Date:   Wed, 23 May 2018 11:11:54 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     rjw@...ysocki.net, edubezval@...il.com, kevin.wangtao@...aro.org,
        leo.yan@...aro.org, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, javi.merino@...nel.org,
        rui.zhang@...el.com, linux-pm@...r.kernel.org,
        daniel.thompson@...aro.org
Subject: Re: [PATCH V3] powercap/drivers/idle_injection: Add an idle
 injection framework

On 22-05-18, 15:42, Daniel Lezcano wrote:
> On 21/05/2018 12:32, Viresh Kumar wrote:
> > On 18-05-18, 16:50, Daniel Lezcano wrote:
> >> Initially, the cpu_cooling device for ARM was changed by adding a new
> >> policy inserting idle cycles. The intel_powerclamp driver does a
> >> similar action.
> >>
> >> Instead of implementing idle injections privately in the cpu_cooling
> >> device, move the idle injection code in a dedicated framework and give
> >> the opportunity to other frameworks to make use of it.
> > 
> > I thought you agreed to move above in the comments section ?
> 
> This is what I did. I just kept the relevant log here.

The fact that you are stating that you tried to update the cooling
device earlier looked like a bit of version history to me, not what
this patch is doing.

But its okay if you really want that to be preserved in git history :)

> >> +static void idle_injection_fn(unsigned int cpu)
> >> +{
> >> +	struct idle_injection_device *ii_dev;
> >> +	struct idle_injection_thread *iit;
> >> +	int run_duration_ms, idle_duration_ms;
> >> +
> >> +	ii_dev = per_cpu(idle_injection_device, cpu);
> >> +
> >> +	iit = per_cpu_ptr(&idle_injection_thread, cpu);
> >> +
> >> +	/*
> >> +	 * Boolean used by the smpboot mainloop and used as a flip-flop
> >> +	 * in this function
> >> +	 */
> >> +	iit->should_run = 0;
> >> +
> >> +	atomic_inc(&ii_dev->count);
> >> +
> >> +	idle_duration_ms = atomic_read(&ii_dev->idle_duration_ms);
> >> +
> >> +	play_idle(idle_duration_ms);
> >> +
> >> +	/*
> >> +	 * The last CPU waking up is in charge of setting the timer. If
> >> +	 * the CPU is hotplugged, the timer will move to another CPU
> >> +	 * (which may not belong to the same cluster) but that is not a
> >> +	 * problem as the timer will be set again by another CPU
> >> +	 * belonging to the cluster. This mechanism is self adaptive.
> >> +	 */
> >> +	if (!atomic_dec_and_test(&ii_dev->count))
> >> +		return;
> >> +
> >> +	run_duration_ms = atomic_read(&ii_dev->run_duration_ms);
> > 
> > This reads as if it is okay to have run_duration_ms set as 0, so we
> > run idle loop only once. Which is fine, but why do you mandate this to
> > be non-zero in idle_injection_start() ?
> 
> It does not make sense to run this function with a run duration set to
> zero because we will immediately go to idle again after exiting idle. So
> the action is exiting. In this context we can't accept to start
> injecting idle cycles.

Right and that's why I said "Which is fine" in my comment above. My
question was more on why we error out in idle_injection_start() if
run_duration_ms is 0.

Just for my understanding, is it a valid usecase where we want to run
the idle loop only once ? i.e. set idle_duration_ms to a non-zero
value but run_duration_ms to 0 ? In that case we shouldn't check for
zero run_duration_ms in idle_injection_start().

> >> +	if (!run_duration_ms)
> >> +		return;
> >> +
> >> +	hrtimer_start(&ii_dev->timer, ms_to_ktime(run_duration_ms),
> >> +		      HRTIMER_MODE_REL_PINNED);
> >> +}
> >> +
> >> +/**
> >> + * idle_injection_set_duration - idle and run duration helper
> >> + * @run_duration_ms: an unsigned int giving the running time in milliseconds
> >> + * @idle_duration_ms: an unsigned int giving the idle time in milliseconds
> >> + */
> >> +void idle_injection_set_duration(struct idle_injection_device *ii_dev,
> >> +				 unsigned int run_duration_ms,
> >> +				 unsigned int idle_duration_ms)
> >> +{
> >> +	atomic_set(&ii_dev->run_duration_ms, run_duration_ms);
> >> +	atomic_set(&ii_dev->idle_duration_ms, idle_duration_ms);
> > 
> > You check for valid values of these in idle_injection_start() but not
> > here, why ?
> 
> By checking against a zero values in the start function is a way to make
> sure we are not starting the idle injection with uninitialized values
> and by setting the duration to zero is a way to stop the idle injection.

Why do we need two ways of stopping the idle injection thread ? Why
isn't just calling idle_injection_stop() the right thing to do in that
case ?

> >> +}
> >> +
> >> +/**
> >> + * idle_injection_get_duration - idle and run duration helper
> >> + * @run_duration_ms: a pointer to an unsigned int to store the running time
> >> + * @idle_duration_ms: a pointer to an unsigned int to store the idle time
> >> + */
> >> +void idle_injection_get_duration(struct idle_injection_device *ii_dev,
> >> +				 unsigned int *run_duration_ms,
> >> +				 unsigned int *idle_duration_ms)
> >> +{
> >> +	*run_duration_ms = atomic_read(&ii_dev->run_duration_ms);
> >> +	*idle_duration_ms = atomic_read(&ii_dev->idle_duration_ms);
> >> +}
> >> +
> >> +/**
> >> + * idle_injection_start - starts the idle injections
> >> + * @ii_dev: a pointer to an idle_injection_device structure
> >> + *
> >> + * The function starts the idle injection cycles by first waking up
> >> + * all the tasks the ii_dev is attached to and let them handle the
> >> + * idle-run periods.
> >> + *
> >> + * Return: -EINVAL if the idle or the running durations are not set.
> >> + */
> >> +int idle_injection_start(struct idle_injection_device *ii_dev)
> >> +{
> >> +	if (!atomic_read(&ii_dev->idle_duration_ms))
> >> +		return -EINVAL;
> >> +
> >> +	if (!atomic_read(&ii_dev->run_duration_ms))
> >> +		return -EINVAL;
> >> +

So according to above comments from me, I am saying that this
particular test isn't really required as we may want to run idle loop
only once.

> >> +	pr_debug("Starting injecting idle cycles on CPUs '%*pbl'\n",
> >> +		 cpumask_pr_args(ii_dev->cpumask));
> >> +
> >> +	idle_injection_wakeup(ii_dev);
> >> +
> >> +	return 0;
> >> +}
> >> +
> >> +/**
> >> + * idle_injection_stop - stops the idle injections
> >> + * @ii_dev: a pointer to an idle injection_device structure
> >> + *
> >> + * The function stops the idle injection by canceling the timer in
> >> + * charge of waking up the tasks to inject idle and unset the idle and
> >> + * running durations.
> >> + */
> >> +void idle_injection_stop(struct idle_injection_device *ii_dev)
> >> +{
> >> +	pr_debug("Stopping injecting idle cycles on CPUs '%*pbl'\n",
> >> +		 cpumask_pr_args(ii_dev->cpumask));
> >> +
> >> +	hrtimer_cancel(&ii_dev->timer);
> > 
> > How are we sure that idle_injection_fn() isn't running at this point
> > and it would start the timer cancelled here again ?
> 
> Nothing will ensure that. We will have an extra idle injection in this
> case. We can invert the set_duration(0,0) and the timer cancellation to
> reduce to reduce the window.

That's what I thought and so its racy. If someone calls
idle_injection_unregister(), then we call this routine and then free
the data structures while they are still getting used by the thread :(

> >> +
> >> +	idle_injection_set_duration(ii_dev, 0, 0);
> > 
> > And why exactly this this required ? Why shouldn't we allow this
> > sequence to work:
> > 
> > idle_injection_set_duration()
> > idle_injection_start()
> > idle_injection_stop()
> > idle_injection_start()
> > idle_injection_stop()
> > idle_injection_start()
> > idle_injection_stop()
> 
> Sorry, I don't get it.
> 
> Who will decide to start() and stop() ?

Some confusion here about the usecase then. How do you see this stuff
getting used and how users (cooling-driver ?) will use it ?

-- 
viresh

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