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

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Date:   Mon, 19 Dec 2016 14:44:38 -0800
From:   Paul Turner <pjt@...nel.org>
To:     Samuel Thibault <samuel.thibault@...-lyon.org>,
        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

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.

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.

>
> Signed-off-by: Samuel Thibault <samuel.thibault@...-lyon.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: stable@...r.kernel.org
> Fixes: 2159197d6677 ("sched/core: Enable increased load resolution on 64-bit kernels")
>
> ---
> This should be backported to 4.7 and 4.8 to fix scheduling priorities
> miscalculations
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6559d19..be84f72 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2657,8 +2657,8 @@ static long calc_cfs_shares(struct cfs_rq *cfs_rq, struct task_group *tg)
>         if (tg_weight)
>                 shares /= tg_weight;
>
> -       if (shares < MIN_SHARES)
> -               shares = MIN_SHARES;
> +       if (shares < scale_load(MIN_SHARES))
> +               shares = scale_load(MIN_SHARES);
>         if (shares > tg->shares)
>                 shares = tg->shares;
>