[PATCH v4 ] sched: fix imbalance flag reset

[Preeti U Murthy]

Hi Vincent,

On 07/10/2014 03:00 PM, 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.
> 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.
> 
> Signed-off-by: Vincent Guittot <vincent.guittot at linaro.org>
> ---
>  kernel/sched/fair.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index d3c73122..a836198 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6615,10 +6615,8 @@ 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 */
> @@ -6629,7 +6627,7 @@ 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;
>  		}
>  	}
> 
> @@ -6703,6 +6701,22 @@ 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;
> 

This patch looks good to me.

Reviewed-by: Preeti U Murthy <preeti at linux.vnet.ibm.com>