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    SubjectRe: [PATCH v2] sched/fair: Fix enqueue_task_fair warning some more
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    On 08/05/2020 19:02, Tao Zhou wrote:
    > On Fri, May 08, 2020 at 05:27:44PM +0200, Vincent Guittot wrote:
    >> On Fri, 8 May 2020 at 17:12, Tao Zhou <zohooouoto@zoho.com.cn> wrote:
    >>>
    >>> Hi Phil,
    >>>
    >>> On Thu, May 07, 2020 at 04:36:12PM -0400, Phil Auld wrote:
    >>>> sched/fair: Fix enqueue_task_fair warning some more

    [...]

    >>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    >>>> index 02f323b85b6d..c6d57c334d51 100644
    >>>> --- a/kernel/sched/fair.c
    >>>> +++ b/kernel/sched/fair.c
    >>>> @@ -5479,6 +5479,13 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
    >>>> /* end evaluation on encountering a throttled cfs_rq */
    >>>> if (cfs_rq_throttled(cfs_rq))
    >>>> goto enqueue_throttle;
    >>>> +
    >>>> + /*
    >>>> + * One parent has been throttled and cfs_rq removed from the
    >>>> + * list. Add it back to not break the leaf list.
    >>>> + */
    >>>> + if (throttled_hierarchy(cfs_rq))
    >>>> + list_add_leaf_cfs_rq(cfs_rq);
    >>>> }
    >>>
    >>> I was confused by why the throttled cfs rq can be on list.
    >>> It is possible when enqueue a task and thanks to the 'threads'.
    >>> But I think the above comment does not truely put the right
    >>> intention, right ?
    >>> If throttled parent is onlist, the child cfs_rq is ignored
    >>> to be added to the leaf cfs_rq list me think.
    >>>
    >>> unthrottle_cfs_rq() follows the same logic if i am not wrong.
    >>> Is it necessary to add the above to it ?
    >>
    >> When a cfs_rq is throttled, its sched group is dequeued and all child
    >> cfs_rq are removed from leaf_cfs_rq list. But the sched group of the
    >> child cfs_rq stay enqueued in the throttled cfs_rq so child sched
    >> group->on_rq might be still set.
    >
    > If there is a throttle of throttle, and unthrottle the child throttled
    > cfs_rq(ugly):
    > ...
    > |
    > cfs_rq throttled (parent A)
    > |
    > |
    > cfs_rq in hierarchy (B)
    > |
    > |
    > cfs_rq throttled (C)
    > |
    > ...
    >
    > Then unthrottle the child throttled cfs_rq C, now the A is on the
    > leaf_cfs_rq list. sched_group entity of C is enqueued to B, and
    > sched_group entity of B is on_rq and is ignored by enqueue but in
    > the throttled hierarchy and not add to leaf_cfs_rq list.
    > The above may be absolutely wrong that I miss something.
    >
    > Another thing :
    > In enqueue_task_fair():
    >
    > for_each_sched_entity(se) {
    > cfs_rq = cfs_rq_of(se);
    >
    > if (list_add_leaf_cfs_rq(cfs_rq))
    > break;
    > }
    >
    > In unthrottle_cfs_rq():
    >
    > for_each_sched_entity(se) {
    > cfs_rq = cfs_rq_of(se);
    >
    > list_add_leaf_cfs_rq(cfs_rq);
    > }
    >
    > The difference between them is that if condition, add if
    > condition to unthrottle_cfs_rq() may be an optimization and
    > keep the same.
    >

    I'm not 100% sure if this is exactly what Tao pointed out here but I
    also had difficulties understanding understanding how this patch works:

    p.se
    |
    __________________|
    |
    V
    cfs_c -> tg_c -> se_c (se->on_rq = 1)
    |
    __________________|
    |
    v
    cfs_b -> tg_b -> se_b
    |
    __________________|
    |
    V
    cfs_a -> tg_a -> se_a
    |
    __________________|
    |
    V
    cfs_r -> tg_r
    |
    V
    rq

    (1) The incomplete update happens with cfs_c at the end of
    enqueue_entity() in the first loop because of 'if ( .... ||
    cfs_bandwidth_used())' (cfs_b->on_list=0 since cfs_a is throttled)

    (2) se_c breaks out of the first loop (se_c->on_rq = 1)

    (3) With the patch cfs_b is added back to the list.
    But only because cfs_a->on_list=1.

    But since cfs_a is throttled it should be cfs_a->on_list=0 as well.
    throttle_cfs_rq()->walk_tg_tree_from(..., tg_throttle_down, ...) should
    include cfs_a when calling list_del_leaf_cfs_rq().

    IMHO, throttle_cfs_rq() calls tg_throttle_down() for the throttled
    cfs_rq too.


    Another thing: Why don't we use throttled_hierarchy(cfs_rq) instead of
    cfs_bandwidth_used() in enqueue_entity() as well?

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