On Mon, 11 May 2020 at 12:39, Dietmar Eggemann <dietmar.eggem...@arm.com> wrote: > > 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
on_list is only there to say if the cfs_rq is already in the list but there is not dependency with the child > 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)? > In your example above if the parent is already on the list then we know where to insert the child. > > 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. Because the cfs_rq is on the list already so we don't have to add it > > 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.
