linux-kernel - Re: [PATCH v1 1/1] wq: Avoid using isolated cpus' timers on unbounded queue_delayed

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Date: Fri, 26 Jan 2024 11:49:02 -1000
From: Tejun Heo <tj@...nel.org>
To: Leonardo Bras <leobras@...hat.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>,
	Marcelo Tosatti <mtosatti@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/1] wq: Avoid using isolated cpus' timers on
 unbounded queue_delayed_work

Hello,

On Thu, Jan 25, 2024 at 10:03:20PM -0300, Leonardo Bras wrote:
..
> AS an optimization, if the current cpu is not isolated, use it's timer
  ^                                                           ^
  As                                                          its

> instead of looking for another candidate.

The sentence reads weird tho. It's always the same timer. We're deciding
which CPU to queue the timer on.

> @@ -1958,10 +1958,24 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
>  	dwork->cpu = cpu;
>  	timer->expires = jiffies + delay;
>  
> -	if (unlikely(cpu != WORK_CPU_UNBOUND))
> -		add_timer_on(timer, cpu);
> -	else
> -		add_timer(timer);
> +	if (likely(cpu == WORK_CPU_UNBOUND)) {
> +		if (!housekeeping_enabled(HK_TYPE_TIMER)) {
> +			/* Reuse the same timer */

This comment is confusing because it's always the same timer.

> +			add_timer(timer);
> +			return;
> +		}
> +
> +		/*
> +		 * If the work is cpu-unbound, and cpu isolation is in place,
> +		 * only use timers from housekeeping cpus.
> +		 * If the current cpu is a housekeeping cpu, use it instead.
> +		 */
> +		cpu = smp_processor_id();
> +		if (!housekeeping_test_cpu(cpu, HK_TYPE_TIMER))
> +			cpu = housekeeping_any_cpu(HK_TYPE_TIMER);
> +	}
> +
> +	add_timer_on(timer, cpu);
>  }

I find the control flow a bit difficult to follow. It's not the end of the
world to have two add_timer_on() calls. Would something like the following
be easier to read?

	if (housekeeping_enabled(HK_TYPE_TIMER)) {
		cpu = smp_processor_id();
		if (!housekeeping_test_cpu(cpu, HK_TYPE_TIMER))
			cpu = housekeeping_any_cpu(HK_TYPE_TIMER);
		add_timer_on(timer, cpu);
	} else {
		if (likely(cpu == WORK_CPU_UNBOUND))
			add_timer(timer, cpu);
		else
			add_timer_on(timer, cpu);
	}

Thanks.

-- 
tejun