linux-kernel - Re: [PATCH RESEND 1/2] tick/nohz: Fix wrong NOHZ idle CPU state

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Message-ID: <aLrN4KQ1vDsqRTwM@localhost.localdomain>
Date: Fri, 5 Sep 2025 13:47:44 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: "Christoph Lameter (Ampere)" <cl@...two.org>
Cc: Adam Li <adamli@...amperecomputing.com>, anna-maria@...utronix.de,
	tglx@...utronix.de, mingo@...hat.com, peterz@...radead.org,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	vschneid@...hat.com, dietmar.eggemann@....com, rostedt@...dmis.org,
	bsegall@...gle.com, mgorman@...e.de, linux-kernel@...r.kernel.org,
	patches@...erecomputing.com
Subject: Re: [PATCH RESEND 1/2] tick/nohz: Fix wrong NOHZ idle CPU state

Le Thu, Sep 04, 2025 at 09:10:29AM -0700, Christoph Lameter (Ampere) a écrit :
> On Thu, 4 Sep 2025, Frederic Weisbecker wrote:
> 
> > The current state is indeed broken and some people have already tried to fix it.
> > The thing is nohz_full don't want dynamic isolation because it is deemed to run a
> > single task. Therefore those tasks must be placed manually in order not to break
> > isolation guarantees by accident.
> >
> > In fact nohz_full doesn't make much sense without isolcpus (or isolated cpuset
> > v2 partitions) and I even intend to make nohz_full depend on domain isolation
> > in the long term.
> 
> I have never used isolcpus with nohz_full. AFAICT isolcpus is depreciated
> and cpusets are unnecessary complex overhead.

isolcpus for domain isolation is indeed in the way for long term deprecation
and the only replacement possible is cpuset, which overhead is only visible
on partition creation and update.

We could argue on the interface, the point is that nohz_full doesn't make sense
without domain isolation.

Thanks.

-- 
Frederic Weisbecker
SUSE Labs