linux-kernel - Re: [PATCH][v3] cpufreq: governor: Fix overflow when calculating idle time

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Date:	Wed, 20 Apr 2016 03:26:44 +0200
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Chen Yu <yu.c.chen@...el.com>
Cc:	linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Len Brown <lenb@...nel.org>
Subject: Re: [PATCH][v3] cpufreq: governor: Fix overflow when calculating idle time

On Tuesday, April 19, 2016 11:57:32 AM Chen Yu wrote:
> It was reported that after Commit 0df35026c6a5 ("cpufreq: governor:
> Fix negative idle_time when configured with CONFIG_HZ_PERIODIC"),
> cpufreq ondemand governor started to act oddly. Without any load,
> with freshly booted system, it pumped cpu frequency up to maximum
> at some point of time and stayed there. The problem is caused by
> jiffies overflow in get_cpu_idle_time:
> 
> After booting up 5 minutes, the jiffies will round up to zero.
> As a result, the following condition in cpu governor will always be
> true:
> 	if (cur_idle_time <= j_cdbs->prev_cpu_idle)
> 		idle_time = 0;
> 
> which caused problems.
> 
> For example, once cur_idle_time has rounded up to zero, meanwhile
> prev_cpu_idle still remains negative(because of jiffies initial value
> of -300HZ, which is very big after converted to unsigned), thus above
> condition is met, thus we get a zero of idle running time during
> this sample, which causes a high busy time, thus governor always
> requests for the highest freq.
> 
> This patch fixes this problem by updating prev_cpu_idle for
> each sample period, even if prev_cpu_idle is bigger than
> cur_idle_time, thus to prevent the scenario of 'prev_cpu_idle always
> bigger than cur_idle_time' from happening.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=115261
> Reported-by: Timo Valtoaho <timo.valtoaho@...il.com>
> Signed-off-by: Chen Yu <yu.c.chen@...el.com>

This looks better than the previous versions to me, but ->

> ---
> v3:
>  - Do not use INITIAL_JIFFIES because it should be transparent
>    to user, meanwhile keep original semanteme to use delta
>    of time slice.
> ---
> v2:
>  - Send this patch to a wider scope, including timing-system maintainers,
>    as well as some modifications in the commit message to make it more clear.
> ---
>  drivers/cpufreq/cpufreq.c          | 4 ++++
>  drivers/cpufreq/cpufreq_governor.c | 8 +++++++-
>  2 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index b87596b..b0479b3 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -132,6 +132,10 @@ struct cpufreq_frequency_table *cpufreq_frequency_get_table(unsigned int cpu)
>  }
>  EXPORT_SYMBOL_GPL(cpufreq_frequency_get_table);
>  
> +/**
> + * The wall time and idle time are both possible to round up,

That's difficult to parse.  I guess you wanted to say that they may overflow?

> + * people should use delta rather than the value itself.
> + */

-> this new comment doesn't really belong to the fix.  You can send a separate
patch adding it.

Moreover -->

>  static inline u64 get_cpu_idle_time_jiffy(unsigned int cpu, u64 *wall)
>  {
>  	u64 idle_time;
> diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
> index 10a5cfe..8de3fba 100644
> --- a/drivers/cpufreq/cpufreq_governor.c
> +++ b/drivers/cpufreq/cpufreq_governor.c
> @@ -197,8 +197,14 @@ unsigned int dbs_update(struct cpufreq_policy *policy)
>  			idle_time = 0;
>  		} else {
>  			idle_time = cur_idle_time - j_cdbs->prev_cpu_idle;
> -			j_cdbs->prev_cpu_idle = cur_idle_time;
>  		}
> +		/*
> +		 * It is possible prev_cpu_idle being bigger than cur_idle_time,
> +		 * when 32bit rounds up if !CONFIG_VIRT_CPU_ACCOUNTING,
> +		 * thus get a 0% idle estimation. So update prev_cpu_idle during
> +		 * each sample period to avoid this situation lasting too long.
> +		 */
> +		j_cdbs->prev_cpu_idle = cur_idle_time;

--> it looks like the bug is that we are comparing signed values as unsigned.

>  
>  		if (ignore_nice) {
>  			u64 cur_nice = kcpustat_cpu(j).cpustat[CPUTIME_NICE];
> 

So what about the simple change below?

---
 drivers/cpufreq/cpufreq_governor.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

Index: linux-pm/drivers/cpufreq/cpufreq_governor.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/cpufreq_governor.c
+++ linux-pm/drivers/cpufreq/cpufreq_governor.c
@@ -146,7 +146,7 @@ unsigned int dbs_update(struct cpufreq_p
 		wall_time = cur_wall_time - j_cdbs->prev_cpu_wall;
 		j_cdbs->prev_cpu_wall = cur_wall_time;
 
-		if (cur_idle_time <= j_cdbs->prev_cpu_idle) {
+		if ((s64)cur_idle_time <= (s64)j_cdbs->prev_cpu_idle) {
 			idle_time = 0;
 		} else {
 			idle_time = cur_idle_time - j_cdbs->prev_cpu_idle;