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Date:   Mon, 12 Apr 2021 12:55:14 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Qais Yousef <qais.yousef@arm.com>
Cc:     valentin.schneider@arm.com, tglx@linutronix.de, mingo@kernel.org,
        bigeasy@linutronix.de, swood@redhat.com, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vincent.donnefort@arm.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/3] cpumask: Introduce DYING mask
Message-ID: <YHQnEgXFi3YAFvIP@hirez.programming.kicks-ass.net>
References: <20210310145258.899619710@infradead.org>
 <20210310150109.151441252@infradead.org>
 <20210321193037.7o3mqcmwjthbos7n@e107158-lin>
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On Sun, Mar 21, 2021 at 07:30:37PM +0000, Qais Yousef wrote:
> On 03/10/21 15:53, Peter Zijlstra wrote:
> > --- a/kernel/cpu.c
> > +++ b/kernel/cpu.c
> > @@ -160,6 +160,9 @@ static int cpuhp_invoke_callback(unsigne
> >  	int (*cb)(unsigned int cpu);
> >  	int ret, cnt;
> >  
> > +	if (bringup != !cpu_dying(cpu))
> 
> nit: this condition is hard to read
> 
> > +		set_cpu_dying(cpu, !bringup);

How's:

	if (cpu_dying(cpu) != !bringup)
		set_cpu_dying(cpu, !bringup);

> since cpu_dying() will do cpumask_test_cpu(), are we saving  much if we
> unconditionally call set_cpu_dying(cpu, !bringup) which performs
> cpumask_{set, clear}_cpu()?

This is hotplug, it's all slow, endlessly rewriting that bit shouldn't
be a problem I suppose.