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Date:   Wed, 11 Oct 2023 11:45:39 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Abel Wu <wuyun.abel@bytedance.com>
Cc:     mingo@kernel.org, vincent.guittot@linaro.org,
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Subject: Re: Re: [PATCH 01/15] sched/fair: Add avg_vruntime
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On Wed, Oct 11, 2023 at 04:30:26PM +0800, Abel Wu wrote:
> On 10/11/23 3:30 PM, Peter Zijlstra Wrote:
> > On Wed, Oct 11, 2023 at 12:15:28PM +0800, Abel Wu wrote:
> > > On 5/31/23 7:58 PM, Peter Zijlstra wrote:
> > > > +/*
> > > > + * Compute virtual time from the per-task service numbers:
> > > > + *
> > > > + * Fair schedulers conserve lag:
> > > > + *
> > > > + *   \Sum lag_i = 0
> > > > + *
> > > > + * Where lag_i is given by:
> > > > + *
> > > > + *   lag_i = S - s_i = w_i * (V - v_i)
> > > 
> > > Since the ideal service time S is task-specific, should this be:
> > > 
> > > 	lag_i = S_i - s_i = w_i * (V - v_i)
> > 
> > It is not, S is the same for all tasks. Remember, the base form is a
> > differential equation and all tasks progress at the same time at dt/w_i
> > while S progresses at dt/W.
> 
> IIUC it's V progresses at dt/W and is same for all tasks, not S which is
> measured in real time (V*w_i).

Clearly I should wake up before replying ;-)

  V = S/W, so dV = dt/W and dS = dt

Anyway, the point is that both V and S are the same across all tasks,
all tasks execute in parallel with infinitely small time increments.

In reality this can't work ofc, so we get the approximations v_i and s_i
and lag is the deviation from the ideal.