Re: [PATCH V4] powercap/drivers/idle_injection: Add an idle injection framework

[Daniel Lezcano]

On 05/06/2018 07:14, Viresh Kumar wrote:
> On 31-05-18, 20:25, Daniel Lezcano wrote:
>> On 29/05/2018 11:31, Viresh Kumar wrote:
>>> On 25-05-18, 11:49, Daniel Lezcano wrote:
>>>> +	/* +	 * 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. +	 */
>>> 
>>> I am afraid that the above comment may not be completely true all
>>> the time. For a quad-core platform, it is possible for 3 CPUs
>>> (0,1,2) to run this function as soon as the kthread is woken up,
>>> but one of the CPUs (3) may be stuck servicing an IRQ, Deadline
>>> or RT activity. Because you do atomic_inc() also in this function
>>> (above) itself, below decrement may return a true value for the
>>> CPU2 and that will restart the hrtimer, while one of the CPUs
>>> never got a chance to increment count in the first place.
>>> 
>>> The fix is simple though, do the increment in
>>> idle_injection_wakeup() and things should be fine then.
>> 
>> Ok.
>> 
>>>> +	if (!atomic_dec_and_test(&ii_dev->count)) +		return; + +
>>>> run_duration_ms = atomic_read(&ii_dev->run_duration_ms); +	if
>>>> (run_duration_ms) { +		hrtimer_start(&ii_dev->timer,
>>>> ms_to_ktime(run_duration_ms), +
>>>> HRTIMER_MODE_REL_PINNED); +		return; +	} + +
>>>> complete(&ii_dev->stop_complete);
>>> 
>>> So you call complete() after hrtimer is potentially restarted.
>>> This can happen if idle_injection_stop() is called right after
>>> the above atomic_read() has finished :)
>>> 
>>> IOW, this doesn't look safe now as well.
>> 
>> It is safe, we just missed a cycle and the stop will block until
>> the next cycle. I did it on purpose and for me it is correct.
> 
> Okay, what about this then:
> 
> Path A                                         Path B
> 
> idle_injection_fn()
> idle_injection_unregister() hrtimer_start()
> idle_injection_stop()
> 
> complete()
> 
> wait_for_completion() kfree(ii_dev);
> 
> Hrtimer is still used here after getting freed.
> 
> Is this not possible ?

As soon as we reach complete(), no timer can be set because of the
condition before.

>>>> +struct idle_injection_device *idle_injection_register(struct
>>>> cpumask *cpumask) +{ +	struct idle_injection_device *ii_dev; +
>>>> int cpu, cpu2; + +	ii_dev = ii_dev_alloc(); +	if (!ii_dev) +
>>>> return NULL; + +	cpumask_copy(ii_dev->cpumask, cpumask); +
>>>> hrtimer_init(&ii_dev->timer, CLOCK_MONOTONIC,
>>>> HRTIMER_MODE_REL); +	ii_dev->timer.function =
>>>> idle_injection_wakeup_fn; + +	for_each_cpu(cpu,
>>>> ii_dev->cpumask) { + +		if (per_cpu(idle_injection_device,
>>>> cpu)) {
>>> 
>>> Maybe add unlikely here ?
>> 
>> For this kind of init function and tight loop, there is no benefit
>> of adding the unlikely / likely. I can add it if you want, but it
>> is useless.
> 
> Okay.
> 
>>>> +			pr_err("cpu%d is already registered\n", cpu); +			goto
>>>> out_rollback_per_cpu; +		} + +		per_cpu(idle_injection_device,
>>>> cpu) = ii_dev; +	} + +	return ii_dev; + +out_rollback_per_cpu: 
>>>> +	for_each_cpu(cpu2, ii_dev->cpumask) { +		if (cpu == cpu2)
>>> 
>>> And is this really required? Perhaps this conditional is making
>>> it worse and just setting the per-cpu variable forcefully to NULL
>>> would be much faster :)
>> 
>> We want undo what was done, setting all variables to NULL resets
>> the values from a previous register and we don't want that.
> 
> Why will that happen ? We are only iterating over a particular
> cpumask and not all possible CPUs. Yes I understand the "undo only
> what we did" part, but the conditional is more expensive than that I
> feel.

Ok, I will remove it. In any case, there is no point of having the
function called twice. If that happens, there is something wrong with
the caller.


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