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. >> 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. [...]
