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

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Message-ID: <aNFpHgRnf-mV-4d8@2a01cb069018a81087c6c9b3bf9471d3.ipv6.abo.wanadoo.fr>
Date: Mon, 22 Sep 2025 17:19:58 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: 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()

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.

Thanks.



> 
> 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 */
> 

-- 
Frederic Weisbecker
SUSE Labs