On Mon, 11 May 2020 at 19:02, Dietmar Eggemann <dietmar.eggem...@arm.com> wrote: > > On 11/05/2020 14:12, Vincent Guittot wrote: > > 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 > > Yes, I agree. But coming back to what the patch does in the example: > > W/ the patch, list_add_leaf_cfs_rq() is now called for cfs_b and since > cfs_b->tg->parent->cfs_a and cfs_a->on_list=1 the 'branch is now > connected' which means 'rq->tmp_alone_branch = &rq->leaf_cfs_rq_list'. > > I.e. assert_list_leaf_cfs_rq() at the end of enqueue_task_fair() is not > barfing anymore. > > W/o the patch, list_add_leaf_cfs_rq() called w/ cfs_c left the 'branch > open', it's not called on cfs_b and since cfs_a->on_list=1, the 3rd > for_each_sched_entity() in enqueue_task_fair() doesn't 'connect the > branch' so the assert fires. > > What I don't immediately see is how can cfs_a be throttled (which causes > cfs_b -> cfs_c being a throttled hierarchy) and be on the list > (cfs_a->on_list=1) at the same time. > > So the only thing how this could happen is when there was a task enqueue > in a parallel cfs_b' (another child of cfs_a) sub hierarchy just before > the example.
Yes. A task has been enqueued on another child (cfs_b') and cfs_a has been be added back to ensure that cfs are correctly ordered > > >> 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. > > True, we go the 2nd if() condition in list_add_leaf_cfs_rq(). > > >>> 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 > > Yes. > > [...]