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

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Message-ID: <20150811000612.GL3956@byungchulpark-X58A-UD3R>
Date:	Tue, 11 Aug 2015 09:06:12 +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 Tue, Aug 11, 2015 at 08:35:29AM +0900, Byungchul Park wrote:
> 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?

i found it through code inspection, and then i checked it with debugger.

> 
> 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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