linux-kernel - Re: [PATCH 2/2] nohz: use delayed iowait accounting to avoid race on idle time stats

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Message-ID: <534F2374.8090304@jp.fujitsu.com>
Date:	Thu, 17 Apr 2014 09:42:28 +0900
From:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
To:	Peter Zijlstra <peterz@...radead.org>
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>,
	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>,
	Denys Vlasenko <vda.linux@...glemail.com>,
	stable@...r.kernel.org
Subject: Re: [PATCH 2/2] nohz: use delayed iowait accounting to avoid race
 on idle time stats

(2014/04/16 18:36), Peter Zijlstra wrote:
> On Wed, Apr 16, 2014 at 03:33:06PM +0900, Hidetoshi Seto wrote:
>> So we need 2 operations:
>>   a) remove regression
> 
> What regression; there's never been talk about a regression, just a bug
> found. AFAICT this 'regression' is ever since we introduced NOHZ or
> somesuch, which is very long ago indeed.
> 
> And since its basically been broken forever, there's no rush what so
> ever.

Well, from a customer's view, when he upgrade his foobar enterprise
linux from version 5 to 6 (for example), he will say "it's a regression"
if something worked well in previous version have broken in new version
without any documents and/or technical notes etc.

That's why I used the word "regression" for this bug.

>>   b) implement new iowait accounting mechanism
>>
>> What Frederic mentioned is that we don't need a) once if we invent
>> the solution for b). But I doubt it because a) is still required
>> for stable environment including some distributor's kernel.
>> It is clear that patches for b) will not be backportable.
>>
>> Still the b) is disease that has no known cure. There is no reason
>> to wait works on b) before starting works for a).
> 
> As stated, there is no a). Its been forever broken. There is no urgency.

I just wrote my patches for my customer like above and for my salary ;-)

Thank you for your comments!
I'll post my v4 patch set soon.


Thanks,
H.Seto



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