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

From: Viresh Kumar
Date: Wed Jun 06 2018 - 00:27:20 EST

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);
+ }