Re: [PATCH 2/6] kthread: Keep unbound kthreads off nohz_full CPUs

[Waiman Long <longman@redhat.com>]

On 11/3/26 9:01 AM, Frederic Weisbecker wrote:
> Le Tue, Nov 03, 2026 at 08:59:36PM +1300, Matthew Ruffell a écrit :
>> From: Caron Wills <caron.wills@pulsar.com>
>>
>> Since commit 041ee6f3727a ("kthread: Rely on HK_TYPE_DOMAIN for
>> preferred affinity management") kthread_fetch_affinity() only removes
>> domain isolated CPUs from a kthread's affinity, assuming that nohz_full
>> CPUs are always domain isolated as well. Up to v6.13 every kthread
>> inherited kthreadd's affinity to housekeeping_cpumask(HK_TYPE_KTHREAD),
>> and from v6.14 the preferred affinity was filtered with HK_TYPE_KTHREAD,
>> so nohz_full CPUs were always excluded.
> This was done on purpose because on all setup we have met so far, it didn't
> make sense to run nohz_full with load balancing. After all the point is to
> run a pinned single task on a single CPU. We have seen too many misconfigured
> CPU isolation workloads due to that constraint not being enforced since the
> origins.
>
> I'm open to counter-arguments of course but you better have a good case.

I do see configurations where nohz_full/managed_irq CPUs are set at boot 
time without isolcpus=domain. But these nohz_full CPUs are there as a 
backup in case some latency critical workloads need a set of fully 
isolated CPUs to run on. Right now, isolcpus=domain can be set 
dynamically by using the cpuset isolated partition feature and they will 
be used when the need arises.

Matthew, is it possible for users to adapt their workloads to 
dynamically domain-isolated the CPUs before running RT tasks on them?

Cheers,
Longman