linux-kernel - Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init

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Message-ID: <20220826013602.GQ6159@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 25 Aug 2022 18:36:02 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     Zhen Lei <thunder.leizhen@...wei.com>,
        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>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/1] rcu: Simplify the code logic of rcu_init_nohz()

On Thu, Aug 25, 2022 at 07:24:05PM +0000, Joel Fernandes wrote:
> On Thu, Aug 25, 2022 at 10:26:54AM -0700, Paul E. McKenney wrote:
> > On Thu, Aug 25, 2022 at 05:23:11PM +0800, Zhen Lei wrote:
> > > When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
> > > CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
> > > available now, due to 'rcu_nocbs' is not specified. Check and initialize
> > > 'rcu_nocb_mask' before using it. This code simplification strictly follows
> > > this logic, compared with old implementations, unnecessary crossovers are
> > > avoided and easy to understand.
> > > 
> > > Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> > 
> > Much nicer, thank you!
> > 
> > As usual, I could not resist the urge to wordsmith.  Could you please
> > check to make sure that I did not mess anything up?
> > 
> > 							Thanx, Paul
> > 
> > ------------------------------------------------------------------------
> > 
> > commit 4ac3b3d1a19943b1522c0b1d0895aefbb80ec294
> > Author: Zhen Lei <thunder.leizhen@...wei.com>
> > Date:   Thu Aug 25 17:23:11 2022 +0800
> > 
> >     rcu: Simplify rcu_init_nohz() cpumask handling
> >     
> >     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_nocb_mask= 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>
> 
> Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>

Thank you, and I will apply this on the next rebase.

							Thanx, Paul

> thanks,
> 
>  - Joel
> 
> 
> > 
> > diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> > index 0a5f0ef414845..c8167be2288fa 100644
> > --- a/kernel/rcu/tree_nocb.h
> > +++ b/kernel/rcu/tree_nocb.h
> > @@ -1210,45 +1210,31 @@ 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;
> > +	const struct cpumask *cpumask = NULL;
> >  
> >  #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) */
> > -
> > -#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) */
> > +	cpumask = cpu_possible_mask;
> > +#elif defined(CONFIG_NO_HZ_FULL)
> > +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > +		cpumask = tick_nohz_full_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,