linux-kernel - Re: commit e9e9250b: sync wakeup bustage when waker is an RT task

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Message-ID: <1274086345.5605.3727.camel@twins>
Date:	Mon, 17 May 2010 10:52:25 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mike Galbraith <efault@....de>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: commit e9e9250b: sync wakeup bustage when waker is an RT task

On Mon, 2010-05-17 at 10:49 +0200, Peter Zijlstra wrote:
> On Mon, 2010-05-17 at 06:38 +0200, Mike Galbraith wrote:
> > What would be the harm/consequence of restoring RT tasks to rq->load so
> > the wake_affine()::sync logic just worked as before without hackery?
> 
> Well, you'd have to constantly adjust the task weight of RT tasks to
> reflect their actual consumption. Not really feasible.
> 
> So the proportional stuff works like:
> 
>  slice_i = w_i / (\Sum_j w_j) * dt
> 
> Giving a RT task a sensible weight we'd have to reverse that:
> 
>  w_i = slice_i/dt * (\Sum_j w_j)

Another point to note is that this requires we track per-RT-task usage
averages, whereas the cpu_power approach simply lumps everything !fair
(one of the things still on the TODO list is account for IRQ overhead)
into a single large bucket and doesn't care.
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