linux-kernel - Re: [RESEND PATCH 2/2] sched/fair: Optimize __update_sched

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Date:   Fri, 31 Mar 2017 09:13:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yuyang Du <yuyang.du@...el.com>
Cc:     Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        umgwanakikbuti@...il.com
Subject: Re: [RESEND PATCH 2/2] sched/fair: Optimize __update_sched_avg()

On Fri, Mar 31, 2017 at 03:41:18AM +0800, Yuyang Du wrote:

> > > 
> > >              p
> > >  c2 = 1024 \Sum y^n
> > >             n=1
> > > 
> > >               inf        inf
> > >     = 1024 ( \Sum y^n - \Sum y^n - y^0 )
> > >               n=0        n=p
> >  
> > It looks surprisingly kinda works :)
> >  
> > > +	c2 = LOAD_AVG_MAX - decay_load(LOAD_AVG_MAX, periods) - 1024;
> >                             ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > But, I'm not sure               this is what you want (just assume p==0).
> > 
> 
> Oh, what I meant is when p != 0, actually p>=1.

Right, note that we'll never get here with p==0. For p==1 we'll have
c2==0, and for p > 1 we'll have whatever section we wanted.

> And thinking about it for a while, it's really what you want, brilliant :)

Right, because:


     inf              inf            inf
  ( \Sum y^n ) y^p = \Sum y^(n+p) = \Sum y^n
     n=0              n=0            n=p