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Date: Thu, 25 Apr 2024 13:51:25 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: hupu@oppo.com
Cc: mingo@redhat.com, juri.lelli@redhat.com, vincent.guittot@linaro.org,
	dietmar.eggemann@arm.com, rostedt@goodmis.org, bsegall@google.com,
	mgorman@suse.de, bristot@redhat.com, vschneid@redhat.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/fair.c: Fix the calculation method of 'lag'
Message-ID: <20240425115125.GZ40213@noisy.programming.kicks-ass.net>
References: <20240424085533.GS40213@noisy.programming.kicks-ass.net>
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On Thu, Apr 25, 2024 at 07:44:59PM +0800, hupu@oppo.com wrote:
> > > From: hupu <hupu@oppo.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!