linux-kernel - Re: [PATCH V7] powercap/drivers/idle_injection: Add an idle injection framework

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Date:   Mon, 18 Jun 2018 13:28:22 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        linux-pm@...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>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>
Subject: Re: [PATCH V7] powercap/drivers/idle_injection: Add an idle injection
 framework

On 18/06/2018 12:38, Viresh Kumar wrote:
> On 18-06-18, 12:35, Daniel Lezcano wrote:
>> On 18/06/2018 12:22, Viresh Kumar wrote:
>>> On 15-06-18, 11:19, Daniel Lezcano wrote:
>>>> +/**
>>>> + * idle_injection_stop - stops the idle injections
>>>> + * @ii_dev: a pointer to an idle injection_device structure
>>>> + *
>>>> + * The function stops the idle injection and waits for the threads to
>>>> + * complete. If we are in the process of injecting an idle cycle, then
>>>> + * this will wait the end of the cycle.
>>>> + *
>>>> + * When the function returns there is no more idle injection
>>>> + * activity. The kthreads are scheduled out and the periodic timer is
>>>> + * off.
>>>> + */
>>>> +void idle_injection_stop(struct idle_injection_device *ii_dev)
>>>> +{
>>>> +	struct idle_injection_thread *iit;
>>>> +	unsigned int cpu;
>>>> +
>>>> +	pr_debug("Stopping injecting idle cycles on CPUs '%*pbl'\n",
>>>> +		 cpumask_pr_args(to_cpumask(ii_dev->cpumask)));
>>>> +
>>>> +	hrtimer_cancel(&ii_dev->timer);
>>>> +
>>>> +	/*
>>>> +	 * We want the guarantee we have a quescient point where
>>>> +	 * parked threads stay in there state while we are stopping
>>>> +	 * the idle injection. After exiting the loop, if any CPU is
>>>> +	 * plugged in, the 'should_run' boolean being false, the
>>>> +	 * smpboot main loop schedules the task out.
>>>> +	 */
>>>> +	cpu_hotplug_disable();
>>>> +
>>>> +	for_each_cpu_and(cpu, to_cpumask(ii_dev->cpumask), cpu_online_mask) {
>>>
>>> Maybe you should do below for all CPUs in the mask. Is the below usecase
>>> possible ?
>>>
>>> - CPU0-4 are part of the mask and are all online.
>>> - hrtimer fires and sets should_run for all of them to 1.
>>
>>     ^^
>> hrtimer_cancel gives you the guarantee, the timer is no longer active
>> and there is no execution in the timer handler. So the timer can no
>> longer fire after hrtimer_cancel() is called (which is a blocking call).
> 
> Right but that isn't called yet in my sequence.
> 
>>> - Right at this time CPU3 goes offline, so the thread gets parked with
>>>   should_run == 1. Is there a reason why this can't happen ?

for this specific case, we can use the park() callback to set should_run
to false, no ?

>>> - Now we unregister the stuff and CPU3 again comes online.
> 
> It gets called here from unregister/stop.
> 
>>> - Because it had should_run as true, we again run the thread and Crash.
>>>
>>> makes sense ?
> 
>>>> +out_rollback_per_cpu:
>>>> +	for_each_cpu(cpu, to_cpumask(ii_dev->cpumask))
>>>> +		per_cpu(idle_injection_device, cpu) = NULL;
>>>
>>> So if two parts of the kernel call this routine with the same cpumask, then the
>>> second call will also overwrite the masks with NULL and return error. That will
>>> screw up things a bit here.
>>
>> Apparently there is a misunderstanding :)
>>
>> https://lkml.org/lkml/2018/5/29/209 (at the end)
> 
> Right, your earlier version was doing the right thing :)
> 


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