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

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Date:   Wed, 6 Jun 2018 09:57:08 +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 05-06-18, 16:54, Daniel Lezcano wrote:
> On 05/06/2018 12:39, Viresh Kumar wrote:
> I don't think you are doing a mistake. Even if this can happen
> theoretically, I don't think practically that is the case.
> 
> The play_idle() has 1ms minimum sleep time.
> 
> The scenario you are describing means:
> 
> 1. the loop in idle_injection_wakeup() takes more than 1ms to achieve

There are many ways in which idle_injection_wakeup() can get called.

- from hrtimer handler, this happens in softirq context, right? So interrupts
  can still block the handler to run ?

- from idle_injection_start(), process context. RT or DL or IRQ activity can
  block the CPU for long durations sometimes.

> 2. at the same time, the user of the idle injection unregisters while
> the idle injection is acting precisely at CPU0 and exits before another
> task was wakeup by the loop in 1. more than 1ms after.
> 
> >From my POV, this scenario can't happen.

Maybe something else needs to be buggy as well to make this crap happen.

> Anyway, we must write rock solid code

That's my point.

> so may be we can use a refcount to
> protect against that, so instead of freeing in unregister, we refput the
> ii_dev pointer.

I think the solution can be a simple change in implementation of
idle_injection_wakeup(), something like this..

+static void idle_injection_wakeup(struct idle_injection_device *ii_dev)
+{
+	struct idle_injection_thread *iit;
+	int cpu;
+
+	for_each_cpu_and(cpu, ii_dev->cpumask, cpu_online_mask)
+		atomic_inc(&ii_dev->count);
+
+       mb(); //I am not sure but I think we need some kind of barrier here ?
+
+	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);
+	}
+}

-- 
viresh