linux-kernel - Re: about definition of iowait

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Message-ID: <20160607120125.GH30154@twins.programming.kicks-ass.net>
Date:	Tue, 7 Jun 2016 14:01:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Cao jin <caoj.fnst@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Izumi, Taku/泉 拓 
	<izumi.taku@...fujitsu.com>
Subject: Re: about definition of iowait

On Tue, Jun 07, 2016 at 07:24:46PM +0800, Cao jin wrote:
> Hi,
> 
> I noticed some problems about iowait entry of /proc/stat: it seems not
> accurate, and sometimes will decrease in SMP.
> 
> For UP, we have a simple definition about iowait, which is:
> The time that the processor is idle, during which there is a task waiting
> for I/O.
> 
> This definition seems don't fit well when step into SMP era, so I think
> maybe it can be extended like:
> For a given CPU, the I/O wait time is the time during which that CPU was
> idle, and there is at least one outstanding disk I/O operation requested by
> a task scheduled on that CPU.
> 
> But tasks can migrate among cpus, so this definition maybe not accurate.
> 
> I also saw some suggestions that it should be a global value, not per cpu,
> but don`t see clear definition, so anyone have suggestions about it?
> (personally guessing, maybe: there is task waiting for outstanding I/O while
> all cpus are idle? But I don't think it is good...)

It is possible to come up with definition for a single global number;
per-cpu iowait numbers are doomed.

But I don't see it matters one way or the other. Personally I would just
make the kernel return 0 and be done with it.