[RFC 1/1] sched/fair: Consider asymmetric scheduler groups in load balancer

[Vincent Guittot]

On Mon, 15 May 2023 at 13:46, Tobias Huschle <huschle at linux.ibm.com> wrote:
>
> The current load balancer implementation implies that scheduler groups,
> within the same domain, all host the same number of CPUs. This is
> reflected in the condition, that a scheduler group, which is load
> balancing and classified as having spare capacity, should pull work
> from the busiest group, if the local group runs less processes than
> the busiest one. This implies that these two groups should run the
> same number of processes, which is problematic if the groups are not
> of the same size.
>
> The assumption that scheduler groups within the same scheduler domain
> host the same number of CPUs appears to be true for non-s390
> architectures. Nevertheless, s390 can have scheduler groups of unequal
> size.
>
> This introduces a performance degredation in the following scenario:
>
> Consider a system with 8 CPUs, 6 CPUs are located on one CPU socket,
> the remaining 2 are located on another socket:
>
> Socket   -----1-----    -2-
> CPU      1 2 3 4 5 6    7 8
>
> Placing some workload ( x = one task ) yields the following
> scenarios:
>
> The first 5 tasks are distributed evenly across the two groups.
>
> Socket   -----1-----    -2-
> CPU      1 2 3 4 5 6    7 8
>          x x x          x x
>
> Adding a 6th task yields the following distribution:
>
> Socket   -----1-----    -2-
> CPU      1 2 3 4 5 6    7 8
> SMT1     x x x          x x
> SMT2                    x

Your description is a bit confusing for me. What you name CPU above
should be named Core, doesn' it ?

Could you share with us your scheduler topology ?

>
> The task is added to the 2nd scheduler group, as the scheduler has the
> assumption that scheduler groups are of the same size, so they should
> also host the same number of tasks. This makes CPU 7 run into SMT
> thread, which comes with a performance penalty. This means, that in
> the window of 6-8 tasks, load balancing is done suboptimally, because
> SMT is used although there is no reason to do so as fully idle CPUs
> are still available.
>
> Taking the weight of the scheduler groups into account, ensures that
> a load balancing CPU within a smaller group will not try to pull tasks
> from a bigger group while the bigger group still has idle CPUs
> available.
>
> Signed-off-by: Tobias Huschle <huschle at linux.ibm.com>
> ---
>  kernel/sched/fair.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 48b6f0ca13ac..b1307d7e4065 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10426,7 +10426,8 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>          * group's child domain.
>          */
>         if (sds.prefer_sibling && local->group_type == group_has_spare &&
> -           busiest->sum_nr_running > local->sum_nr_running + 1)
> +           busiest->sum_nr_running * local->group_weight >
> +                       local->sum_nr_running * busiest->group_weight + 1)

This is the prefer_sibling path. Could it be that you should disable
prefer_siling between your sockets for such topology ? the default
path compares the number of idle CPUs when groups has spare capacity


>                 goto force_balance;
>
>         if (busiest->group_type != group_overloaded) {
> --
> 2.34.1
>