Re: [PATCH v4 01/12] sched: fix imbalance flag reset

[Wanpeng Li]

Hi Vincent,
On 7/29/14, 1:51 AM, Vincent Guittot wrote:
> The imbalance flag can stay set whereas there is no imbalance.
>
> Let assume that we have 3 tasks that run on a dual cores /dual cluster system.
> We will have some idle load balance which are triggered during tick.
> Unfortunately, the tick is also used to queue background work so we can reach
> the situation where short work has been queued on a CPU which already runs a
> task. The load balance will detect this imbalance (2 tasks on 1 CPU and an idle
> CPU) and will try to pull the waiting task on the idle CPU. The waiting task is
> a worker thread that is pinned on a CPU so an imbalance due to pinned task is
> detected and the imbalance flag is set.

The waiting task is the third task or one the '2 tasks on 1 CPU' ?

Regards,
Wanpeng Li

> Then, we will not be able to clear the flag because we have at most 1 task on
> each CPU but the imbalance flag will trig to useless active load balance
> between the idle CPU and the busy CPU.
>
> We need to reset of the imbalance flag as soon as we have reached a balanced
> state. If all tasks are pinned, we don't consider that as a balanced state and
> let the imbalance flag set.
>
> Reviewed-by: Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> ---
>   kernel/sched/fair.c | 23 +++++++++++++++++++----
>   1 file changed, 19 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 923fe32..7eb9126 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6672,10 +6672,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>   		if (sd_parent) {
>   			int *group_imbalance = &sd_parent->groups->sgc->imbalance;
>   
> -			if ((env.flags & LBF_SOME_PINNED) && env.imbalance > 0) {
> +			if ((env.flags & LBF_SOME_PINNED) && env.imbalance > 0)
>   				*group_imbalance = 1;
> -			} else if (*group_imbalance)
> -				*group_imbalance = 0;
>   		}
>   
>   		/* All tasks on this runqueue were pinned by CPU affinity */
> @@ -6686,7 +6684,7 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>   				env.loop_break = sched_nr_migrate_break;
>   				goto redo;
>   			}
> -			goto out_balanced;
> +			goto out_all_pinned;
>   		}
>   	}
>   
> @@ -6760,6 +6758,23 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>   	goto out;
>   
>   out_balanced:
> +	/*
> +	 * We reach balance although we may have faced some affinity
> +	 * constraints. Clear the imbalance flag if it was set.
> +	 */
> +	if (sd_parent) {
> +		int *group_imbalance = &sd_parent->groups->sgc->imbalance;
> +
> +		if (*group_imbalance)
> +			*group_imbalance = 0;
> +	}
> +
> +out_all_pinned:
> +	/*
> +	 * We reach balance because all tasks are pinned at this level so
> +	 * we can't migrate them. Let the imbalance flag set so parent level
> +	 * can try to migrate them.
> +	 */
>   	schedstat_inc(sd, lb_balanced[idle]);
>   
>   	sd->nr_balance_failed = 0;

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