linux-kernel - Re: [PATCH] sched: ensure a task has a non-normalized vruntime when returning back to cfs

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Message-ID: <20150810233529.GI3956@byungchulpark-X58A-UD3R>
Date:	Tue, 11 Aug 2015 08:35:29 +0900
From:	Byungchul Park <byungchul.park@....com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: ensure a task has a non-normalized vruntime when
 returning back to cfs

On Mon, Aug 10, 2015 at 02:14:07PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 10, 2015 at 06:02:55PM +0900, byungchul.park@....com wrote:
> > From: Byungchul Park <byungchul.park@....com>
> > 
> > current code ensures a task has a normalized vruntime when switching off
> > from fair class, but it does not ensure the task has a non-normalized
> > vruntime when switching back to the fair class.
> > 
> > this is an example breaking this consistency.
> > 
> > 1. a task is in fair class and !queue
> > 2. change its class to rt class (still !queue)
> > 3. change its class to fair class again (stll !queue)
> 
> Just curious, did you manage to trigger this in practise or did you find
> it through code inspection only?

hello,

not only through code inspection, but also checked it through gdb debugger.
when returning back to fair class (step 3), vruntime was still normalized.
more exactly, it has a very large value which means negative value.

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