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

Frederic Weisbecker
2021-11-17T21:58:42Z

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
> >