linux-kernel - Re: [PATCH 2/3] cpumask: Introduce DYING mask

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Message-ID: <20210412111657.hu7hbaay2zxzdxm5@e107158-lin.cambridge.arm.com>
Date:   Mon, 12 Apr 2021 12:16:57 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     valentin.schneider@....com, tglx@...utronix.de, mingo@...nel.org,
        bigeasy@...utronix.de, swood@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vincent.donnefort@....com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] cpumask: Introduce DYING mask

On 04/12/21 12:55, Peter Zijlstra wrote:
> 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);

Slightly better I suppose :)

> 
> > 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.

True. Beside I doubt there's a performance hit really, cpu_dying() will read
the bit in the cpumask anyway, unconditionally writing will be as fast since
both will fetch the cacheline anyway?

Regardless, not really a big deal. It's not really the hardest thing to stare
at here ;-)

Thanks

--
Qais Yousef