linux-kernel - Re: [PATCH 3/6] driver core: cpu: optimize print_cpus

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Date: Tue, 14 May 2024 23:02:00 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Yury Norov <yury.norov@...il.com>, linux-kernel@...r.kernel.org, Greg
 Kroah-Hartman <gregkh@...uxfoundation.org>, "Paul E. McKenney"
 <paulmck@...nel.org>, "Rafael J. Wysocki" <rafael@...nel.org>, Anna-Maria
 Behnsen <anna-maria@...utronix.de>, Ben Segall <bsegall@...gle.com>,
 Daniel Bristot de Oliveira <bristot@...hat.com>, Dietmar Eggemann
 <dietmar.eggemann@....com>, Frederic Weisbecker <frederic@...nel.org>,
 Imran Khan <imran.f.khan@...cle.com>, Ingo Molnar <mingo@...hat.com>,
 Johannes Weiner <hannes@...xchg.org>, Juri Lelli <juri.lelli@...hat.com>,
 Leonardo Bras <leobras@...hat.com>, Mel Gorman <mgorman@...e.de>, Peter
 Zijlstra <peterz@...radead.org>, Rik van Riel <riel@...riel.com>, Steven
 Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>, Valentin
 Schneider <vschneid@...hat.com>, Vincent Guittot
 <vincent.guittot@...aro.org>, Waiman Long <longman@...hat.com>, Yury Norov
 <yury.norov@...il.com>, Zefan Li <lizefan.x@...edance.com>,
 cgroups@...r.kernel.org
Subject: Re: [PATCH 3/6] driver core: cpu: optimize print_cpus_isolated()

On Mon, May 13 2024 at 15:01, Yury Norov wrote:
> The function may be called with housekeeping_cpumask == cpu_possible_mask,

How so? There is no cpumask argument in the function signature. Can you
please be precise?

> and in such case the 'isolated' cpumask would be just empty.
>
> We can call cpumask_clear() in that case, and save CPU cycles.
>
> @@ -282,8 +282,10 @@ static ssize_t print_cpus_isolated(struct device *dev,
>  	if (!alloc_cpumask_var(&isolated, GFP_KERNEL))
>  		return -ENOMEM;
>  
> -	cpumask_andnot(isolated, cpu_possible_mask,
> -		       housekeeping_cpumask(HK_TYPE_DOMAIN));
> +	if (cpu_possible_mask != housekeeping_cpumask(HK_TYPE_DOMAIN))
> +		cpumask_andnot(isolated, cpu_possible_mask, housekeeping_cpumask(HK_TYPE_DOMAIN));
> +	else
> +		cpumask_clear(isolated);
>  	len = sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(isolated));
>  
>  	free_cpumask_var(isolated);

Seriously? You need clear() to emit an empty string via %*pbl?

	if (cpu_possible_mask != housekeeping_cpumask(HK_TYPE_DOMAIN)) {
        	if (!alloc_cpumask_var(&isolated, GFP_KERNEL))
                	return -ENOMEM;
                cpumask_andnot(isolated, cpu_possible_mask, housekeeping_cpumask(HK_TYPE_DOMAIN));
                len = sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(isolated));
	  	free_cpumask_var(isolated);
	} else {
        	len = sysfs_emit(buf, "\n");
        }

That actually would make sense and spare way more CPU cycles, no?

Is it actually worth the larger text size? Not really convinced about that.

Thanks,

        tglx