Re: [PATCH 2/4] PM / devfreq: add possibility for delayed work

[Kamil Konieczny]

Hi,

On 09.12.2019 20:27, Matthias Kaehlcke wrote:
> Hi,
> 
> On Mon, Dec 09, 2019 at 03:44:23PM +0100, Kamil Konieczny wrote:
>> Current devfreq workqueue uses deferred timer. Introduce sysfs
>> file delayed_timer and use it for change from deferred to
>> delayed work. The default is to use old deferred one, which
>> saves power, but can miss increased demand for higher bus
>> frequency if timer was assigned to idle cpu.
>>
>> Signed-off-by: Kamil Konieczny <k.konieczny@xxxxxxxxxxx>
>> ---
>>  Documentation/ABI/testing/sysfs-class-devfreq | 10 ++++
>>  drivers/devfreq/devfreq.c                     | 46 ++++++++++++++++++-
>>  include/linux/devfreq.h                       |  2 +
>>  3 files changed, 57 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/ABI/testing/sysfs-class-devfreq b/Documentation/ABI/testing/sysfs-class-devfreq
>> index 9758eb85ade3..07bfd0df6a4a 100644
>> --- a/Documentation/ABI/testing/sysfs-class-devfreq
>> +++ b/Documentation/ABI/testing/sysfs-class-devfreq
>> @@ -30,6 +30,16 @@ Description:
>>  		target_freq when get_cur_freq() is not implemented by
>>  		devfreq driver.
>>  
>> +What:		/sys/class/devfreq/.../delayed_timer
>> +Date:		December 2019
>> +Contact:	Kamil Konieczny <k.konieczny@xxxxxxxxxxx>
>> +Description:
>> +		This ABI shows or clears timer type used by devfreq
>> +		workqueue. When 0, it uses default deferred timer.
>> +		When set to 1 devfreq will use delayed timer. Example
>> +		useage:
>> +			echo 1 > /sys/class/devfreq/.../delayed_timer
>> +
>>  What:		/sys/class/devfreq/.../target_freq
>>  Date:		September 2012
>>  Contact:	Rajagopal Venkat <rajagopal.venkat@xxxxxxxxxx>
>> diff --git a/drivers/devfreq/devfreq.c b/drivers/devfreq/devfreq.c
>> index 955949c6fc1f..c277d1770fef 100644
>> --- a/drivers/devfreq/devfreq.c
>> +++ b/drivers/devfreq/devfreq.c
>> @@ -445,7 +445,11 @@ void devfreq_monitor_start(struct devfreq *devfreq)
>>  	if (devfreq->governor->interrupt_driven)
>>  		return;
>>  
>> -	INIT_DEFERRABLE_WORK(&devfreq->work, devfreq_monitor);
>> +	if (devfreq->delayed_timer)
>> +		INIT_DELAYED_WORK(&devfreq->work, devfreq_monitor);
>> +	else
>> +		INIT_DEFERRABLE_WORK(&devfreq->work, devfreq_monitor);
>> +
>>  	if (devfreq->profile->polling_ms)
>>  		queue_delayed_work(devfreq_wq, &devfreq->work,
>>  			msecs_to_jiffies(devfreq->profile->polling_ms));
>> @@ -698,6 +702,7 @@ struct devfreq *devfreq_add_device(struct device *dev,
>>  	devfreq->last_status.current_frequency = profile->initial_freq;
>>  	devfreq->data = data;
>>  	devfreq->nb.notifier_call = devfreq_notifier_call;
>> +	devfreq->delayed_timer = false;
> 
> devfreq is zero initialized (allocated with kzalloc()), hence this is
> unnecessary.

ok, I will remove it

> 
>>  
>>  	if (!devfreq->profile->max_state && !devfreq->profile->freq_table) {
>>  		mutex_unlock(&devfreq->lock);
>> @@ -1288,6 +1293,44 @@ static ssize_t available_governors_show(struct device *d,
>>  }
>>  static DEVICE_ATTR_RO(available_governors);
>>  
>> +static ssize_t delayed_timer_show(struct device *dev,
>> +				  struct device_attribute *attr, char *buf)
>> +{
>> +	int i;
>> +
>> +	i = to_devfreq(dev)->delayed_timer ? 1 : 0;
>> +	return sprintf(buf, "%d\n", i);
> 
> get rid of 'i' and just use df->delayed_timer in sprintf().

ok

> 
>> +}
>> +
>> +static ssize_t delayed_timer_store(struct device *dev,
>> +				   struct device_attribute *attr,
>> +				   const char *buf, size_t count)
>> +{
>> +	struct devfreq *df = to_devfreq(dev);
>> +	bool old_timer;
> 
> Not a great name, the variable doesn't hold a timer. I'd suggest something
> like 'prev_val'.

ok, will change to 'bool prev, value;'

> 
>> +	int value, ret;
>> +
>> +	if (!df->governor)
>> +		return -EINVAL;
>> +
>> +	ret = kstrtoint(buf, 10, &value);
>> +	if (ret || (value != 1 && value != 0))
>> +		return -EINVAL;
> 
> use kstrtobool() instead of partially re-implementing it.

ok

>> +
>> +	mutex_lock(&df->lock);
>> +	old_timer = df->delayed_timer;
>> +	df->delayed_timer = value == 0 ? false : true;
> 
> What's wrong with:
> 
> df->delayed_timer = value;
> 
> ?

ok, when I change type of value and use above kstrtobool, this will be ok

> 
>> +	mutex_unlock(&df->lock);
> 
> Does the locking as is actually provide any value? The use case seems to
> be concurrent setting of the sysfs attribute. The lock is released after
> the assignment, hence the value of 'df->delayed_timer' could be overwritten
> before the condition below is evaluated.

Good point, before send I spotted these lines and "optimized" code so it got worse...

> If you want to protect against this case you need something like this:
> 
> // don't release the lock before evaluating the condition
> 
>> +	if (old_timer != df->delayed_timer) {

even better: if (prev != value) {
so no need for ugly mutex_unlock in both if-else paths

>   	   	mutex_unlock(&df->lock);

>> +		devfreq_monitor_stop(df);
>> +		devfreq_monitor_start(df);
>> +	}
>   	else {
> 		mutex_unlock(&df->lock);
> 	}
> 
> I don't pretend it's pretty ;-)

Thank you for review.

-- 
Best regards,
Kamil Konieczny
Samsung R&D Institute Poland