Re: [PATCH 1/4] sched/fair: make sure to try to detach at least one movable task

Dietmar Eggemann
2022-09-12T08:44:42Z

On 25/08/2022 14:27, Vincent Guittot wrote:

s/sched/fair: make/sched/fair: Make

> During load balance, we try at most env->loop_max time to move a task.
> But it can happen that the loop_max LRU tasks (ie tail of
> the cfs_tasks list) can't be moved to dst_cpu because of affinity.
> In this case, loop in the list until we found at least one.
>
> The maximum of detached tasks remained the same as before.

Not sure how this relates to the patch? Isn't this given by the
`env->imbalance <= 0` check at the end of detach_tasks()?

>
> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> ---
> kernel/sched/fair.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index da388657d5ac..02b7b808e186 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8052,8 +8052,12 @@ static int detach_tasks(struct lb_env *env)
> p = list_last_entry(tasks, struct task_struct, se.group_node);
>
> env->loop++;
> - /* We've more or less seen every task there is, call it quits */
> - if (env->loop > env->loop_max)
> + /*
> + * We've more or less seen every task there is, call it quits

I never understood this `more or less`. Either we have seen all tasks or
not?

> + * unless we haven't found any movable task yet.
> + */
> + if (env->loop > env->loop_max &&
> + !(env->flags & LBF_ALL_PINNED))
> break;
>
> /* take a breather every nr_migrate tasks */
> @@ -10182,7 +10186,9 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>
> if (env.flags & LBF_NEED_BREAK) {
> env.flags &= ~LBF_NEED_BREAK;
> - goto more_balance;
> + /* Stop if we tried all running tasks */

Would say s/running/runnable but I see that we do use running/runnable
interchangeably.

> + if (env.loop < busiest->nr_running)
> + goto more_balance;
> }
>
> /*

IMHO, there will be some interaction with the `All tasks on this
runqueue were pinned by CPU affinity` check at the end of load_balance()?