linux-kernel - Re: [RFC 0/2] drivers: spi/i2c: account completions as iowait

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Message-ID: <20141102165943.GT10501@worktop.programming.kicks-ass.net>
Date:	Sun, 2 Nov 2014 17:59:43 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Wolfram Sang <wsa@...-dreams.de>
Cc:	linux-kernel@...r.kernel.org, linux-i2c@...r.kernel.org,
	linux-spi@...r.kernel.org, Mark Brown <broonie@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Balbir Singh <bsingharora@...il.com>
Subject: Re: [RFC 0/2] drivers: spi/i2c: account completions as iowait

On Sun, Nov 02, 2014 at 02:58:07PM +0100, Wolfram Sang wrote:

> However, researching the net, users currently interpret iowait entirely as
> blkio wait. Furthermore, io_schedule() calls delayacct_blkio_{start|end}() which
> worked fine for my tests with I2C but might show that iowait was really meant as
> blkiowait? So, should other subsystems use it?

I would tend to agree with that; historically this has always been about
blkio, not device io.

> To make it more confusing, some people (like Peter Zijlstra [1]) seem to like
> iowait gone, so maybe it is all not worth it?

Yeah, iowait accounting is terminally broken :-) Mostly because the
iowait is accounted per-cpu but that is a very tenuous relation because
the IO devices are not per IO and blocking tasks are not associated with
any particular CPU -- after all they're not consuming CPU time.

If people really think its worth; we could invent new IO-wait measure
that do make sense -- maybe, but the current thing is complete bollocks.
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