linux-kernel - Re: [PATCH v2] fix idle ticks in cpu summary line of /proc/stat

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Date:	Tue, 13 Mar 2012 14:02:35 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Michal Hocko <mhocko@...e.cz>
CC:	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] fix idle ticks in cpu summary line of /proc/stat

On 03/13/2012 01:37 PM, Michal Hocko wrote:

> OK, so the updated version of the patch looks like this. I am sorry but
> I had time to only compile test this...
> ---
> From d12247f14c5f8b00ae97a87442f62e49227a759b Mon Sep 17 00:00:00 2001
> From: Michal Hocko <mhocko@...e.cz>
> Date: Mon, 12 Mar 2012 13:11:38 +0100
> Subject: [PATCH] nohz: fix idle ticks in cpu summary line of /proc/stat
> 
> Git commit 09a1d34f8535ecf9 "nohz: Make idle/iowait counter update
> conditional" introduced a bug in regard to cpu hotplug. The effect is
> that the number of idle ticks in the cpu summary line in /proc/stat is
> still counting ticks for offline cpus.
> 
> Reproduction is easy, just start a workload that keeps all cpus busy,
> switch off one or more cpus and then watch the idle field in top.
> On a dual-core with one cpu 100% busy and one offline cpu you will get
> something like this:
> 
> %Cpu(s): 48.7 us,  1.3 sy,  0.0 ni, 50.0 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
> 
> The problem is that an offline cpu still has ts->idle_active == 1.
> To fix this we should make sure that the cpu is online when calling
> get_cpu_idle_time_us and get_cpu_iowait_time_us.
> 
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
> Reported-by: Martin Schwidefsky <schwidefsky@...ibm.com>
> Signed-off-by: Michal Hocko <mhocko@...e.cz>
> ---
>  fs/proc/stat.c |   14 ++++++++++----
>  1 files changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/fs/proc/stat.c b/fs/proc/stat.c
> index 121f77c..62bda24 100644
> --- a/fs/proc/stat.c
> +++ b/fs/proc/stat.c
> @@ -24,10 +24,13 @@
> 
>  static u64 get_idle_time(int cpu)
>  {
> -	u64 idle, idle_time = get_cpu_idle_time_us(cpu, NULL);
> +	u64 idle, idle_time = -1ULL;
> +
> +	if (cpu_online(cpu))
> +		idle_time = get_cpu_idle_time_us(cpu, NULL);
> 
>  	if (idle_time == -1ULL) {
> -		/* !NO_HZ so we can rely on cpustat.idle */
> +		/* !NO_HZ or cpu offline so we can rely on cpustat.idle */
>  		idle = kcpustat_cpu(cpu).cpustat[CPUTIME_IDLE];
>  		idle += arch_idle_time(cpu);
>  	} else
> @@ -38,10 +41,13 @@ static u64 get_idle_time(int cpu)
> 
>  static u64 get_iowait_time(int cpu)
>  {
> -	u64 iowait, iowait_time = get_cpu_iowait_time_us(cpu, NULL);
> +	u64 iowait, iowait_time = -1ULL;
> +
> +	if (cpu_online(cpu))
> +		iowait_time = get_cpu_iowait_time_us(cpu, NULL);
> 
>  	if (iowait_time == -1ULL)
> -		/* !NO_HZ so we can rely on cpustat.iowait */
> +		/* !NO_HZ or cpu offline so we can rely on cpustat.iowait */
>  		iowait = kcpustat_cpu(cpu).cpustat[CPUTIME_IOWAIT];
>  	else
>  		iowait = usecs_to_cputime64(iowait_time);



Yeah, this looks much better..

Reviewed-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
 
Regards,
Srivatsa S. Bhat

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