linux-kernel - Re: [PATCH] cpufreq: Fix wrong time unit conversion

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Date:	Sat, 07 Sep 2013 23:55:06 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	paulmck@...ux.vnet.ibm.com,
	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Viresh Kumar <viresh.kumar@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Carsten Emde <C.Emde@...dl.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Clark Williams <williams@...hat.com>,
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH] cpufreq: Fix wrong time unit conversion

On Saturday, September 07, 2013 12:02:26 PM Paul E. McKenney wrote:
> On Sat, Sep 07, 2013 at 06:35:08PM +0200, Frederic Weisbecker wrote:
> > The time spent by a CPU under a given frequency is stored in jiffies unit
> > in the cpu var cpufreq_stats_table->time_in_state[i], i being the index of
> > the frequency.
> > 
> > This is what is displayed in the following file on the right column:
> > 
> >      cat /sys/devices/system/cpu/cpuX/cpufreq/stats/time_in_state
> >      2301000 19835820
> >      2300000 3172
> >      [...]
> > 
> > Now cpufreq converts this jiffies unit delta to clock_t before returning it
> > to the user as in the above file. And that conversion is achieved using the API
> > cputime64_to_clock_t().
> > 
> > Although it accidentally works on traditional tick based cputime accounting, where
> > cputime_t maps directly to jiffies, it doesn't work with other types of cputime
> > accounting such as CONFIG_VIRT_CPU_ACCOUNTING_* where cputime_t can map to nsecs
> > or any granularity preffered by the architecture.
> > 
> > For example we get a buggy zero delta on full dyntick configurations:
> > 
> >      cat /sys/devices/system/cpu/cpuX/cpufreq/stats/time_in_state
> >      2301000 0
> >      2300000 0
> >      [...]
> > 
> > Fix this with using the proper jiffies_64_t to clock_t conversion.
> > 
> > Reported-by: Carsten Emde <C.Emde@...dl.org>
> > Tested-by: Carsten Emde <C.Emde@...dl.org>
> > Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> > Cc: Carsten Emde <C.Emde@...dl.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Clark Williams <williams@...hat.com>
> > Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > Cc: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> 
> Good catch!
> 
> Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Applied, will push for 3.12-rc1.

Thanks Frederic!

> 
> > ---
> >  drivers/cpufreq/cpufreq_stats.c |    2 +-
> >  1 files changed, 1 insertions(+), 1 deletions(-)
> > 
> > diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
> > index d37568c..10e6138 100644
> > --- a/drivers/cpufreq/cpufreq_stats.c
> > +++ b/drivers/cpufreq/cpufreq_stats.c
> > @@ -81,7 +81,7 @@ static ssize_t show_time_in_state(struct cpufreq_policy *policy, char *buf)
> >  	for (i = 0; i < stat->state_num; i++) {
> >  		len += sprintf(buf + len, "%u %llu\n", stat->freq_table[i],
> >  			(unsigned long long)
> > -			cputime64_to_clock_t(stat->time_in_state[i]));
> > +			jiffies_64_to_clock_t(stat->time_in_state[i]));
> >  	}
> >  	return len;
> >  }
> 
-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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