Re: [RESEND PATCH 2/2] sched/fair: Optimize __update_sched_avg()

[Peter Zijlstra]

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