linux-kernel - Re: [PATCH V6] powercap/drivers/idle_injection: Add an idle injection framework

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Message-ID: <16770447-afe3-0fd7-19f9-1bd52c4c8ced@linaro.org>
Date:   Tue, 12 Jun 2018 15:02:14 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     viresh.kumar@...aro.org, 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>,
        Andrea Parri <andrea.parri@...rulasolutions.com>
Subject: Re: [PATCH V6] powercap/drivers/idle_injection: Add an idle injection
 framework

On 12/06/2018 14:52, Peter Zijlstra wrote:
> On Tue, Jun 12, 2018 at 02:44:29PM +0200, Daniel Lezcano wrote:
>> On 12/06/2018 14:30, Peter Zijlstra wrote:
>>> On Tue, Jun 12, 2018 at 02:00:11PM +0200, Daniel Lezcano wrote:
>>>> +static void __idle_injection_wakeup(struct idle_injection_device *ii_dev)
>>>> +{
>>>> +	struct idle_injection_thread *iit;
>>>> +	struct cpumask tmp;
>>>> +	unsigned int cpu;
>>>> +
>>>> +	cpumask_and(&tmp, ii_dev->cpumask, cpu_online_mask);
>>>
>>> You should not be having a cpumask on the stack. Those things can be
>>> ginormous.
>>
>> Ok, the kernel code uses of cpumask_t on the stack when dealing with
>> cpumask_and. I assume it is also not recommended.
> 
> Yes, that should all get fixed. It's mostly legacy code I suppose. It's
> been at least 10 years I think since we merged the whole
> CPUMASK_OFFSTACK stuff.
> 
>> What would be the best practice ? Allocate a per cpumask at init time as
>> a temporary mask to work with ?
> 
> In this case, you can do:

That is what we had before but we change the code to set the count
before waking up the task, so compute the cpumask_weight of the
resulting AND right before this loop.

> +       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);
> +       }
> 


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