linux-kernel - Re: [PATCH 4/6] rcu/nocb: Create nocb kthreads on all CPUs as long as the "rcu

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Message-ID: <20211117215753.GB365507@lothringen>
Date:   Wed, 17 Nov 2021 22:57:53 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 4/6] rcu/nocb: Create nocb kthreads on all CPUs as long
 as the "rcu_nocb=" is passed

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.

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
> >