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

[Zhang, Qiang1]

> 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.

>
>* 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.

Thanks
Zqiang

>
>Thanks.