linux-kernel - Re: [PATCH v4 2/2] rcu: Simplify the code logic of rcu_init

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Message-ID: <20220825161955.GO6159@paulmck-ThinkPad-P17-Gen-1>
Date:   Thu, 25 Aug 2022 09:19:55 -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 v4 2/2] rcu: Simplify the code logic of rcu_init_nohz()

On Tue, Aug 23, 2022 at 08:13:00PM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/8/23 0:35, Paul E. McKenney wrote:
> > On Wed, Aug 17, 2022 at 09:42:53AM +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>
> > 
> > This looks like a nice simplification, but I will wait for your response
> > on Patch 1/1 before trying it out.
> 
> How about I post v5 and just do this simplification? Patch 1/2 seems to require
> further discussion and in-depth analysis, which may take a long time.

Excellent choice, thank you!


							Thanx, Paul

> >> ---
> >>  kernel/rcu/tree_nocb.h | 32 +++++++++-----------------------
> >>  1 file changed, 9 insertions(+), 23 deletions(-)
> >>
> >> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> >> index ff763e7dc53551f..3c59b12f4465af1 100644
> >> --- a/kernel/rcu/tree_nocb.h
> >> +++ b/kernel/rcu/tree_nocb.h
> >> @@ -1209,44 +1209,30 @@ 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 (!cpumask_available(rcu_nocb_mask)) {
> >> -		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);
> >>  	}
> >>  
> >>  	if (!cpumask_available(rcu_nocb_mask))
> >>  		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,
> >> -- 
> >> 2.25.1
> >>
> > .
> > 
> 
> -- 
> Regards,
>   Zhen Lei