linux-kernel - Re: [PATCH 3/3] nohz: Fix iowait overcounting if iowait task migrates

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Message-ID: <53595B36.70606@redhat.com>
Date:	Thu, 24 Apr 2014 20:43:02 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>,
	linux-kernel@...r.kernel.org
CC:	Frederic Weisbecker <fweisbec@...il.com>,
	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>
Subject: Re: [PATCH 3/3] nohz: Fix iowait overcounting if iowait task migrates

On 04/24/2014 10:19 AM, Hidetoshi Seto wrote:
> As I already mentioned in previous discussion with Peter, I have
> concern here that this change might have impact on performance.
> Especially in case if system is a kind of io-busy box, originally
> there may be no iowait time (and possibly also no idle time).
> For such case this change adds extra execution cost to manage
> value of iowait_exittime which might not used.

Everything has some cost. Correctness usually trumps a few extra
locked bus cycles.

Wouldn't it be nice if we'd know whether anyone even needs
the stats? If no one going to read /proc/stat on the box,
there's no point in going to all the trouble to maintain the counters...

> And if we successfully found a way to get the iowait_exittime
> within reasonable negligible cheap cost, then why we don't use
> it for NOHZ=n kernels too? 

Kernels without NOHZ maintain the counters based on timer interrupt
sampling. It should still work fine.

> As Frederic already pointed, seqcount must be better choice.

Yes, I'm switching to seqcounts.
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