Re: [PATCH 19/33] sched/isolation: Remove HK_TYPE_TICK test from cpu_is_isolated()

[Waiman Long <llong@redhat.com>]

On 9/22/25 11:19 AM, Frederic Weisbecker wrote:
> Le Tue, Sep 02, 2025 at 10:28:58AM -0400, Waiman Long a écrit :
>> On 8/29/25 11:48 AM, Frederic Weisbecker wrote:
>>> It doesn't make sense to use nohz_full without also isolating the
>>> related CPUs from the domain topology, either through the use of
>>> isolcpus= or cpuset isolated partitions.
>>>
>>> And now HK_TYPE_DOMAIN includes all kinds of domain isolated CPUs.
>>>
>>> This means that HK_TYPE_KERNEL_NOISE (of which HK_TYPE_TICK is only an
>>> alias) is always a superset of HK_TYPE_DOMAIN.
>> That may not be true. Users can still set "isolcpus=" and "nohz_full=" with
>> disjoint set of CPUs whether cpuset is used for additional isolated CPUs or
>> not.
> There can be domain isolated CPU that are not nohz_full indeed. But OTOH nohz_full
> CPUs that are not domain isolated don't make much sense. I know such settings
> exist but it's usually a half working misconfiguration. I wish I had
> forbidden that since the early days but this was from times when we didn't know
> about all the potential usages.

The assumption is true for cpuset isolated partition, but users may 
still set conflicting nohz_full and isolcpus parameters by mistake. We 
should probably a warning if this happens and document that clearly to 
enforce such rule.

Cheers,
Longman