Re: [PATCH] sched/fair: handle case of task_h_load() returning 0

[Valentin Schneider <valentin.schneider@arm.com>]

On 02/07/20 15:42, Vincent Guittot wrote:
> task_h_load() can return 0 in some situations like running stress-ng
> mmapfork, which forks thousands of threads, in a sched group on a 224 cores
> system. The load balance doesn't handle this correctly because
> env->imbalance never decreases and it will stop pulling tasks only after
> reaching loop_max, which can be equal to the number of running tasks of
> the cfs. Make sure that imbalance will be decreased by at least 1.
>
> misfit task is the other feature that doesn't handle correctly such
> situation although it's probably more difficult to face the problem
> because of the smaller number of CPUs and running tasks on heterogenous
> system.
>
> We can't simply ensure that task_h_load() returns at least one because it
> would imply to handle underrun in other places.
>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>

I dug some more into this; if I got my math right, this can be reproduced
with a single task group below the root. Forked tasks get max load, so this
can be tried out with either tons of forks or tons of CPU hogs.

We need

  p->se.avg.load_avg * cfs_rq->h_load
  -----------------------------------  < 1
    cfs_rq_load_avg(cfs_rq) + 1

Assuming homogeneous system with tasks spread out all over (no other tasks
interfering), that should boil down to

  1024 * (tg.shares / nr_cpus)
  ---------------------------  < 1
  1024 * (nr_tasks_on_cpu)

IOW

  tg.shares / nr_cpus < nr_tasks_on_cpu

If we get tasks nicely spread out, a simple condition to hit this should be
to have more tasks than shares.

I can hit task_h_load=0 with the following on my Juno (pinned to one CPU to
make things simpler; big.LITTLE doesn't yield equal weights between CPUs):

  cgcreate -g cpu:tg0

  echo 128 > /sys/fs/cgroup/cpu/tg0/cpu.shares

  for ((i=0; i<130; i++)); do
      # busy loop of your choice
      taskset -c 0 ./loop.sh &
      echo $! > /sys/fs/cgroup/cpu/tg0/tasks
  done

> ---
>  kernel/sched/fair.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 6fab1d17c575..62747c24aa9e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4049,7 +4049,13 @@ static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
>               return;
>       }
>
> -	rq->misfit_task_load = task_h_load(p);
> +	/*
> +	 * Make sure that misfit_task_load will not be null even if
> +	 * task_h_load() returns 0. misfit_task_load is only used to select
> +	 * rq with highest load so adding 1 will not modify the result
> +	 * of the comparison.
> +	 */
> +	rq->misfit_task_load = task_h_load(p) + 1;

For here and below; wouldn't it be a tad cleaner to just do

        foo = max(task_h_load(p), 1);

Otherwise, I think I've properly convinced myself we do want to have
that in one form or another. So either way:

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>

>  }
>
>  #else /* CONFIG_SMP */
> @@ -7664,6 +7670,16 @@ static int detach_tasks(struct lb_env *env)
>                           env->sd->nr_balance_failed <= env->sd->cache_nice_tries)
>                               goto next;
>
> +			/*
> +			 * Depending of the number of CPUs and tasks and the
> +			 * cgroup hierarchy, task_h_load() can return a null
> +			 * value. Make sure that env->imbalance decreases
> +			 * otherwise detach_tasks() will stop only after
> +			 * detaching up to loop_max tasks.
> +			 */
> +			if (!load)
> +				load = 1;
> +
>                       env->imbalance -= load;
>                       break;