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Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics

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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@ens-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

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Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics Paul Turner <pjt@kernel.org> - 2016-12-19 23:50 +0100
  Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics Samuel Thibault <samuel.thibault@ens-lyon.org> - 2016-12-20 00:10 +0100
    Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics Paul Turner <pjt@kernel.org> - 2016-12-20 00:30 +0100
      Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics Samuel Thibault <samuel.thibault@ens-lyon.org> - 2016-12-20 00:40 +0100
        Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics Paul Turner <pjt@kernel.org> - 2016-12-20 00:40 +0100
          Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics Samuel Thibault <samuel.thibault@ens-lyon.org> - 2016-12-20 00:50 +0100
            Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics Dietmar Eggemann <dietmar.eggemann@arm.com> - 2016-12-20 14:10 +0100
              Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics Peter Zijlstra <peterz@infradead.org> - 2016-12-20 14:20 +0100
              Re: [PATCH] sched/fair: fix calc_cfs_shares fixed point arithmetics Samuel Thibault <samuel.thibault@ens-lyon.org> - 2016-12-20 14:30 +0100

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