linux-kernel - Re: [PATCH] sched/fair.c: Fix the calculation method of 'lag'

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Message-ID: <20240425115125.GZ40213@noisy.programming.kicks-ass.net>
Date: Thu, 25 Apr 2024 13:51:25 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: hupu@...o.com
Cc: mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/fair.c: Fix the calculation method of 'lag'

On Thu, Apr 25, 2024 at 07:44:59PM +0800, hupu@...o.com wrote:
> > > From: hupu <hupu@...o.com>
> > >
> > > I think the 'lag' calculation here is inaccurate.
> > >
> > > Assume that delta needs to be subtracted from v_i to ensure that the
> > > vlag of task i after placement is the same as before.
> >
> > Why ?!? v_i is the unkown, it makes no sense to complicate things by
> > adding extra unknowns.
> >
> > > At this time, the
> > > vlag of task i after placement should be:
> > > vl'_i = V' - (v_i - delta)
> >
> > But but but, you can't have V' without knowing v_i.
> >
> 
> Thank you for your patient guidance. I overlooked a important fact that
> v_i is unknown in the process of proof. Below is the complete proof
> process, and it turns out that you are correct.

*phew*, thanks for checking!