linux-kernel - Re: [PATCH 1/2] nohz: use seqlock to avoid race on idle time stats

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Date:	Tue, 25 Mar 2014 10:57:32 +0530
From:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
CC:	Preeti Murthy <preeti.lkml@...il.com>,
	linux-kernel@...r.kernel.org,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>
Subject: Re: [PATCH 1/2] nohz: use seqlock to avoid race on idle time stats

On 03/24/2014 01:51 PM, Hidetoshi Seto wrote:
> (2014/03/24 16:45), Preeti Murthy wrote:
>> Hi Hidetoshi,
>>
>> The patch looks good to me except the comments around the monotonicity
>> of the return value of the idle stats observer. I am unable to relate them
>> to the dependency on nr_iowait_cpu.
>>
>> I see that when the reader queries for the idle stats and calls
>> get_cpu_idle_time_us(), the nr_iowait_cpu might be 0. When he later
>> queries get_cpu_iowait_time_us(), it may be >0 . Hence we will be
>> accounting for the idle time in both idle time and iowait time. This
>> is definitely a problem. But I do not understand what this has got to
>> do with the monotonicity of the time
>> returned. This is just for my understanding.
> 
> Thank you for your comment!
> 
> Ah yes, I think I could write better comments around here to clarify
> the monotonicity problem. (It will be happy if someone can give me
> such better sentence for here :-D)
> 
> One important point is that readers do not update idle stats when they
> use these function. i.e.
> 
> given:
>    idle stats:  idle=1000, iowait=100
>    stamp at idle entry: entry=50
>    nr tasks waiting io: nr_iowait=1
> 
> 1st reader:
>    query @ now=60
>      idle=1000
>      iowait=110 (=100+(60-50))
> 
> (here nr_iowait changed to 0)
> 
> 2nd reader:
>    query @ now=70
>      idle=1020 (=1000+(70-50))
>      iowait=100
> 
> So you will see iowait is decreased from 110 to 100.

Ah ok! I get it. Please do add my Reviewed-by to it.

> 
> I hope this short story helps you.

Thanks!

Regards
Preeti U Murthy
> 
> Thanks,
> H.Seto
> 

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