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

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Date:	Sat, 7 Sep 2013 12:02:26 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>,
	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 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>

> ---
>  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;
>  }
> -- 
> 1.7.5.4
> 

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