Re: [PATCH v2] rcu: Keeping rcu-related kthreads running on housekeeping CPUS

[Joel Fernandes]

On Fri, Feb 10, 2023 at 12:26 AM Zhang, Qiang1 <qiang1.zhang@xxxxxxxxx> wrote:
>
> > For kernels built with CONFIG_NO_HZ_FULL=y and CONFIG_RCU_NOCB_CPU=y,
> > when passing cpulist to "isolcpus=", "nohz_full=" and "rcu_nocbs="
> > bootparams, after system starting, the rcu-related kthreads(include
> > rcu_gp, rcuog*, rcuop* kthreads etc) will running on housekeeping
> > CPUs, but for cpulist contains CPU0, the result will deferent, these
> > rcu-related kthreads will be restricted to running on CPU0.
> >
> > Although invoke kthread_create() to spwan rcu-related kthreads and
> > when it's starting, invoke set_cpus_allowed_ptr() to allowed cpumaks
> > is housekeeping_cpumask(HK_TYPE_KTHREAD), but due to these rcu-related
> > kthreads are created before starting other CPUS, that is to say, at
> > this time, only CPU0 is online, when these rcu-related kthreads running
> > and set allowed cpumaks is housekeeping cpumask, if find that only CPU0
> > is online and CPU0 exists in "isolcpus=", "nohz_full=" and "rcu_nocbs="
> > bootparams, invoke set_cpus_allowed_ptr() will return error.
> >
> > set_cpus_allowed_ptr()
> >  ->__set_cpus_allowed_ptr()
> >    ->__set_cpus_allowed_ptr_locked
> >      {
> >                 bool kthread = p->flags & PF_KTHREAD;
> >                 ....
> >                 if (kthread || is_migration_disabled(p))
> >                         cpu_valid_mask = cpu_online_mask;
> >                 ....
> >                 dest_cpu = cpumask_any_and_distribute(cpu_valid_mask, ctx->new_mask);
> >                 if (dest_cpu >= nr_cpu_ids) {
> >                         ret = -EINVAL;
> >                         goto out;
> >                 }
> >                 ....
> >      }
> >
> > At this time, only CPU0 is set in the cpu_online_mask, the ctx->new_mask
> > is housekeeping cpumask and not contains CPU0, this will result dest_cpu
> > is illegal cpu value, the set_cpus_allowed_ptr() will return -EINVAL and
> > failed to set housekeeping cpumask.
> >
> > This commit therefore add additional cpus_allowed_ptr() call in CPU hotplug
> > path. and reset the CPU affinity of rcuboost, rcuog, rcuop kthreads after
> > all other CPUs are online.
> >
> > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx>
> >
> >Good catch! But based on that and your other fix, I suspect that
> >nohz_full=0... has never been seriously used.
> >
> >A few points:
> >
> >* This is a problem for kthreads in general. And since HK_TYPE_KTHREAD =
> >  HK_TYPE_RCU and both are going to be merged in the future, I think we should
> >  stop handling the RCU kthreads housekeeping affinity from RCU but let the
> >  kthread code handle that and also fix the issue from the kthread code.
> >  RCU boost may be an exception since we try to enforce some node locality
> >  within the housekeeping range.
>
> Agree.  indeed, these works that set housekeeping CPU affinity should not be handled by RCU,
> and not only RCU-related kthreads are affected, other kthreads created earlier also have the
> same problem.

Agreed as well.

> >* If CPU 0 is isolated and it is the boot CPU, we should wait for a secondary
> >  CPU to boot before activating nohz_full at all. Unfortunately the nohz_full
> >  code is not yet ready for runtime housekeeping cpumask change but work is
> >  in progress (I'm saying that for 10 years...)
> >
> >* I'm tempted to revert 08ae95f4fd3b (nohz_full: Allow the boot CPU to be
> >  nohz_full) since it doesn't work and nobody ever complained?
>
> Yes if remove 08ae95f4fd3b, this problem will disappear.

Just checking. So what's the next step, revert this?

Thanks.