linux-kernel - Re: Re: [PATCH 03/15] sched/fair: Add lag based placement

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Message-ID: <20231013073739.GA12118@noisy.programming.kicks-ass.net>
Date:   Fri, 13 Oct 2023 09:37:39 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Abel Wu <wuyun.abel@...edance.com>
Cc:     mingo@...nel.org, vincent.guittot@...aro.org,
        linux-kernel@...r.kernel.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, corbet@....net,
        qyousef@...alina.io, chris.hyser@...cle.com,
        patrick.bellasi@...bug.net, pjt@...gle.com, pavel@....cz,
        qperret@...gle.com, tim.c.chen@...ux.intel.com, joshdon@...gle.com,
        timj@....org, kprateek.nayak@....com, yu.c.chen@...el.com,
        youssefesmat@...omium.org, joel@...lfernandes.org, efault@....de,
        tglx@...utronix.de
Subject: Re: Re: [PATCH 03/15] sched/fair: Add lag based placement

On Thu, Oct 12, 2023 at 03:04:47PM +0800, Abel Wu wrote:
> On 10/11/23 9:24 PM, Peter Zijlstra Wrote:

> > > > +		 * we should inflate the lag before placement such that the
> > > > +		 * effective lag after placement comes out right.
> > > > +		 *
> > > > +		 * As such, invert the above relation for vl'_i to get the vl_i
> > > > +		 * we need to use such that the lag after placement is the lag
> > > > +		 * we computed before dequeue.
> > > > +		 *
> > > > +		 *   vl'_i = vl_i - w_i*vl_i / (W + w_i)
> > > > +		 *         = ((W + w_i)*vl_i - w_i*vl_i) / (W + w_i)
> > > > +		 *
> > > > +		 *   (W + w_i)*vl'_i = (W + w_i)*vl_i - w_i*vl_i
> > > > +		 *                   = W*vl_i
> > > > +		 *
> > > > +		 *   vl_i = (W + w_i)*vl'_i / W
> > 
> > And then we obtain the scale factor: (W + w_i)/W, which is >1, right?
> 
> Yeah, I see. But the scale factor is only for the to-be-placed entity.
> Say there is an entity k on the tree:
> 
> 	vl_k	= V - v_k
> 
> adding the to-be-placed entity i affects this by:
> 
> 	define delta := w_i*vl_i / (W + w_i)
> 
> 	vl'_k	= V' - v_k
> 		= V - delta - (V - vl_k)
> 		= vl_k - delta
> 
> hence for any entity on the tree, its lag is offsetted by @delta. So
> I wonder if we should simply do offsetting rather than scaling.

I don't see the point, the result is the same and computing delta seems
numerically less stable.