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

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Date:   Wed, 6 Jun 2018 16:15:28 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...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 06-06-18, 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 :)

-- 
viresh