On 11/05/2020 11:36, Vincent Guittot wrote:
> On Mon, 11 May 2020 at 10:40, Dietmar Eggemann <dietmar.eggem...@arm.com>
> wrote:
>>
>> 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 <zohooou...@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
[...]
>> 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
>>
>
> In your example, which cfs_ rq has been throttled ? cfs_a ?
Yes, cfs_a. 0xffffa085e48ce000 in Phil's trace.
>
>> (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)
>
> so cfs_c is added with the 1st loop
Yes.
>> (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.
>
> hmm I don't understand the link between cfs_b been added and cfs_a->on_list=1
cfs_b, 0xffffa085e48ce000 is the one which is now added in the 2. loop.
Isn't the link between cfs_b and cfs_a the first if condition in
list_add_leaf_cfs_rq():
if (cfs_rq->tg->parent &&
cfs_rq->tg->parent->cfs_rq[cpu]->on_list)
to 'connect the branch' or not (default, returning false case)?
> cfs_b is added with 2nd loop because its throttle_count > 0 due to
> cfs_a been throttled (purpose of this patch)
>
>>
>> But since cfs_a is throttled it should be cfs_a->on_list=0 as well.
>
> So 2nd loop breaks because cfs_a is throttled
Yes.
> The 3rd loop will add cfs_a
Yes, but in the example, cfs_a->on_list=1, so we bail out of
list_add_leaf_cfs_rq() early.
I don't grasp how can cfs_a->on_list=1, when cfs_a is throttled and
cfs_b, cfs_c are in a throttled hierarchy?
>> 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?
>
> Mainly to be conservative because as this patch demonstrates, there
> are a lot of possible use cases and combinations and I can't ensure
> that it is always safe to use the throttled_hierarchy.
Maybe this deserves a comment then.
