Re: [PATCH] sched: make update_sd_pick_busiest return true on a busier sd

[Vincent Guittot]

On 22 July 2014 20:45, Rik van Riel <riel@xxxxxxxxxx> wrote:
> Currently update_sd_pick_busiest only returns true when an sd
> is overloaded, or for SD_ASYM_PACKING when a domain is busier
> than average and a higher numbered domain than the target.
>
> This breaks load balancing between domains that are not overloaded,
> in the !SD_ASYM_PACKING case. This patch makes update_sd_pick_busiest
> return true when the busiest sd yet is encountered.
>
> On a 4 node system, this seems to result in the load balancer finally
> putting 1 thread of a 4 thread test run of "perf bench numa mem" on
> each node, where before the load was generally not spread across all
> nodes.
>
> Behaviour for SD_ASYM_PACKING does not seem to match the comment,
> in that groups with below average load average are ignored, but I
> have no hardware to test that so I have left the behaviour of that
> code unchanged.
>
> Cc: mikey@xxxxxxxxxxx
> Cc: peterz@xxxxxxxxxxxxx
> Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fea7d33..ff4ddba 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5942,16 +5942,20 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>          * numbered CPUs in the group, therefore mark all groups
>          * higher than ourself as busy.
>          */
> -       if ((env->sd->flags & SD_ASYM_PACKING) && sgs->sum_nr_running &&
> -           env->dst_cpu < group_first_cpu(sg)) {
> -               if (!sds->busiest)
> -                       return true;
> +       if (env->sd->flags & SD_ASYM_PACKING) {
> +               if (sgs->sum_nr_running && env->dst_cpu < group_first_cpu(sg)) {
> +                       if (!sds->busiest)
> +                               return true;
>
> -               if (group_first_cpu(sds->busiest) > group_first_cpu(sg))
> -                       return true;
> +                       if (group_first_cpu(sds->busiest) > group_first_cpu(sg))
> +                               return true;
> +               }
> +
> +               return false;
>         }
>
> -       return false;
> +       /* See above: sgs->avg_load > sds->busiest_stat.avg_load */
> +       return true;

Hi Rik,

I can see one issue with a default return set to true.  You increase
the number of time where we will not effectively migrate a task
because we don't ensure that we will take the overloaded group if
there is one. We can be in a situation where a group is overloaded but
the load_balance will select a not overloaded group with an average
load higher than sched_domain average value just because it is checked
after.

Regarding your issue with "perf bench numa mem" that is not spread on
all nodes, SD_PREFER_SIBLING flag (of DIE level) should do the job by
reducing the capacity of  "not local DIE" group at NUMA level to 1
task during the load balance computation. So you should have 1 task
per sched_group at NUMA level.

Vincent


>  }
>
>  #ifdef CONFIG_NUMA_BALANCING
>
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