linux-kernel - Re: [PATCH] sched/fair: fix calc_cfs

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Message-ID: <20161219234525.GI2895@var.home>
Date:   Tue, 20 Dec 2016 00:45:25 +0100
From:   Samuel Thibault <samuel.thibault@...-lyon.org>
To:     Paul Turner <pjt@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

Paul Turner, on Mon 19 Dec 2016 15:32:15 -0800, wrote:
> On Mon, Dec 19, 2016 at 3:29 PM, Samuel Thibault
> <samuel.thibault@...-lyon.org> wrote:
> > Paul Turner, on Mon 19 Dec 2016 15:26:19 -0800, wrote:
> >> >> > -       if (shares < MIN_SHARES)
> >> >> > -               shares = MIN_SHARES;
> >> > ...
> >> >> >         return shares;
> >> >
> >> > This will only make sure that the returned shares is 2, not 2048.
> >>
> >> This is intentional.  The MIN_SHARES you are seeing here is overloaded.
> >> Every "1" unit of share is "SCHED_LOAD_RESOLUTION" bits internally.
> >
> > I'm not talking about the SCHED_LOAD_RESOLUTION scaling, but about the
> > SCHED_FIXEDPOINT_SHIFT scaling, which is what
> > 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
> > modified on 64bit platforms.
> 
> .... From that commit:
> 
> """
> -#if 0 /* BITS_PER_LONG > 32 -- currently broken: it increases power
> usage under light load  */
> +#ifdef CONFIG_64BIT
>  # define SCHED_LOAD_RESOLUTION 10
>  # define scale_load(w)         ((w) << SCHED_LOAD_RESOLUTION)
>  # define scale_load_down(w)    ((w) >> SCHED_LOAD_RESOLUTION)

Errgl, sorry, I was referring to the old naming.  This stuff has seen
so much patching over and over in the past revisions...  It though you
were referring to SCHED_CAPACITY_SCALE.  The code I was reading now uses
SCHED_LOAD_RESOLUTION, so that's why I read your "SCHED_LOAD_RESOLUTION"
as "the other scaling".

> The MIN_SHARES you are seeing here is overloaded.
> In the unscaled case this needs to be MIN_SHARES, and in the scaled
> case, the subdivision of the scaled values must still be >=2.

Ok, now I understand.  I have to say this overloading is confusing.

Samuel