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SubjectRe: [PATCH v2 1/2] drivers: devfreq: change devfreq workqueue mechanism
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Hi Matthias,

On 2/12/19 9:12 PM, Matthias Kaehlcke wrote:
> On Tue, Feb 12, 2019 at 12:20:42PM +0100, Lukasz Luba wrote:
>> Hi Matthias,
>>
>> On 2/11/19 10:42 PM, Matthias Kaehlcke wrote:
>>> 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?
>> The devfreq supports suspend from v4.20-rc6, which picks OPP for a
>> device based on its DT 'opp-suspend'. For the devices which do not
>> choose the suspend OPP it is possible to enter that state with any
>> frequency. Queuing work for calling governor during suspend which
>> calculates the device's frequency for the next period is IMO not needed,
>> The 'next period' is actually suspend and is not related to
>> 'predicted' load by the governor.
>
> If I am not mistaken the monitor can still be running after a device
> was suspended:
>
> devfreq_suspend
> list_for_each_entry(devfreq, &devfreq_list, node)
> devfreq_suspend_device
> devfreq->governor->event_handler(devfreq,
> DEVFREQ_GOV_SUSPEND, NULL);
>
> According to the comment of devfreq_monitor_suspend() the function is
> supposed to be called by the governor in response to
> DEVFREQ_GOV_SUSPEND, however this doesn't seem to be universally the case:
>
> git grep devfreq_monitor_suspend
> drivers/devfreq/governor_simpleondemand.c: devfreq_monitor_suspend(devfreq);
> drivers/devfreq/tegra-devfreq.c: devfreq_monitor_suspend(devfreq);
>
> i.e. the other governors don't seem to call devfreq_monitor_suspend().
>
> Am I missing something?
Probably not.
Good catch, these governors should support case DEVFREQ_GOV_SUSPEND.
The system suspend which calls 'devfreq_suspend' does it when the
workqueues are frozen and sets the desired OPP for later resume.
The other use use cases (like pm_suspend) might assume that these
governors are ready for DEVFREQ_GOV_SUSPEND...
Do you like to write a patch for them (I can test it) or should I do it?

Regards,
Lukasz
>
> Thanks
>
> Matthias
>
>

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