linux-kernel - Re: [RFC][PATCH -rt] sched: fully ignore RT tasks for CFS load-balancing

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Date:	Wed, 26 Aug 2009 21:04:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Darren Hart <dvhltc@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC][PATCH -rt] sched: fully ignore RT tasks for CFS
 load-balancing

On Wed, 2009-08-26 at 11:52 -0700, Darren Hart wrote:
> Peter Zijlstra wrote:
> > Since -rt runs IRQs and SoftIRQs as RT tasks and load-balancing is done
> > from softirq context, there is always at least one RT task (and very
> > likely multiple) running when we balance.
> > 
> > The current (and totally broken) interaction between RT tasks and CFS
> > load-balancing makes it so that we'll try to evacuate a significant
> > amount of tasks due to RT tasks being runnable.
> > 
> > Solve this in another broken way by not accounting RT tasks at all.
> > 
> > This will likely break another class of cases, but until we can properly
> > fix this, we might as well do this.
> 
> Peter,
> 
> Do we know why this became a problem lately?  I don't recall seeing load 
> balancing issues like this way back around 2.6.21 or 22 when CFS was 
> released.  Is the cpu_load stuff fairly recent?

The earlier problem solved by the previous patch was caused by tglx
tinkering with the cpu_load computation.

This particular problem I'm not sure about, afaict it should have been
present a long long time.
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