Re: [PATCH 4/6] rcu/nocb: Create nocb kthreads on all CPUs as long as the "rcu_nocb=" is passed

From: "Paul E. McKenney" <paulmck@kernel.org>
To: Frederic Weisbecker <frederic@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Uladzislau Rezki <urezki@gmail.com>,
	Neeraj Upadhyay <quic_neeraju@quicinc.com>,
	Boqun Feng <boqun.feng@gmail.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Joel Fernandes <joel@joelfernandes.org>,
	rcu@vger.kernel.org
Subject: Re: [PATCH 4/6] rcu/nocb: Create nocb kthreads on all CPUs as long as the "rcu_nocb=" is passed
Date: Wed, 17 Nov 2021 14:28:02 -0800	[thread overview]
Message-ID: <20211117222802.GQ641268@paulmck-ThinkPad-P17-Gen-1> (raw)
In-Reply-To: <20211117215753.GB365507@lothringen>

On Wed, Nov 17, 2021 at 10:57:53PM +0100, Frederic Weisbecker wrote:
> On Wed, Nov 17, 2021 at 11:27:10AM -0800, Paul E. McKenney wrote:
> > On Wed, Nov 17, 2021 at 04:56:35PM +0100, Frederic Weisbecker wrote:
> > > In order to be able to (de-)offload any CPU using cpuset in the future,
> > > create a NOCB kthread for all possible CPUs. For now this is done only
> > > as long as the "rcu_nocb=" kernel parameter is passed to avoid
> > > the unnecessary overhead for most users.
> > 
> > The "nohz_full=" kernel parameter would also cause these kthreads to
> > be created, correct?  (Yeah, a nit, but...)
> > 
> > And some fallout of my forgetfulness below.  :-/
> 
> Ah right, that too!
> > >  
> > > @@ -1268,7 +1265,7 @@ static void __init rcu_spawn_nocb_kthreads(void)
> > >  	int cpu;
> > >  
> > >  	if (rcu_nocb_is_setup) {
> > > -		for_each_online_cpu(cpu)
> > > +		for_each_possible_cpu(cpu)
> > 
> > Gah...  I had forgotten.  :-/
> > 
> > Some firmware lies about the OS instance's age.  Other firmware lies
> > about the number of CPUs, sometimes claiming large multiples of the
> > actual number of CPUs.
> > 
> > So this needs to stay "for_each_online_cpu".  Otherwise, Paul Gortmaker
> > will once again be afflicted with hundreds of unnecessary rcuo kthreads.
> > 
> > The later calls to rcutree_prepare_cpu() from rcutree_prepare_cpu()
> > will take care of any CPUs that first come online later.
> 
> Ok that makes sense.
> 
> > 
> > >  			rcu_spawn_cpu_nocb_kthread(cpu);
> > >  	}
> > >  }
> > > @@ -1303,7 +1300,7 @@ static void __init rcu_organize_nocb_kthreads(void)
> > >  	 * Should the corresponding CPU come online in the future, then
> > >  	 * we will spawn the needed set of rcu_nocb_kthread() kthreads.
> > >  	 */
> > > -	for_each_cpu(cpu, rcu_nocb_mask) {
> > > +	for_each_possible_cpu(cpu) {
> > 
> > This needs to change, but to for_each_online_cpu() instead of
> > for_each_possible_cpu().  That handles the case where the boot CPU is
> > not initially offloaded, but where the sysadm later needs to offload it.
> 
> I'm less sure about that one. This is called early from rcu_init_nohz(). Only
> the boot CPU should be online at that point. We really need to integrate all
> possible CPUs within the nocb groups.

Ah, yes, this is creating the data-structure linkages, not the kthreads.
Your for_each_possible_cpu() is quite correct, good!

							Thanx, Paul

> Thanks.
> 
> > 
> > >  		rdp = per_cpu_ptr(&rcu_data, cpu);
> > >  		if (rdp->cpu >= nl) {
> > >  			/* New GP kthread, set up for CBs & next GP. */
> > > @@ -1327,7 +1324,8 @@ static void __init rcu_organize_nocb_kthreads(void)
> > >  				pr_cont(" %d", cpu);
> > >  		}
> > >  		rdp->nocb_gp_rdp = rdp_gp;
> > > -		list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
> > > +		if (cpumask_test_cpu(cpu, rcu_nocb_mask))
> > > +			list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
> > >  	}
> > >  	if (gotnocbs && dump_tree)
> > >  		pr_cont("%s\n", gotnocbscbs ? "" : " (self only)");
> > > -- 
> > > 2.25.1
> > > 