linux-kernel - Re: [PATCH v2] pm: Add runtime PM statistics

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Message-Id: <201008060145.28969.rjw@sisk.pl>
Date:	Fri, 6 Aug 2010 01:45:28 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Kevin Hilman <khilman@...prootsystems.com>
Cc:	Arjan van de Ven <arjan@...ux.intel.com>,
	Arjan van de Ven <arjan@...radead.org>,
	"linux-pm@...ts.linux-foundation.org" 
	<linux-pm@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Alan Stern <stern@...land.harvard.edu>,
	Ming Lei <tom.leiming@...il.com>
Subject: Re: [PATCH v2] pm: Add runtime PM statistics

On Friday, August 06, 2010, Kevin Hilman wrote:
> Arjan van de Ven <arjan@...ux.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.

Patch please?

Rafael
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