Hello! On Thu 09-12-21 10:23:33, yukuai (C) wrote: > 在 2021/11/30 1:11, Jan Kara 写道: > > On Fri 26-11-21 15:47:24, Michal Koutný wrote: > > > Hello. > > > > > > On Thu, Nov 25, 2021 at 06:28:09PM +0100, Jan Kara <jack@xxxxxxx> wrote: > > > [...] > > > +Cc cgroups ML > > > https://lore.kernel.org/linux-block/20211125172809.GC19572@xxxxxxxxxxxxxx/ > > > > > > > > > I understand there are more objects than blkcgs but I assume it can > > > eventually boil down to blkcg references, so I suggest another > > > alternative. (But I may easily miss the relations between BFQ objects, > > > so consider this only high-level opinion.) > > > > > > > After some poking, looking into crashdumps, and applying some debug patches > > > > the following seems to be happening: We have a process P in blkcg G. Now > > > > G is taken offline so bfq_group is cleaned up in bfq_pd_offline() but P > > > > still holds reference to G from its bfq_queue. Then P submits IO, G gets > > > > inserted into service tree despite being already offline. > > > > > > (If G is offline, P can only be zombie, just saying. (I guess it can > > > still be Q's IO on behalf of G.)) > > > > > > IIUC, the reference to G is only held by P. If the G reference is copied > > > into another structure (the service tree) it should get another > > > reference. My naïve proposal would be css_get(). (1) > > > > So I was looking into this puzzle. The answer is following: > > > > The process P (podman, pid 2571) is currently attached to the root cgroup > > but it has io_context with BFQ queue that points to the already-offline G > > as a parent. The bio is thus associated with the root cgroup (via > > bio->bi_blkg) but BFQ uses io_context to lookup the BFQ queue where IO > > should be queued and then uses its parent to determine blkg which it should > > be charged and thus gets to the dying cgroup. > > After some code review, we found that the root cause of the problem > semms to be different. > > If the process is moved from group G to root group, and a new io is > issued from the process, then bfq should detect this and changing > bfq_queue's parent to root bfq_group: > > bfq_insert_request > bfq_init_rq > bfq_bic_update_cgroup > serial_nr = __bio_blkcg(bio)->css.serial_nr; -> from root group > bic->blkcg_serial_nr == serial_nr -> this do not pass，because > bic->blkcg_serial_nr is still from group G So in the crashdump I have available, I can see that _bio_blkcg(bio)->css.serial_nr is 4. Also bic->blkcg_serial_nr is 4. But bic->bfqq[1] is a bfq_queue that has its entity->parent set to already offlined bfq_group. Not sure how that is possible... > __bfq_bic_change_cgroup -> bfq_queue parent will be changed to root group > > And we think the following path is possible to trigger the problem: > > 1) process P1 and P2 is currently in cgroup C1, corresponding to > bfq_queue q1, q2 and bfq_group g1. And q1 and q2 are merged: > q1->next_bfqq = q2. I agree shared queues are some factor in this - the problematic bfq_queue pointing to the dead bfq_group has 'coop' flag set, pid == -1, bic == NULL. So clearly it has been merged with another bfq_queue. > 2) move P1 from C1 to root_cgroup, q1->next_bfqq is still q2 > and flag BFQQF_split_coop is not set yet. There's no next_bfqq in the kernel I'm looking into... Generally the merge code seems to be working somewhat differently to what you describe (I'm looking into 5.16-rc3 kernel). > 3) P2 exit, q2 won't exit because it's still referenced through > queue merge. > > 4) delete C1, g1 is offlined > > 5) issue a new io in q1, q1's parent entity will change to root, > however the io will end up in q1->next_bfqq = q2, and thus the > offlined g1 is inserted to service tree through q2. > > 6) P1 exit, q2 exit, and finially g1 is freed, while g1 is still > in service tree of it's parent. > > We confirmed this by our reproducer through a simple patch: > stop merging bfq_queues if their parents are different. > > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c > index 1ce1a99a7160..14c1d1c3811e 100644 > --- a/block/bfq-iosched.c > +++ b/block/bfq-iosched.c > @@ -2626,6 +2626,11 @@ bfq_setup_merge(struct bfq_queue *bfqq, struct > bfq_queue *new_bfqq) > while ((__bfqq = new_bfqq->new_bfqq)) { > if (__bfqq == bfqq) > return NULL; > + if (__bfqq->entity.parent != bfqq->entity.parent) { > + if (bfq_bfqq_coop(__bfqq)) > + bfq_mark_bfqq_split_coop(__bfqq); > + return NULL; > + } > new_bfqq = __bfqq; > } > > @@ -2825,8 +2830,16 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct > bfq_queue *bfqq, > if (bfq_too_late_for_merging(bfqq)) > return NULL; > > - if (bfqq->new_bfqq) > - return bfqq->new_bfqq; > + if (bfqq->new_bfqq) { > + struct bfq_queue *new_bfqq = bfqq->new_bfqq; > + > + if(bfqq->entity.parent == new_bfqq->entity.parent) > + return new_bfqq; > + > + if(bfq_bfqq_coop(new_bfqq)) > + bfq_mark_bfqq_split_coop(new_bfqq); > + return NULL; > + } > > Do you think this analysis is correct? Honestly, I'm not sure. At this point I'm not sure why the bfqq pointed to from bic didn't get reparented to the new cgroup when bio was submitted... Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
