Re: [PATCH v2] pm: Add runtime PM statistics

[Arjan van de Ven <arjan@linux.intel.com>]

On 8/5/2010 4:20 PM, Kevin Hilman wrote:
> Arjan van de Ven<arjan@linux.intel.com>  writes:
>
>    
>> +
>> +/**
>> + * update_pm_runtime_accounting - Update the time accounting of power
>> states
>> + * @dev: Device to update the accounting for
>> + *
>> + * In order to be able to have time accounting of the various power states
>> + * (as used by programs such as PowerTOP to show the effectiveness of
>> runtime
>> + * PM), we need to track the time spent in each state.
>> + * update_pm_runtime_accounting must be called each time before the
>> + * runtime_status field is updated, to account the time in the old state
>> + * correctly.
>> + */
>> +void update_pm_runtime_accounting(struct device *dev)
>> +{
>> +    unsigned long now = jiffies;
>> +    int delta;
>> +
>> +    delta = now - dev->power.accounting_timestamp;
>> +
>> +    if (delta<  0)
>> +        delta = 0;
>> +
>> +    dev->power.accounting_timestamp = now;
>> +
>> +    if (dev->power.disable_depth>  0)
>> +        return;
>> +
>> +    if (dev->power.runtime_status == RPM_SUSPENDED)
>> +        dev->power.suspended_jiffies += delta;
>> +    else
>> +        dev->power.active_jiffies += delta;
>> +}
>>      
> By using jiffies, I think we might miss events in drivers that are doing
> runtime PM transitions in short bursts.  On embedded systems with slow
> HZ, there could potentially be lots of transitions between ticks.
>
> It would be nicer to use clocksource-based time so transitions between
> jiffies could still be factored into the accounting.
>    

you're absolutely right that the current mechanism is more "sampling 
accuracy" (similar to most /proc info that shows up with top and such).

on the "slow HZ".. there is no more valid reason to not set HZ to 
1000... so we'll get 1 msec sampling rate basically.

the problem with a more accurate clocksource is that it's expensive. And 
more... the path to such clocksource itself might be subject to power 
management ;-)