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

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Date:   Thu, 7 Jun 2018 10:18:27 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        Viresh Kumar <viresh.kumar@...aro.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>,
        "open list:POWER MANAGEMENT CORE" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection
 framework

On 06/06/2018 17:02, Peter Zijlstra wrote:
> On Wed, Jun 06, 2018 at 03:42:08PM +0200, Daniel Lezcano wrote:
>> On 06/06/2018 14:23, Peter Zijlstra wrote:
>>> On Tue, Jun 05, 2018 at 11:16:40AM +0200, Daniel Lezcano wrote:
>>>> +	atomic_t idle_duration_ms;
>>>> +	atomic_t run_duration_ms;
>>>
>>>> +	idle_duration_ms = atomic_read(&ii_dev->idle_duration_ms);
>>>
>>>> +	run_duration_ms = atomic_read(&ii_dev->run_duration_ms);
>>>
>>>> +	atomic_set(&ii_dev->run_duration_ms, run_duration_ms);
>>>> +	atomic_set(&ii_dev->idle_duration_ms, idle_duration_ms);
>>>
>>>> +	*run_duration_ms = atomic_read(&ii_dev->run_duration_ms);
>>>> +	*idle_duration_ms = atomic_read(&ii_dev->idle_duration_ms);
>>>
>>>> +	if (!atomic_read(&ii_dev->idle_duration_ms))
>>>
>>>> +	if (!atomic_read(&ii_dev->run_duration_ms))
>>>
>>> What is the point of atomic_t here ?!
>>
>> idle_duration and run_duration can be changed from different places at
>> the same time. The atomic is here to ensure the read/write are consistent.
>>
>> Do you think it is pointless ?
> 
> Yes, atomic_read() / atomic_set() are no more atomic than READ_ONCE() /
> WRITE_ONCE().

So IIUC, neither atomic or WRITE|READ_ONCE are necessary in this code
because of the wake_up_process() barrier is enough, right ?


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