Re: [PATCH 3/6] sched/numa: Avoid task migration for small numa improvement

[Ingo Molnar <mingo@kernel.org>]

* Srikar Dronamraju <srikar@linux.vnet.ibm.com> wrote:

> If numa improvement from the task migration is going to be very
> minimal, then avoid task migration.
> 
> specjbb2005 / bops/JVM / higher bops are better
> on 2 Socket/2 Node Intel
> JVMS  Prev    Current  %Change
> 4     200892  210118   4.59252
> 1     325766  313171   -3.86627
> 
> 
> on 2 Socket/4 Node Power8 (PowerNV)
> JVMS  Prev     Current  %Change
> 8     89011.9  91027.5  2.26442
> 1     211338   216460   2.42361
> 
> 
> on 2 Socket/2 Node Power9 (PowerNV)
> JVMS  Prev    Current  %Change
> 4     190261  191918   0.870909
> 1     195305  207043   6.01009
> 
> 
> on 4 Socket/4 Node Power7
> JVMS  Prev     Current  %Change
> 8     57651.1  58462.1  1.40674
> 1     111351   108334   -2.70945
> 
> 
> dbench / transactions / higher numbers are better
> on 2 Socket/2 Node Intel
> count  Min      Max      Avg      Variance  %Change
> 5      12254.7  12331.9  12297.8  28.1846
> 5      11851.8  11937.3  11890.9  33.5169   -3.30872
> 
> 
> on 2 Socket/4 Node Power8 (PowerNV)
> count  Min      Max      Avg      Variance  %Change
> 5      4997.83  5030.14  5015.54  12.947
> 5      4791     5016.08  4962.55  85.9625   -1.05652
> 
> 
> on 2 Socket/2 Node Power9 (PowerNV)
> count  Min      Max      Avg      Variance  %Change
> 5      9331.84  9375.11  9352.04  16.0703
> 5      9353.43  9380.49  9369.6   9.04361   0.187767
> 
> 
> on 4 Socket/4 Node Power7
> count  Min      Max      Avg      Variance  %Change
> 5      147.55   181.605  168.963  11.3513
> 5      149.518  215.412  179.083  21.5903   5.98948
> 
> Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
> ---
> Changelog v1->v2:
>  - Handle trivial changes due to variable name change. (Rik Van Riel)
>  - Drop changes where subsequent better cpu find was rejected for
>    small numa improvement (Rik Van Riel).
> 
>  kernel/sched/fair.c | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 5cf921a..a717870 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -1568,6 +1568,13 @@ static bool load_too_imbalanced(long src_load, long dst_load,
>  }
>  
>  /*
> + * Maximum numa importance can be 1998 (2*999);
> + * SMALLIMP @ 30 would be close to 1998/64.
> + * Used to deter task migration.
> + */
> +#define SMALLIMP	30
> +
> +/*
>   * This checks if the overall compute and NUMA accesses of the system would
>   * be improved if the source tasks was migrated to the target dst_cpu taking
>   * into account that it might be best if task running on the dst_cpu should
> @@ -1600,7 +1607,7 @@ static void task_numa_compare(struct task_numa_env *env,
>  		goto unlock;
>  
>  	if (!cur) {
> -		if (maymove || imp > env->best_imp)
> +		if (maymove && moveimp >= env->best_imp)
>  			goto assign;
>  		else
>  			goto unlock;
> @@ -1643,16 +1650,22 @@ static void task_numa_compare(struct task_numa_env *env,
>  			       task_weight(cur, env->dst_nid, dist);
>  	}
>  
> -	if (imp <= env->best_imp)
> -		goto unlock;
> -
>  	if (maymove && moveimp > imp && moveimp > env->best_imp) {
> -		imp = moveimp - 1;
> +		imp = moveimp;
>  		cur = NULL;
>  		goto assign;
>  	}
>  
>  	/*
> +	 * If the numa importance is less than SMALLIMP,
> +	 * task migration might only result in ping pong
> +	 * of tasks and also hurt performance due to cache
> +	 * misses.
> +	 */
> +	if (imp < SMALLIMP || imp <= env->best_imp + SMALLIMP / 2)
> +		goto unlock;
> +
> +	/*
>  	 * In the overloaded case, try and keep the load balanced.
>  	 */
>  	load = task_h_load(env->p) - task_h_load(cur);

So what is this 'NUMA importance'? Seems just like a random parameter which generally isn't a 
good idea.

Also, same review feedback as I gave for the previous patches.

Thanks,

	Ingo