在 2021/12/14 1:33, Jan Kara 写道:

Hello!

On Thu 09-12-21 10:23:33, yukuai (C) wrote:

quoted

在 2021/11/30 1:11, Jan Kara 写道:

quoted

On Fri 26-11-21 15:47:24, Michal Koutný wrote:

quoted

Hello.

On Thu, Nov 25, 2021 at 06:28:09PM +0100, Jan Kara [off-list ref] wrote:
[...]
+Cc cgroups ML
https://lore.kernel.org/linux-block/20211125172809.GC19572-4I4JzKEfoa/jFM9bn6wA6Q@public.gmane.org/


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.)

quoted

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...

quoted

    __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.

quoted

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).

Hi, Jan

Sorry, It should be new_bfqq, which will be set if the queue is merged
to other queue.

quoted

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...

After queue merging, bic is set to the new bfqq, for example:

__bfq_insert_request
  new_bfqq = bfq_setup_cooperator
  bfq_merge_bfqqs -> before this is done, bic is set to old bfqq
   bic_set_bfqq(bic, new_bfqq, 1); -> bic is set to new bfqq
  rq->elv.priv[1] = new_bfqq;

I think current problem is that if bfq_queue is merged, task migration
won't break such cooperation，thus issue io from one cgroup may endup to
a bfq_queue that is from another cgroup, which might be offlined.

Thanks,
Kuai

								Honza