Re: [PATCH 1/2] sched: fix and clean up calculate_imbalance

[Vincent Guittot]

On 29 July 2014 17:27, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> On Tue, Jul 29, 2014 at 04:59:10PM +0200, Peter Zijlstra wrote:
>> On Tue, Jul 29, 2014 at 11:04:50AM +0200, Vincent Guittot wrote:
>> > > In situations where all the domains are overloaded, or where only the
>> > > busiest domain is overloaded, that code is also superfluous, since
>> > > the normal env->imbalance calculation will figure out how much to move.
>> > > Remove the load_above_capacity calculation.
>> >
>> > IMHO, we should not remove that part which is used by prefer_sibling
>> >
>> > Originally, we had 2 type of busiest group: overloaded or imbalanced.
>> > You add a new one which has only a avg_load higher than other so you
>> > should handle this new case and keep the other ones unchanged
>>
>> Right, so we want that code for overloaded -> overloaded migrations such
>> as not to cause idle cpus in an attempt to balance things. Idle cpus are
>> worse than imbalance.
>>
>> But in case of overloaded/imb -> !overloaded migrations we can allow it,
>> and in fact want to allow it in order to balance idle cpus.
>
> Which would be patch 3/2

FWIW, you can add my ack for the new version of the patchset which
includes this patch

>
> ---
> Subject: sched,fair: Allow calculate_imbalance() to move idle cpus
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Date: Tue Jul 29 17:15:11 CEST 2014
>
> Allow calculate_imbalance() to 'create' idle cpus in the busiest group
> if there are idle cpus in the local group.
>
> Suggested-by: Rik van Riel <riel@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/n/tip-7k95k4i2tjv78iivstggiude@xxxxxxxxxxxxxx
> ---
>  kernel/sched/fair.c |   11 +++++------
>  1 file changed, 5 insertions(+), 6 deletions(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6273,12 +6273,11 @@ static inline void calculate_imbalance(s
>                 return fix_small_imbalance(env, sds);
>         }
>
> -       if (busiest->group_type == group_overloaded) {
> -               /*
> -                * Don't want to pull so many tasks that a group would go idle.
> -                * Except of course for the group_imb case, since then we might
> -                * have to drop below capacity to reach cpu-load equilibrium.
> -                */
> +       /*
> +        * If there aren't any idle cpus, avoid creating some.
> +        */
> +       if (busiest->group_type == group_overloaded &&
> +           local->group_type   == group_overloaded) {
>                 load_above_capacity =
>                         (busiest->sum_nr_running - busiest->group_capacity_factor);
>
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