linux-kernel - Re: iowait v.s. idle accounting is "inconsistent"

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Message-ID: <20190705113806.GP3402@hirez.programming.kicks-ass.net>
Date:   Fri, 5 Jul 2019 13:38:06 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alan Jenkins <alan.christopher.jenkins@...il.com>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        Doug Smythies <dsmythies@...us.net>, linux-pm@...r.kernel.org
Subject: Re: iowait v.s. idle accounting is "inconsistent" - iowait is too low

On Fri, Jul 05, 2019 at 12:25:46PM +0100, Alan Jenkins wrote:
> Hi, scheduler experts!
> 
> My cpu "iowait" time appears to be reported incorrectly.  Do you know why
> this could happen?

Because iowait is a magic random number that has no sane meaning.
Personally I'd prefer to just delete the whole thing, except ABI :/

Also see the comment near nr_iowait():

/*
 * IO-wait accounting, and how its mostly bollocks (on SMP).
 *
 * The idea behind IO-wait account is to account the idle time that we could
 * have spend running if it were not for IO. That is, if we were to improve the
 * storage performance, we'd have a proportional reduction in IO-wait time.
 *
 * This all works nicely on UP, where, when a task blocks on IO, we account
 * idle time as IO-wait, because if the storage were faster, it could've been
 * running and we'd not be idle.
 *
 * This has been extended to SMP, by doing the same for each CPU. This however
 * is broken.
 *
 * Imagine for instance the case where two tasks block on one CPU, only the one
 * CPU will have IO-wait accounted, while the other has regular idle. Even
 * though, if the storage were faster, both could've ran at the same time,
 * utilising both CPUs.
 *
 * This means, that when looking globally, the current IO-wait accounting on
 * SMP is a lower bound, by reason of under accounting.
 *
 * Worse, since the numbers are provided per CPU, they are sometimes
 * interpreted per CPU, and that is nonsensical. A blocked task isn't strictly
 * associated with any one particular CPU, it can wake to another CPU than it
 * blocked on. This means the per CPU IO-wait number is meaningless.
 *
 * Task CPU affinities can make all that even more 'interesting'.
 */