From: Kevin Hilman <khilman@deeprootsystems.com>
To: Arjan van de Ven <arjan@linux.intel.com>
Cc: "Rafael J. Wysocki" <rjw@sisk.pl>, Arjan van de Ven <arjan@infradead.org>,
        "linux-pm\@lists.linux-foundation.org" 
	<linux-pm@lists.linux-foundation.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Alan Stern <stern@rowland.harvard.edu>,
        Ming Lei <tom.leiming@gmail.com>
Subject: Re: [PATCH v2] pm: Add runtime PM statistics
Organization: Deep Root Systems, LLC
References: <20100710095254.5ab7291c@infradead.org>
	<201007112326.07541.rjw@sisk.pl>
	<20100711221646.35f11b70@infradead.org>
	<201007132328.28853.rjw@sisk.pl> <4C3F2CEF.2090404@linux.intel.com>
	<87bp9gpryb.fsf@deeprootsystems.com>
	<4C5B5077.1060901@linux.intel.com>
Date: Fri, 06 Aug 2010 16:37:56 -0700
In-Reply-To: <4C5B5077.1060901@linux.intel.com> (Arjan van de Ven's message of
	"Thu, 05 Aug 2010 16:59:51 -0700")
Message-ID: <87wrs3mhwr.fsf@deeprootsystems.com>
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Arjan van de Ven <arjan@linux.intel.com> writes:

> 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... 

Probably, especially with tickless idle, but not so sure there is total
agreement on this in the embedded world though...

> 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 ;-)

What about using read_persistent_clock() then?  Then the arch/platform
definition of this will determine the max sampling rate.

Kevin


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