Re: [PATCH] sched/fair: Don't pull task if local group is more loaded than busiest group

Li, Aubrey
2020-03-26T01:57:13Z

On 2020/3/25 21:58, Vincent Guittot wrote:
> Le mercredi 25 mars 2020 à 20:46:28 (+0800), Aubrey Li a écrit :
>> A huge number of load imbalance was observed when the local group
>> type is group_fully_busy, and the average load of local group is
>> greater than the selected busiest group, so the imbalance calculation
>> returns a negative value actually. Fix this problem by comparing the
>
> Do you see any wrong task migration ? because if imbalance is < 0, detach_tasks should return early

I didn't see wrong task migration, in this case the busiest group has only one CPU intensive
workload running.

>
>> average load before local group type check.
>>
>> Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
>> ---
>> kernel/sched/fair.c | 14 +++++++-------
>> 1 file changed, 7 insertions(+), 7 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index c1217bf..c524369 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -8862,17 +8862,17 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
>> goto out_balanced;
>>
>> /*
>> + * If the local group is more loaded than the selected
>> + * busiest group don't try to pull any tasks.
>> + */
>> + if (local->avg_load >= busiest->avg_load)
>
> If local is not overloaded, local->avg_load is null because it has not been already computed.
>
> You should better add such test in calculate_imbalance after we computed local->avg_load and set imbalance to NULL

This makes more sense, will refine the patch for this.

Thanks,
-Aubrey

>
> something like:
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 9e544e702f66..62f0861cefc0 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9152,6 +9152,15 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>
> sds->avg_load = (sds->total_load * SCHED_CAPACITY_SCALE) /
> sds->total_capacity;
> +
> + /*
> + * If the local group is more loaded than the selected
> + * busiest group don't try to pull any tasks.
> + */
> + if (local->avg_load >= busiest->avg_load) {
> + env->imbalance =0;
> + return;
> + }
> }
>
> /*
>
>
>> + goto out_balanced;
>> +
>> + /*
>> * When groups are overloaded, use the avg_load to ensure fairness
>> * between tasks.
>> */
>> if (local->group_type == group_overloaded) {
>> - /*
>> - * If the local group is more loaded than the selected
>> - * busiest group don't try to pull any tasks.
>> - */
>> - if (local->avg_load >= busiest->avg_load)
>> - goto out_balanced;
>> -
>> /* XXX broken for overlapping NUMA groups */
>> sds.avg_load = (sds.total_load * SCHED_CAPACITY_SCALE) /
>> sds.total_capacity;
>> --
>> 2.7.4
>>