linux-kernel - Re: [PATCH 1/2] nohz: use seqlock to avoid race on idle time stats v2

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Date:	Sat, 5 Apr 2014 16:56:54 +0200
From:	Denys Vlasenko <vda.linux@...glemail.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Fernando Luis Vazquez Cao <fernando_b1@....ntt.co.jp>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	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 U Murthy <preeti@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/2] nohz: use seqlock to avoid race on idle time stats v2

On Sat, Apr 5, 2014 at 12:08 PM, Frederic Weisbecker <fweisbec@...il.com> wrote:
>> > Iowait makes sense but not per cpu. Eventually it's a global
>> > stat. Or per task.
>>
>> There a lot of situations where admins want to know
>> how much, on average, their CPUs are idle because
>> they wait for IO.
>>
>> If you are running, say, a Web cache,
>> you need to know that stat in order to be able to
>> conjecture "looks like I'm IO bound, perhaps caching
>> some data in RAM will speed it up".
>
> But then accounting iowait time waited until completion on the CPU
> that the task wakes up should work for that.
>
> Doesn't it?

It can easily make iowait count higher than idle count,
or even higher than idle+sys+user+nice count.

IOW, it can show that the system is way more
than 100% IO bound, which doesn't make sense.


> So we save, per task, the time when the task went to sleep. And when it wakes up
> we just flush the pending time since that sleep time to the waking CPU:
> iowait[$CPU] += NOW() - tsk->sleeptime;
>
>> Is such counter meaningful for the admin?
>
> Well, you get the iowait time accounting.

Admin wants to know "how often do I have CPU idled
because they have nothing to do until IO is complete?"

Merely knowing how much tasks waited for IO
doesn't answer that question.
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