Re: [PATCH -next 02/11] block, bfq: apply news apis where root group is not expected

[yukuai (C)]

在 2022/04/13 18:59, Jan Kara 写道:
> On Wed 13-04-22 11:50:44, Jan Kara wrote:
> > On Sat 05-03-22 17:11:56, Yu Kuai wrote:
> > > 'entity->sched_data' is set to parent group's sched_data, thus it's NULL
> > > for root group. And for_each_entity() is used widely to access
> > > 'entity->sched_data', thus aplly news apis if root group is not
> >                               ^^ apply
Hi,

Thanks for spotting this.
> > > expected. Prepare to count root group into 'num_groups_with_pending_reqs'.
> > >
> > > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > > ---
> > >   block/bfq-iosched.c |  2 +-
> > >   block/bfq-iosched.h | 22 ++++++++--------------
> > >   block/bfq-wf2q.c    | 10 +++++-----
> > >   3 files changed, 14 insertions(+), 20 deletions(-)
> > >
> > > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> > > index 69ddf6b0f01d..3bc7a7686aad 100644
> > > --- a/block/bfq-iosched.c
> > > +++ b/block/bfq-iosched.c
> > > @@ -4393,7 +4393,7 @@ void bfq_bfqq_expire(struct bfq_data *bfqd,
> > >   	 * service with the same budget.
> > >   	 */
> > >   	entity = entity->parent;
> > > -	for_each_entity(entity)
> > > +	for_each_entity_not_root(entity)
> > >   		entity->service = 0;
> > >   }
> >
> > So why is it a problem to clear the service for root cgroup here?

This is not a problem in theory, however 'entity->service' should always
be 0 for root_group. Thus I think there is no need to do this.

> > > diff --git a/block/bfq-wf2q.c b/block/bfq-wf2q.c
> > > index f8eb340381cf..c4cb935a615a 100644
> > > --- a/block/bfq-wf2q.c
> > > +++ b/block/bfq-wf2q.c
> > > @@ -815,7 +815,7 @@ void bfq_bfqq_served(struct bfq_queue *bfqq, int served)
> > >   		bfqq->service_from_wr += served;
> > >   
> > >   	bfqq->service_from_backlogged += served;
> > > -	for_each_entity(entity) {
> > > +	for_each_entity_not_root(entity) {
> > >   		st = bfq_entity_service_tree(entity);
> >
> > Hum, right so how come this was not crashing? Because entity->sched_data is
> > indeed NULL for bfqd->root_group->entity and so bfq_entity_service_tree()
> > returned some bogus pointer? Similarly for the cases you are changing
> > below?
>
> Oh, I see now. Because for_each_entity() currently does not iterate through
> root cgroup because it has root_group->my_entity set to NULL and thus as a
> result immediate children of root_group will have their parent set to NULL
> as well.

Yes, currently for_each_entity() and for_each_entity_not_root() are the
same, they will stop before root_group.

With patch 5, for_each_entity_not_root() will stay the same, while
for_each_entity() will access root_group's entity in addition. And
because bfq_entity_service_tree() will access 'entity->sched_data', thus
I change to the new api here to avoid null-ptr-deref after patch 5.

Same reasons for below changes.

Thanks,
Kuai