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

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Message-ID: <dc75acab-36c1-4340-9cf2-35a35361c32b@redhat.com>
Date: Tue, 2 Sep 2025 10:28:58 -0400
From: Waiman Long <llong@...hat.com>
To: Frederic Weisbecker <frederic@...nel.org>,
 LKML <linux-kernel@...r.kernel.org>
Cc: 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 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.

Cheers,
Longman

>
> Therefore if a CPU is not HK_TYPE_KERNEL_NOISE, it can't be
> HK_TYPE_DOMAIN either. Testing the latter is then enough.
>
> Simplify cpu_is_isolated() accordingly.
>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> ---
>   include/linux/sched/isolation.h | 3 +--
>   1 file changed, 1 insertion(+), 2 deletions(-)
>
> diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
> index c02923ed4cbe..8d6d26d3fdf5 100644
> --- a/include/linux/sched/isolation.h
> +++ b/include/linux/sched/isolation.h
> @@ -82,8 +82,7 @@ static inline void housekeeping_init(void) { }
>   
>   static inline bool cpu_is_isolated(int cpu)
>   {
> -	return !housekeeping_test_cpu(cpu, HK_TYPE_DOMAIN) ||
> -	       !housekeeping_test_cpu(cpu, HK_TYPE_TICK);
> +	return !housekeeping_test_cpu(cpu, HK_TYPE_DOMAIN);
>   }
>   
>   #endif /* _LINUX_SCHED_ISOLATION_H */