linux-kernel - Re: [PATCH 19/33] sched/isolation: Remove HK_TYPE_TICK test from cpu_is

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Message-ID: <1c77c072-ddf1-4894-a5bb-73bf13a838b9@redhat.com>
Date: Mon, 22 Sep 2025 17:59:12 -0400
From: Waiman Long <llong@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>, Waiman Long <llong@...hat.com>
Cc: LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...hat.com>,
 Marco Crivellari <marco.crivellari@...e.com>, Michal Hocko
 <mhocko@...e.com>, Peter Zijlstra <peterz@...radead.org>,
 Tejun Heo <tj@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH 19/33] sched/isolation: Remove HK_TYPE_TICK test from
 cpu_is_isolated()


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