Re: [PATCH v2] sched/fair: Fix util_avg of new tasks for asymmetric systems

[Quentin Perret <quentin.perret@arm.com>]

Hi,

On Tuesday 12 Jun 2018 at 12:22:15 (+0100), Quentin Perret wrote:
> When a new task wakes-up for the first time, its initial utilization
> is set to half of the spare capacity of its CPU. The current
> implementation of post_init_entity_util_avg() uses SCHED_CAPACITY_SCALE
> directly as a capacity reference. As a result, on a big.LITTLE system, a
> new task waking up on an idle little CPU will be given ~512 of util_avg,
> even if the CPU's capacity is significantly less than that.
> 
> Fix this by computing the spare capacity with arch_scale_cpu_capacity().
> 
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Quentin Perret <quentin.perret@arm.com>
> 
> ---
> v2: added "Acked-by: Vincent Guittot <vincent.guittot@linaro.org>"
> ---
>  kernel/sched/fair.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e497c05aab7f..f19432c17017 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -735,11 +735,12 @@ static void attach_entity_cfs_rq(struct sched_entity *se);
>   * To solve this problem, we also cap the util_avg of successive tasks to
>   * only 1/2 of the left utilization budget:
>   *
> - *   util_avg_cap = (1024 - cfs_rq->avg.util_avg) / 2^n
> + *   util_avg_cap = (cpu_scale - cfs_rq->avg.util_avg) / 2^n
>   *
> - * where n denotes the nth task.
> + * where n denotes the nth task and cpu_scale the CPU capacity.
>   *
> - * For example, a simplest series from the beginning would be like:
> + * For example, for a CPU with 1024 of capacity, a simplest series from
> + * the beginning would be like:
>   *
>   *  task  util_avg: 512, 256, 128,  64,  32,   16,    8, ...
>   * cfs_rq util_avg: 512, 768, 896, 960, 992, 1008, 1016, ...
> @@ -751,7 +752,8 @@ void post_init_entity_util_avg(struct sched_entity *se)
>  {
>  	struct cfs_rq *cfs_rq = cfs_rq_of(se);
>  	struct sched_avg *sa = &se->avg;
> -	long cap = (long)(SCHED_CAPACITY_SCALE - cfs_rq->avg.util_avg) / 2;
> +	long cpu_scale = arch_scale_cpu_capacity(NULL, cpu_of(rq_of(cfs_rq)));
> +	long cap = (long)(cpu_scale - cfs_rq->avg.util_avg) / 2;
>  
>  	if (cap > 0) {
>  		if (cfs_rq->avg.util_avg != 0) {
> -- 
> 2.17.1
> 

Is there anything else I should do for this patch ?

Thanks,
Quentin