take first crack at this
version.  ;-)

							Thanx, Paul

> > Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> >  kernel/rcu/tree_nocb.h | 35 ++++++++++++-----------------------
> >  1 file changed, 12 insertions(+), 23 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 0a5f0ef41484518..8b6dceeabde0b4d 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1210,45 +1210,34 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
> >  void __init rcu_init_nohz(void)
> >  {
> >  	int cpu;
> > -	bool need_rcu_nocb_mask = false;
> > -	bool offload_all = false;
> >  	struct rcu_data *rdp;
> > -
> > -#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> > -	if (!rcu_state.nocb_is_setup) {
> > -		need_rcu_nocb_mask = true;
> > -		offload_all = true;
> > -	}
> > -#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
> > +	const struct cpumask *cpumask = NULL;
> >  
> >  #if defined(CONFIG_NO_HZ_FULL)
> > -	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) {
> > -		need_rcu_nocb_mask = true;
> > -		offload_all = false; /* NO_HZ_FULL has its own mask. */
> > -	}
> > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > +		cpumask = tick_nohz_full_mask;
> > +#endif
> > +
> > +#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
> > +	if (!rcu_state.nocb_is_setup && !cpumask)
> > +		cpumask = cpu_possible_mask;
> > +#endif
> >  
> > -	if (need_rcu_nocb_mask) {
> > +	if (cpumask) {
> >  		if (!cpumask_available(rcu_nocb_mask)) {
> >  			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
> >  				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
> >  				return;
> >  			}
> >  		}
> > +
> > +		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
> >  		rcu_state.nocb_is_setup = true;
> >  	}
> >  
> >  	if (!rcu_state.nocb_is_setup)
> >  		return;
> >  
> > -#if defined(CONFIG_NO_HZ_FULL)
> > -	if (tick_nohz_full_running)
> > -		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
> > -#endif /* #if defined(CONFIG_NO_HZ_FULL) */
> > -
> > -	if (offload_all)
> > -		cpumask_setall(rcu_nocb_mask);
> > -
> >  	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
> >  		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
> >  		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
> > 
> 
> -- 
> Regards,
>   Zhen Lei