linux-kernel - Re: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection framework

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Date:   Thu, 7 Jun 2018 16:11:02 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        Eduardo Valentin <edubezval@...il.com>,
        Javi Merino <javi.merino@...nel.org>,
        Leo Yan <leo.yan@...aro.org>,
        Kevin Wangtao <kevin.wangtao@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rui Zhang <rui.zhang@...el.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        "open list:POWER MANAGEMENT CORE" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection
 framework

On 06/06/2018 12:45, Viresh Kumar wrote:
> On 06-06-18, 12:22, Daniel Lezcano wrote:
>> (mb() are done in the atomic operations AFAICT).
> 
> AFAIU, it is required to make sure the operations are seen in a particular order
> on another CPU and the compiler doesn't reorganize code to optimize it.
> 
> For example, in our case what if the compiler reorganizes the atomic-set
> operation after wakeup-process ? But maybe that wouldn't happen across function
> calls and we should be safe then.
> 
>> What about:
>>
>> 	get_online_cpus();
>>
>> 	nr_tasks = cpumask_weight(
>> 		cpumask_and(ii_dev->cpumask, cpu_online_mask);
>> 			
>> 	atomic_set(&ii_dev->count, nr_tasks);
>>
>> 	for_each_cpu_and(cpu, ii_dev->cpumask, cpu_online_mask) {
>> 		iit = per_cpu_ptr(&idle_injection_thread, cpu);
>> 		iit->should_run = 1;
>> 		wake_up_process(iit->tsk);
>> 	}
>>
>> 	put_online_cpus();
>> ?
> 
> Looks good this time.
> 
>> I'm wondering if we can have a CPU hotplugged right after the
>> 'put_online_cpus', resulting in the 'should park' flag set and then the
>> thread goes in the kthread_parkme instead of jumping back the idle
>> injection function and decrease the count, leading up to the timer not
>> being set again.
> 
> True. That looks like a valid problem to me as well.
> 
> What about starting the hrtimer right from this routine itself, after taking
> into account running-time, idle-time, delay, etc ? That would get rid of the
> count stuff, this get_online_cpus(), etc.
> 
> Even if we restart the next play-idle cycle a bit early or with some delay, sky
> wouldn't fall :)

We won't be able to call completion() in this case.



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