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Date:   Mon, 11 Feb 2019 13:42:52 -0800
From:   Matthias Kaehlcke <mka@chromium.org>
To:     Lukasz Luba <l.luba@partner.samsung.com>
Cc:     linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        b.zolnierkie@samsung.com, myungjoo.ham@samsung.com,
        cw00.choi@samsung.com, kyungmin.park@samsung.com,
        m.szyprowski@samsung.com, s.nawrocki@samsung.com, tkjos@google.com,
        joel@joelfernandes.org, chris.diamand@arm.com
Subject: Re: [PATCH v2 1/2] drivers: devfreq: change devfreq workqueue
 mechanism
Message-ID: <20190211214252.GR117604@google.com>
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Hi Lukasz,

On Mon, Feb 11, 2019 at 04:30:04PM +0100, Lukasz Luba wrote:
> There is no need for creating another workqueue in the system,
> the existing one should meet the requirements.
> This patch removes devfreq's custom workqueue and uses system one.
> It switches from queue_delayed_work() to schedule_delayed_work().
> It also does not wake up the system when it enters suspend (this
> functionality stays the same).
> 
> Signed-off-by: Lukasz Luba <l.luba@partner.samsung.com>
> ---
>  drivers/devfreq/devfreq.c | 25 ++++++-------------------
>  1 file changed, 6 insertions(+), 19 deletions(-)
> 
> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
> index 0ae3de7..882e717 100644
> --- a/drivers/devfreq/devfreq.c
> +++ b/drivers/devfreq/devfreq.c
> @@ -31,13 +31,6 @@
>  
>  static struct class *devfreq_class;
>  
> -/*
> - * devfreq core provides delayed work based load monitoring helper
> - * functions. Governors can use these or can implement their own
> - * monitoring mechanism.
> - */
> -static struct workqueue_struct *devfreq_wq;
> -
>  /* The list of all device-devfreq governors */
>  static LIST_HEAD(devfreq_governor_list);
>  /* The list of all device-devfreq */
> @@ -391,8 +384,8 @@ static void devfreq_monitor(struct work_struct *work)
>  	if (err)
>  		dev_err(&devfreq->dev, "dvfs failed with (%d) error\n", err);
>  
> -	queue_delayed_work(devfreq_wq, &devfreq->work,
> -				msecs_to_jiffies(devfreq->profile->polling_ms));
> +	schedule_delayed_work(&devfreq->work,
> +			      msecs_to_jiffies(devfreq->profile->polling_ms));
>  	mutex_unlock(&devfreq->lock);
>  }
>  
> @@ -409,7 +402,7 @@ void devfreq_monitor_start(struct devfreq *devfreq)
>  {
>  	INIT_DEFERRABLE_WORK(&devfreq->work, devfreq_monitor);
>  	if (devfreq->profile->polling_ms)
> -		queue_delayed_work(devfreq_wq, &devfreq->work,
> +		schedule_delayed_work(&devfreq->work,
>  			msecs_to_jiffies(devfreq->profile->polling_ms));
>  }
>  EXPORT_SYMBOL(devfreq_monitor_start);
> @@ -473,7 +466,7 @@ void devfreq_monitor_resume(struct devfreq *devfreq)
>  
>  	if (!delayed_work_pending(&devfreq->work) &&
>  			devfreq->profile->polling_ms)
> -		queue_delayed_work(devfreq_wq, &devfreq->work,
> +		schedule_delayed_work(&devfreq->work,
>  			msecs_to_jiffies(devfreq->profile->polling_ms));
>  
>  	devfreq->last_stat_updated = jiffies;
> @@ -516,7 +509,7 @@ void devfreq_interval_update(struct devfreq *devfreq, unsigned int *delay)
>  
>  	/* if current delay is zero, start polling with new delay */
>  	if (!cur_delay) {
> -		queue_delayed_work(devfreq_wq, &devfreq->work,
> +		schedule_delayed_work(&devfreq->work,
>  			msecs_to_jiffies(devfreq->profile->polling_ms));
>  		goto out;
>  	}
> @@ -527,7 +520,7 @@ void devfreq_interval_update(struct devfreq *devfreq, unsigned int *delay)
>  		cancel_delayed_work_sync(&devfreq->work);
>  		mutex_lock(&devfreq->lock);
>  		if (!devfreq->stop_polling)
> -			queue_delayed_work(devfreq_wq, &devfreq->work,
> +			schedule_delayed_work(&devfreq->work,
>  			      msecs_to_jiffies(devfreq->profile->polling_ms));
>  	}
>  out:
> @@ -1430,12 +1423,6 @@ static int __init devfreq_init(void)
>  		return PTR_ERR(devfreq_class);
>  	}
>  
> -	devfreq_wq = create_freezable_workqueue("devfreq_wq");
> -	if (!devfreq_wq) {
> -		class_destroy(devfreq_class);
> -		pr_err("%s: couldn't create workqueue\n", __FILE__);
> -		return -ENOMEM;
> -	}
>  	devfreq_class->dev_groups = devfreq_groups;
>  
>  	return 0;

As commented on v1, the change from a custom to a system workqueue
seems reasonable to me. However this patch also changes from a
freezable workqueue to a non-freezable one. C&P of my comments on v1:

``WQ_FREEZABLE``
    A freezable wq participates in the freeze phase of the system
    suspend operations.  Work items on the wq are drained and no
    new work item starts execution until thawed.

I'm not entirely sure what the impact of this is.

I imagine suspend is potentially quicker because the wq isn't drained,
but could works that execute during the suspend phase be a problem?

Cheers

Matthias