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

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Message-ID: <532FEB03.20600@jp.fujitsu.com>
Date:	Mon, 24 Mar 2014 17:21:23 +0900
From:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
To:	Preeti Murthy <preeti.lkml@...il.com>
CC:	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>, preeti@...ux.vnet.ibm.com
Subject: Re: [PATCH 1/2] nohz: use seqlock to avoid race on idle time stats

(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.

I hope this short story helps you.

Thanks,
H.Seto

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