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

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Message-ID: <20161219230713.GD2895@var.home>
Date:   Tue, 20 Dec 2016 00:07:13 +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 14:44:38 -0800, wrote:
> On Mon, Dec 19, 2016 at 2:40 PM, Samuel Thibault
> <samuel.thibault@...-lyon.org> wrote:
> > 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
> >
> > exposed yet another miscalculation in calc_cfs_shares: MIN_SHARES is unscaled,
> > and must thus be scaled before being manipulated against "shares" amounts.
> 
> It's actually intentional that MIN_SHARES is un-scaled here, this is
> necessary to support the goal of sub-partitioning groups with small
> shares.

Uh? you mean it's normal that MIN_SHARES is here compared as such
against "shares" while e.g. in sched_group_set_shares or effective_load
it is scaled before comparing with "shares"?

> E.g.  A group with shares=2 and 5 threads will internally provide 2048
> units of weight for the load-balancer to account for their
> distribution.

But here "shares" is already scaled, so

> > -       if (shares < MIN_SHARES)
> > -               shares = MIN_SHARES;
...
> >         return shares;

This will only make sure that the returned shares is 2, not 2048.

Samuel