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

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Message-ID: <4F7592A6.8030500@linux.vnet.ibm.com>
Date:	Fri, 30 Mar 2012 16:31:58 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Martin Schwidefsky <schwidefsky@...ibm.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Michal Hocko <mhocko@...e.cz>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] fix idle ticks in cpu summary line of /proc/stat

On 03/30/2012 03:53 PM, Martin Schwidefsky wrote:

> Hi Thomas,
> 
> On Thu, 29 Mar 2012 12:42:22 +0200 (CEST)
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> On Tue, 13 Mar 2012, 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>
>>
>> Martin, does that solve the problem for you ?
> 
> Yes, the patch does solve the immediate problem. I just tested if again
> on x86 and s390, works fine. I would like to have another change though.
> The idle_sleep calculation in the scheduler is fine for x86 but for s390
> the arch_idle_time delivers a much better precision. A patch would look
> like this:
> --
> Subject: [PATCH] use arch_idle_time for idle and iowait times if available
> 
> From: Martin Schwidefsky <schwidefsky@...ibm.com>
> 
> Git commit a25cac5198d4ff28 "proc: Consider NO_HZ when printing idle and
> iowait times" changes the code for /proc/stat to use get_cpu_idle_time_us
> and get_cpu_iowait_time_us if the system is running with nohz enabled.
> For architectures which define arch_idle_time (currently s390 only)
> this is a change for the worse. The result of arch_idle_time is supposed
> to be the exact sleep time of the target cpu and should be used instead
> of the value kept by the scheduler.
> 
> Signed-off-by: Martin Schwidefsky <schwidefsky@...ibm.com>


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

> ---
>  fs/proc/stat.c |   34 ++++++++++++++++++++++++++++------
>  1 file changed, 28 insertions(+), 6 deletions(-)
> 
> --- a/fs/proc/stat.c
> +++ b/fs/proc/stat.c
> @@ -18,9 +18,30 @@
>  #ifndef arch_irq_stat
>  #define arch_irq_stat() 0
>  #endif
> -#ifndef arch_idle_time
> -#define arch_idle_time(cpu) 0
> -#endif
> +
> +#ifdef arch_idle_time
> +
> +static cputime64_t get_idle_time(int cpu)
> +{
> +	cputime64_t idle;
> +
> +	idle = kcpustat_cpu(cpu).cpustat[CPUTIME_IDLE];
> +	if (cpu_online(cpu) && !nr_iowait_cpu(cpu))
> +		idle += arch_idle_time(cpu);
> +	return idle;
> +}
> +
> +static cputime64_t get_iowait_time(int cpu)
> +{
> +	cputime64_t iowait;
> +
> +	iowait = kcpustat_cpu(cpu).cpustat[CPUTIME_IOWAIT];
> +	if (cpu_online(cpu) && nr_iowait_cpu(cpu))
> +		iowait += arch_idle_time(cpu);
> +	return iowait;
> +}
> +
> +#else
> 
>  static u64 get_idle_time(int cpu)
>  {
> @@ -29,11 +50,10 @@ static u64 get_idle_time(int cpu)
>  	if (cpu_online(cpu))
>  		idle_time = get_cpu_idle_time_us(cpu, NULL);
> 
> -	if (idle_time == -1ULL) {
> +	if (idle_time == -1ULL)
>  		/* !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
> +	else
>  		idle = usecs_to_cputime64(idle_time);
> 
>  	return idle;
> @@ -55,6 +75,8 @@ static u64 get_iowait_time(int cpu)
>  	return iowait;
>  }
> 
> +#endif
> +
>  static int show_stat(struct seq_file *p, void *v)
>  {
>  	int i, j;
> 


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