linux-kernel - Re: [PATCH 1/4] sched: move IO scheduling accounting from io_schedule_timeout() to _

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Message-ID: <20161031164556.GC26364@mtj.duckdns.org>
Date:   Mon, 31 Oct 2016 10:45:56 -0600
From:   Tejun Heo <tj@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tejun Heo <htejun@...com>, torvalds@...ux-foundation.org,
        akpm@...ux-foundation.org, mingo@...hat.com, axboe@...nel.dk,
        tytso@....edu, jack@...e.com, adilger.kernel@...ger.ca,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com, mingbo@...com
Subject: Re: [PATCH 1/4] sched: move IO scheduling accounting from
 io_schedule_timeout() to __schedule()

Hello,

On Sat, Oct 29, 2016 at 05:21:26AM +0200, Peter Zijlstra wrote:
> On Fri, Oct 28, 2016 at 03:12:32PM -0400, Tejun Heo wrote:
> > Hello, Peter.
> > 
> > On Fri, Oct 28, 2016 at 09:07:02PM +0200, Peter Zijlstra wrote:
> > > One alternative is to inherit the iowait state of the task we block on.
> > > That'll not get rid of the branches much, but it will remove the new
> > > mutex APIs.
> > 
> > Yeah, thought about that briefly but we don't necessarily track mutex
> 
> This one I actually fixed and should be in -next. And it would be
> sufficient to cover the use case here.

Tracking the owners of mutexes and rwsems does help quite a bit.  I
don't think it's as simple as inheriting io sleep state from the
current owner tho.  The owner might be running or in a non-IO sleep
when others try to grab the mutex.  It is an option to ignore those
cases but this would have a real possibility to lead to surprising
results in some corner cases.  If we choose to propagate dynamically,
it becomes an a lot more complex problem and I don't think it'd be
justfiable.

Unless there can be a simple enough and reliable solution, I think
it'd be better to stick with explicit marking.

Thanks.

-- 
tejun