linux-kernel - Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

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Date:	Tue, 20 Aug 2013 08:31:53 -0700
From:	Arjan van de Ven <arjan@...ux.intel.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Fernando Luis Vázquez Cao 
	<fernando_b1@....ntt.co.jp>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/4] nohz: Synchronize sleep time stats with seqlock

On 8/20/2013 1:44 AM, Peter Zijlstra wrote:
> On Tue, Aug 20, 2013 at 03:59:36PM +0900, Fernando Luis Vázquez Cao wrote:
>> That said, if deemed acceptable, option A is the one I would
>> choose.
>
> Right, so I think we can do A without much extra cost mostly because we
> already have 2 atomics in the io_schedule() path. If we replace those
> two atomic operations with locks and modify nr_iowait and the other
> stats under the same lock, and ensure all those variables (including the
> lock) live in the same cacheline we should have the same cost we have
> now.
>
> Of course, if we can get away with completely removing all of that
> (which I think Arjan suggested was a real possibility) then that would
> be ever so much better still :-)

I'm quite ok with removing that.

however note that "top" also reports per cpu iowait...
and that's a userspace expectation

>

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