Re: [RFC PATCH 2/7] sched/fair: Handle throttle path for task based throttle

[Josh Don]

Hi Aaron,

>  static int tg_throttle_down(struct task_group *tg, void *data)
>  {
>         struct rq *rq = data;
>         struct cfs_rq *cfs_rq = tg->cfs_rq[cpu_of(rq)];
> +       struct task_struct *p;
> +       struct rb_node *node;
> +
> +       cfs_rq->throttle_count++;
> +       if (cfs_rq->throttle_count > 1)
> +               return 0;
>
>         /* group is entering throttled state, stop time */
> -       if (!cfs_rq->throttle_count) {
> -               cfs_rq->throttled_clock_pelt = rq_clock_pelt(rq);
> -               list_del_leaf_cfs_rq(cfs_rq);
> +       cfs_rq->throttled_clock_pelt = rq_clock_pelt(rq);
> +       list_del_leaf_cfs_rq(cfs_rq);
>
> -               SCHED_WARN_ON(cfs_rq->throttled_clock_self);
> -               if (cfs_rq->nr_queued)
> -                       cfs_rq->throttled_clock_self = rq_clock(rq);
> +       SCHED_WARN_ON(cfs_rq->throttled_clock_self);
> +       if (cfs_rq->nr_queued)
> +               cfs_rq->throttled_clock_self = rq_clock(rq);
> +
> +       WARN_ON_ONCE(!list_empty(&cfs_rq->throttled_limbo_list));
> +       /*
> +        * rq_lock is held, current is (obviously) executing this in kernelspace.
> +        *
> +        * All other tasks enqueued on this rq have their saved PC at the
> +        * context switch, so they will go through the kernel before returning
> +        * to userspace. Thus, there are no tasks-in-userspace to handle, just
> +        * install the task_work on all of them.
> +        */
> +       node = rb_first(&cfs_rq->tasks_timeline.rb_root);
> +       while (node) {
> +               struct sched_entity *se = __node_2_se(node);
> +
> +               if (!entity_is_task(se))
> +                       goto next;
> +
> +               p = task_of(se);
> +               task_throttle_setup_work(p);
> +next:
> +               node = rb_next(node);
> +       }

I'd like to strongly push back on this approach. This adds quite a lot
of extra computation to an already expensive path
(throttle/unthrottle). e.g. this function is part of the cgroup walk
and so it is already O(cgroups) for the number of cgroups in the
hierarchy being throttled. This gets even worse when you consider that
we repeat this separately across all the cpus that the
bandwidth-constrained group is running on. Keep in mind that
throttle/unthrottle is done with rq lock held and IRQ disabled.

In K Prateek's last RFC, there was discussion of using context
tracking; did you consider that approach any further? We could keep
track of the number of threads within a cgroup hierarchy currently in
kernel mode (similar to h_nr_runnable), and thus simplify down the
throttling code here.

Best,
Josh