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 19:05:35 -0300
From: Leonardo Bras <leobras@...hat.com>
To: Tejun Heo <tj@...nel.org>
Cc: Leonardo Bras <leobras@...hat.com>,
	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

On Fri, Jan 26, 2024 at 11:49:02AM -1000, Tejun Heo wrote:
> 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.
> 

Hello,

Thanks for pointing that out, I will improve it in the v2.



> > @@ -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.

Thanks, I will point out this being the last cpu used to handle the 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.

I am not really against it, but for me it's kind of weird to have that many 
calls to add_timer_on() if we can avoid it. 

I would rather go with:

###
if (unlikely(cpu != WORK_CPU_UNBOUND)) {
	add_timer_on(timer, cpu);
	return;
}

if (!housekeeping_enabled(HK_TYPE_TIMER)) {
	add_timer(timer);
	return;
}

cpu = smp_processor_id();
if (!housekeeping_test_cpu(cpu, HK_TYPE_TIMER))
	cpu = housekeeping_any_cpu(HK_TYPE_TIMER);

add_timer_on(timer, cpu);
###

What do you think?

Thanks,
Leo

> 
> -- 
> tejun
>